Fm Crystal Radio Receivers 3o5q

FM Crystal Radio Receivers The notion of "crystal radio" is strongly associated with huge antennas and radio broadcasting on long and medium bands, in this article, the author describes the experimentally tested detector circuits of VHF receivers designed to listening to a FM stations. The very possibility of receiving VHF FM detector was discovered accidentally. One day I was walking in the Terletskiypark in Moscow, Novogireevo, I decided to listen to the broadcast - I had a simple crystal set without resonant tank (this circuit is described in the "Radio", 2001, № 1, Fig. 3). The receiver had a telescopic antenna with length of about 1.4 m. Wonder whether it is possible to receive radio broadcast with this short antenna? It was possible to hear, but weakly, simultaneous operation of two stations. But what is surprised me is the volume of receiving was rise and fall periodically almost to zero after every 5...7 m, and it was different for each radio station! It is known that in the LW and MW bands, where the wavelengths are hundreds of meters, it is impossible. I had to stop at the point of receive with maximum volume of one of the stations and listen attentively. It turned out - this is "Radio Nostalgie", 100.5 MHz, broadcasting from the near city Balashikha. There were no line of sight between antennas. How does the FM transmission could be received by using the AM detector? Further calculations and experiments shows that it is quite possible and is not depends on the receiver. A simple portable FM crystal receiver is made exactly the same way as an indicator of the electric field, but instead of measuring device it is necessary to connect a high-impedance headphones. It makes sense to add an adjustment of coupling between the detector circuit and the resonant tank to adjust the maximum volume and quality of the receiving signal. The simplest Crystal radio The circuit diagram of the receiver suitable for these requirements is shown in Fig. 1. This circuit is very close to the circuit of the receiver mentioned above. Only the VHF resonant tank has been added to the circuit.

Fig. 1. VD1, VD2 - GD507A - an old USSR Germanium high-frequency diodes with the capacitance of 0.8 pF (at the reverse voltage of 5V), the recovery time of reverse resistance is no more than 0.1 uS (at the Idirect pulse=10 mA, Ureverse pulse=20 V, Icutoff=1 mA)

The device contains a telescopic antenna WA1, directly connected to the resonant tank L1C1. The antenna is also an element of the resonant tank, so to get the maximum power of the signal it must be adjust both the length of the antenna and the frequency of the tank circuit. In some cases, especially when the length of the antenna is about 1/4 of the wavelength, it is useful to connect the antenna to a tap of the tuning coil L1 (find the suitable tap of the coil by finding the maximum volume of the signal). The coupling with the detector can be adjust by trimmer C2. Actually the detector is made of two highfrequency germanium diodes VD1 and VD2. The circuit is completely identical to the voltage doubling rectifier circuit, but the detected voltage would be doubled if only the trimmer capacitor C2 value is high, but then the load of the resonant circuit L1C1 would be excessive, and its quality factor Q will be low. As a result, the signal voltage in the circuit tank L1C1 will be lower and the audio volume will be lower too. In our case, the capacitance of the coupling capacitor C2 is small enough and voltage doubling does not occur. For optimal matching the detector circuit with the tank circuit the impedance of the coupling capacitor must be equal to the geometric mean between the input resistance of the detector and the resonant resistance of the tank circuit L1C1. Under this condition, the detector is getting the maximum power of the high-frequency signal, and this is corresponding to the maximum audio volume. The capacitor C3 is shunting the higher frequencies at the output of the detector. The load of the detector is headphones with the dc resistance of not less than 4K ohms. The whole unit is assembled in a small metal or plastic housing. The telescopic antenna with the length not less then 1m is attached to the upper part of the housing, and the connector or the jack for the phones is attached th the bottom of the housing. Note that the phone cord is the second half of the dipole antenna (a counterweight). The coil L1 is frameless, it contains 5 turns of enameled copper wire with diameter of 0.6...1 mm wound on a mandrel with diameter of 7...8 mm. You can adjust the necessary inductance by stretching or compressing the turns of the coil L1. It's better use the variable capacitor C1 with an air dielectric, for example, type 1KPVM with two or three movable and one or two fixed plates. Its maximum capacity is small and can be in range of 7...15 pF. If the variable capacitor has more plates (the capacitance is higher), it is advisable to remove any of the plates, or connect the variable capacitor in series with a constant capacitor or a trimmer, it will reduce the maximum capacity. The capacitor C2 is ceramic trimmer capacitor, such as a KPK or KPK-M with the capacity of 2...7 pF. Other trimmers capacitors could be used too. The trimmer capacitor C2 can be replaced with a variable capacitor, similar to C1, and it could be used to adjust the coupling "on the fly" to optimize radio receiving capabilities. Diodes VD1 and VD2, can be GD507B, D18, D20 (it is old USSR Germanium high-frequency diodes. This diodes can be replaced with modern Schottky diodes). The shunting capacitor C3 is ceramic, its capacity is not critical and can have a value in range from 100 to 4700 pF. Adjustment of the receiver is simple. Tune the radio by turning the knob on the variable capacitor C1 and adjust the capacitor C2 to get the maximum audio volume. The tune of the resonant tank L1C1 will

be changed, so all operations must be repeated a few more times, and at the same time find the best place for the radio receiving. It is doesn't necessarily the same place where the electric field has maximum strength. This should be discussed in more detail and explain why this receiver can receive FM signals. Interference and conversion of FM into AM If the tank circuit L1C1 of our receiver (Fig. 1) will be set up so that the carrier frequency of FM signal falls on the slope of the resonance curve, the FM can be converted into AM. Let's find the value of Q of the tank circuit. Assuming that the bandwidth of the tank circuit L1C1 is equal to twice the frequency deviation, we obtain Q = F0 /Δ2f = 700 for both the upper and the lower VHF band. The actual Q of the tank circuit in a crystal radio probably will be less than 700 because of the low Qfactor of its own Q (About 150...200) and because the resonant tank is shunted by the antenna and by the input impedance of the detector. Nevertheless, a weak transformation of FM into AM is possible, thus, the receiver will barely work if its tank circuit detune a little up or down in frequency. However, there is much more powerful factor contributing to the transformation of FM into AM, - it is an interference. It's very rarely when the receiver is in the line of sight of radio station, in most cases the line of sight is obscured by buildings, hills, trees and other reflective objects. A few radio beams scattered by these objects comes to the antenna of the receiver. Even in the line of sight to the antenna comes some reflected signals (and of course, direct signal comes too). The total signal depends on both the amplitudes and phases of summing components. The two signals are summed if they are in phase, i.e., the difference of their ways is multiple of an integer of the wavelength, and the two signals are subtracted if they are in opposite phase, when the difference of their ways is the same number of wavelengths plus half wavelength. But the wavelength, as well as the frequency varies at FM! The difference of the beams and their relative phase shift will vary. If the difference of ways is large, then even a small change in frequency leads to significant shifts in the phases. An elementary geometric calculation leads to the relation: Δf/f0 = λ/4ΔC, or ΔC = f0/λ/4Δf, where ΔC - the difference of the ways of the , it's required for the phase shift ±Π/2, to get the full sum of AM signal, Δf - frequency deviation. The full AM is the total variation of the amplitude signal from the sum of the amplitudes of the two signals to their difference. The formula can be further simplified if we consider that the multiply of frequency by the wave length f0λ is equal to the speed of light c: ΔC = c/4Δf. Now it is easy to calculate that to get a full AM of the two-beam FM signal, the sufficient difference between the ways of beams is about a kilometer. If the difference of ways is smaller, the depth of AM proportionally decreases. Well, but if the difference of ways is more? Then, during one period of the modulating audio signal the total amplitude of the interfering signal will several times through the highs and lows, and distortion will be very strong when converting FM into AM, up to complete indistinct of the sound when you receive the FM by an AM detector. Interference with FM broadcast reception is an extremely harmful phenomenon. It is not only produces a concomitant parasitic AM of a signal, as it is described above, but it is produces the parasitic phase

modulation, what leads to distortion even if we got a good FM receiver. That's why it is so important to place the antenna in the right location, where the only one signal prevails. It is always better to use a directional antenna, because it increases the magnitude of the direct signal and reduces reflections coming from other directions. Only in this case with a very simple detector radio receiver the interference played a useful role and allowed us to listen to the radio broadcast, but the radio broadcast can be heard weakly or with significant distortions, and the radio broadcast can't be heard everywhere, but only in certain places. This explains the periodic changes in the volume of the radio broadcast in the Terletskiypark. Crystal Detector Radio Receiver with a frequency detector A radical way to improve reception is to use a frequency detector instead of an amplitude detector. In Figure 2 is shown a circuit of a portable detector radio receiver with a simple frequency detector, based on a single high-frequency germanium transistor VT1. The germanium transistors is used because it's junctions works at a low voltage about 0.15 Volts, this allows to detect very weak signals. The junctions of silicon transitions works at a voltage approximately 0.5 V, and the sensitivity of the receiver with a silicon transistor is much lower.

Fig. 2. VT1 - GT313A - an old USSR Germanium high-frequency transistor with hfe=10...230 (at DC: Uke=3 V, Ie=15 mA), hfe=3..10 (at f=100 mHz, Ukb=5 V, Ie=5 mA) As in the previous design, the antenna is connected to the input tank circuit L1C1, the variable capacitor C1 is used for the tuning function. The signal from the input tank circuit goes to the base of the transistor VT1. The other tank circuit, L2C2, is inductively coupled with the input tank circuit L1C1. The tank circuit L2C2 is tuneble with the variable capacitor C2. Because of the inductive coupling between this two tanks the oscillation in the resonant tank L2C2 is phase shifted by 90° relative to the signal across the input tnak circuit L1C1. From the tap of the coil L2 the signal goes to the emitter of the transistor VT1. A by capacitor C3 and high impedance headphones BF1 is connected to the collector of the transistor VT1. The transistor begins to turn on when its base and emitter has the positive half-wave of the signal, and the instantaneous voltage on the emitter is greater then its base voltage. At the same time the

smoothed detected current es through the headphone in the collector network. But the positive half-wave of the signal is only partially overlapping when the phase shift of the signal is 90° in the resonant tanks, so the detected current reaches the maximum value determined by the signal level. With frequency modulation, depending on the frequency deviation, the phase shift is also changing, corresponding to the phase-frequency response of the tank circuit L2C2. When the frequency deviates in one direction then the phase shift decreases and the half-waves of the signal at the base and emitter is overlapped more, as a result, the detected current increases. When the frequency deviation goes in the opposite direction, its decreases the overlap of half-waves of the signal and the current decreases. So the frequency detection of the signal occurs. The gain of the detector depends directly on the quality factor Q of the resonant tank L2C2, the quality factor Q should be as high as possible (in the limit of 700, as we calculated earlier), therefore the coupling with the emitter of the transistor is weak. Of course, such a simple detector does not suppress the AM of the received signal. In fact, its detected current is proportional to the signal level at the input, this is an obvious disadvantage. But anyway it's the very simple circuit. Just like the previous circuit, the receiver is built in a small housing, on the top of the housing a telescoping antenna is mounted, and the headphone socket in the bottom the housing. The knobs of the variable capacitors is located on the front . These variable capacitors should not be combined into one unit, because a louder volume and a better quality of reception can be obtained with separate tuning. The coils L1, L2 if frameless, they wound with the copper wire 0.7 mm (AWG 21) in diameter on the mandrel of diameter 8 mm. L1 contains 5 turns, L2 - 5+2 turns. If possible, the coil L2 wound with silver plated wire to improve the quality factor Q, the diameter of the wires is not critical. The inductance of the coils is adjusted by compressing or stretching of the coils L1 and L2 to get the FM radio stations in the middle of the variable capacitors tuning range. The distance between the coils L1 and L2 is in the range of 15...20 mm (the axis of the coils is parallel), the distance is adjusted by bending their terminals, soldered to the variable capacitors. With this receiver can be done a lot of interesting experiments, exploring the possibility of reception of VHF radio broadcasts with the detector receiver, exploring the propagation of radio waves in urban areas, etc.Can be done experiments to further improve the receiver. However, the sound quality in a high-impedance headphones with membranes is poor. Because of it a better receiver was developed, which provides better sound quality and allows you to use a different external antennas, connected to the receiver by feedline. Radio receiver powered by the energy of radio waves Experimenting with a simple crystal radio set, repeatedly had to make sure that the power of the detected signal is sufficiently enough (tens or hundreds of microwatts) to provide a very loud sound in the headphones. But the quality of reception is not good because there is no frequency detector. This problem is partially solved in the second receiver (Fig. 2), but the signal strength is also used inefficiently

because the transistor is powered by quadrature high-frequency signal. Therefore it was decided to use two detectors in the receiver: the envelope detector - to power the transistor, and the frequency detector - to improve signal detection.

Fig. 3. C1, C2 - 2.2...15 pF, C3 - 0.15 uF, C4 - 1 uF, C5 - 1 nF, R1 - 130 k The circuit diagram of the receiver is shown in Fig. 3. An external antenna (dipoles) connected to the receiver by a two-wire line, made of ribbon VHF cable with the impedance of 240...300 ohms. The impedance matching between the cable and the antenna is performed automatically, and the impedance matching of the input tank circuit L1C1 is performed by selecting a suitable tap of the coil L1. Generally speaking, unbalanced connection of the feeder to the input tank circuit reduces the noiseproofing of the antenna feeder system, but because the low sensitivity of the receiver, it doesn't matter. There is a well-known methods of balanced connections for a feeder with the use of a coupling coil or a balun. The author's folded dipole was made of a conventional isolated connecting wire, the dipole was placed on the balcony, in a place with a maximum field strength. The length of the feeder does not exceed 5 m. With such a small length the losses in the feeder is negligible, and therefore, the balanced line can be successfully used. The input tank circuit L1C1 is tuned to a frequency of a signal, and a high frequency voltage across L1C1 is rectified by an amplitude detector, based on the high-frequency diode VD1. Since the amplitude of FM signal has a constant value, there is practically no requirements for smoothing the rectified DC voltage. However, to remove possible parasitic amplitude modulation in case of multipath propagation of radio signals (see above story about the interference), the capacitance of the smoothing capacitor C4 is selected sufficiently large. A rectified DC voltage is used to power transistor VT1. For the control of the current consumption and for a signal level indication is used an analog current meter PA1. A quadrature frequency demodulator of the receiver is implemented with the transistor VT1 and phase shifter tank circuit L2C2. The high-frequency signal from the tap of the coil L1 is applied to the base of the transistor VT1 through the coupling capacitor C3, and it's signal is applied to the emitter of the

transistor VT1 from the tap of the coil L2 of the phase-shifting tank circuit L2C2. The work of the detector is exactly the same as in the previous design. To increase the gain of the frequency demodulator, on the base of the transistor VT1 is applied an offset voltage through the resistor R1, and because of it the coupling capacitor C3 is used. Note that the capacitor C3 has sufficient capacitance (0.15 uF) - this capacitance is chosen to shunt the low-frequency currents, i.e., for grounding the base of the transistor VT1 for the sound frequencies. This increases the gain of the transistor and increases the volume of reception. The primary winding of the output transformer T1 in the collector circuit of the transistor VT1 is used to match the high output impedance of the transistor to the low impedance of the headphones. A stereo headphones TDS-1 (8..16 ohms) or TDS-6 (8 ohms) can be used with this radio. Both the earpieces (left and right channels) are connected in parallel. The by capacitor C5 is used to filter the highfrequency currents in the collector circuit. The button SB1 is used to short the collector circuit of the transistor VT1 while tuning the input tank circuit and the search for a signal. The sound in the headphones at the same time disappears, but the sensitivity of the indicator PA1 is significantly increased. The design of the receiver can be very different, but anyway it needs the front with the knobs of the two variable capacitors C1 and C2 (each capacitor has individual knob) and the button SB1. To reduce hand effect on the tuning, it is desirable to make the front of a metal plate or a copper clad laminates. It can work also as a common wire of the receiver. Rotors of the variable capacitors should have good electrical with the . The antenna socket X1 and the phone jack X2 can be placed either on the front or on the side or back of the receiver. Its dimensions are dependent on the available components. So let's say a few words about them. The capacitors C1 and C2 is KPV type with a maximum capacity of 15...25 pF. The capacitors C3-C5 are ceramic. The coils L1 and L2 are frameless (see Figure 4), wound on a mandrel of diameter 8 mm, L1 contain 5, L2 contains 7 turns. The length of the winding is 10...15 mm (do some tuning by adjusting the length). The enameled copper wire of 0.6...0.8 mm (AWG 20..23) is used, but it is better to use a silver-plated wire, especially for the coil L2. The taps are made from 1 and 1.5 turns (L1) and from 1 turn (L2). The coils can be arranged coaxially or axis parallel to each other. The distance between the coils (10...20 mm) is adjusted. The receiver will work even in the absence of inductive coupling between the coils - the capacitive coupling through the junction capacitance of the transistor is enough. The audio transformer T1 is TAG-3, it has a winding ratio of 10:1 or 20:1.

Fig. 4.

The transistor VT1 can be replaced by any germanium transistor with maximum operating frequency ft not lower than 400 MHz. A p-n-p transistor can be used too, for example, GT313A, in this case the polarity of the indicator PA1 and the diode VD1 should be reversed. The diode can be any germanium type, a high-frequency. As the indicator PA1 any ammeter with a current range of 50..150 mA can be used. Tune the tank circuits to the frequency of a radio station, adjust the taps of the coils and the distance between the coils to get the best result (maximum volume and best quality of the reception). It is useful to adjust the value of the resistor R1 for maximum volume. On the balcony the receiver with the antenna described above provided high quality reception of two stations with the strongest signal from the radio center at the distance not less than 4 km and with no direct line of sight (obscured by buildings). Collector current of the transistor was 30...50 mA. Of course, the possible design of VHF crystal radios is not limited to described above. On the contrary, this circuit should be considered only as the first experiments in this interesting field. When using an efficient antenna, placed on a roof and targeted at a radio station, it is possible to obtain sufficient signal strength, even at a considerable distance from the station. This provides a high-quality reception on a headphones, and in some cases, you can get loudspeaking reception. It is possible to improving this receivers by using a more efficient detection circuit and using a high-quality resonant tanks, in particular, spiral resonators as resonant circuits.

Power supply 1. NiCd battery charger for flashlight.

2. FET based voltage regulator.

3. Constant current sources.

4. Constant current source.

5. Op-amp as voltage regulator circuit.

6. Constant current source with cascode of n-p-n transistors.

7. Two-terminal constant current source.

8. Constant current sources.

9. Battery charger circuit diagram.

10. DC-DC voltage converter circuit schematic.

11. Very good battery charger circuit.

12. Replacement for high voltage zener diode.

13. Simple Geiger counter circuit diagram.

14. Battery charger circuit.

15. DC-DC converter based on CD4000.

16. Constant current source.

17. Replacement for zenerdiod.

18. Voltage regulator with current limiter - constant current source circuit diagram.

19. Voltage regulator with solar battery for charger.

20. Power supply with 50/60 Hz noise suppression circuit.

21. Voltage regulator with suppression for main harmonic circuit.

22. Charger powered by free energy.

23. Logic gate (7400) based voltage regulator.

24. Current source controlled by voltage.

25. Constant current source based on voltage regulator IC.

26. Switch Mode Power Supply.

27. Isolated Power supply for digital clock.

28. DC-DC converter with current multiplier.

29. DC-DC voltage converter based on Voltage Quadrupler circuit with IC CD4093.

30. Constant current source based on ТL431 IC.

31. Voltage to current convertrt.

32. Shunt voltage regulator circuit.

33. Unusual rectifier circuit diagram.

34. Powerful shunt voltage regulator.

35. Thyristor-based switching power supply.

36. Voltage regulator with current source.

37. Voltage regulator - charger.

38. DC-DC voltage converter.

39. Voltage-to-current converter drives LED.

40. DC to DC converter drives blue LED.

41. Current source, controlled by voltage.

42. Led step-up converter for flashlight.

43. IC voltage regulators connected in parallel.

44. Voltage regulator with improved stability.

45. Voltage regulator with double pulse frequency.

46. Current source.

47. Circuit with negative resistance.

48. Voltage converters with current coupling .

1. Schmitt trigger circuit diagram.

2. Audio compressor.

3. Comparator with level-dependent hysteresis.

4. Capacitive sensor.

5. Voltage to current converter.

6. Bridge with current stabilisation circuit.

7. Two-terminal circuit with negative resistance.

8. Window comparator circuit diagram.

9. Phase detector circui.

10. Switch mode phase detectors.

11. Balanced phase detector circuit diagram.

12. High Input Impedance AC Amplifier.

13. Trigger based on opto-isolator circuit schematic.

14. Frequency doubler.

15. Stepper motor controller based on IC 7474 circuit schematic.

16. Stepper motor working in synchronous mode.

17. Triangular wave to sine wave converter.

18. Bidirectional intercom circuit.

19. Frequency divider with variable division ratio.

20. Sawtooth wave to sine wave converter circuit.

21. Changing band of variable capacitor by transformer.

22. Welding transformer circuit diagram.

23. Metal detector circuit.

24. Superregenerative metal detector circuit.

25. Sensitive capacitive sensor circuit.

26. Voltage to frequency converter.

27. Regenerative capacitance multiplier.

28. Equalization of output resistance.

29. Wien bridge notch filter circuit schematic.

30. Compensate capacity load to avoid self-excitation.

31. Notch filter circuit.

32. Notch filter based double T-shaped bridge circuit.

33. Adjustable notch filter circuit based on bridge differential unit.

34. Notch filter circuit.

35. Active notch filter circuit.

36. Notch filter circuit with Wien-Robinson bridge.

37. Notch filter circuit.

38. Adjustable notch filter circuit.

39. Adjustable notch filter circuit.

40. Variable capacitor based on Op-Am.

41. Phase modulator based on op-amp.

42. Increasing amplitude of single pulse.

43. Equivalent of resistor with high resistance.

44. Resonant filter based on rejection filter.

45. Resistance to period converter circuit.

46. Sine wave to sawtooth wave converter.

47. Frequency divider based on DIAC.

48. Nonlinear sawtooth wave to sine wave converter.

49. Sine wave former circuit diagram.

50. Regenerative notch filter circuit.

51. Replacement of the high-resistance resistor on a low resistance.

52. Sawtooth wave to sine wave converter circuit.

53. Voltage to current converter.

54. Neutralization feedthrough capacitance.

55. Regenerative frequency divider circuit.

1. Duty-cycle to dc converter.

2. Frequency divider (Fin<200мГц; IC's - MC10137 and 7400).

3. Replacement for Lambda-diode.

4. Voltage limiter.

5. Frequency doubler circuit diagram.

6. Triangle-wave Generator.

7. Frequency comparator circuit diagram. (with IC's 74121 and 7474)

8. Analog of DIAC circuit schematic.

9. Comparator with two edges.

10. Frequency divider.

11. Buffer/inverter gate made of the trigger CD4013.

12. Circuit finding difference of two frequencies and phase detector circuit.

13. bounce eliminator circuit.

14. Narrow bandfilte.

15. Current stabilizer for Zener diode.

16. Restore signal by DC.

17. Phase changer with constant amplitude of signal at output.

18. Phase filter circuit.

19. Edge detection circuit diagram.

20. Frequency comparator circuit diagram.

21. Comparator-monostablemultivibrator based on LM139 voltage comparator .

22. Monostablemultivibrator with wide range of pulses (CD4000).

23. Monostablemultivibrator doubles number of pulses (CD4000).

24. Monostablemultivibrator based on inductor (CD4000).

25. Monostablemultivibrator based on inductor (CD4000).

26. Monostablemultivibrator based on inductor and trigger (CD4000).

27. Duty-cycle indicator circuit. If Duty-cycle=50% then U1=U2.

28. Analog of zener diode with low operating voltage.

29. Synchronous detector.

30. Output stage of phase detecto.

31. Gyrator circuit.

32. Detector of the frequency. (CD4000)

33. Regenerative filter circuit.

34. Comparator-monostablemultivibrator.

35. Filter for carrier frequency.

36. Voice frequency doubler circuit.

37. Notch filter with Q multiplier circuit.

38. Flashes lights based on neon lamps circuit diagram.

39. Output stage of a DC to DC converter circuit.

40. Simple ADC with potential divider circuit based on CD4000 series. See details

41. Simple ADC with current summator circuit based on CD4000 series. See details

42. 3D image on the screen of the oscilloscope.

How to calculate coil inductance - Coil Inductance Calculator The inductance of a coil depends on its geometrical characteristics, the number of turns and the method of winding the coil. The larger the diameter, length, and the larger number of winding turns, the greater its inductance.

If the coil is tightly wound, turn to turn, then it will have more inductance than a not tightly wound coil, with gaps between the turns. Sometimes you need to wind a coil with a given geometry, and you don't have a wire with required diameter, then if use a thicker wire you should increase slightly number of turns, and if use a thinner wire it takes to reduce the number of turns of the coil to get the required inductance. All of the above considerations are related to winding coils without ferrite cores. Inductance of single-layer coils on cylindrical winding forms can be calculated by the formula: L=(D/10)2*n2/(4.5*D+10*l) D= L=

18

n=

20

l=

(1)

20

μH

Where L - inductance of the coil, μH; D - diameter of the coil (diameter of the former), mm; l - length of the coil, mm; n - number of turns of windings. There is could be two tasks in the calculation: A. The geometry of the coil is given, find the inductance; B. The inductance of the coil is given, calculate the number of turns and the diameter of the wire. In the case "A" all data are given, it is easy to find the inductance.

Example 1. Let's calculate the inductance of the coil shown in the figure above. Put the values in the formula 1: L=(18/10)2*202/(4.5*18+10*20) = 4.6 μH In the second case the coil diameter and the length of the wound are known. The length of the wound depends on the number of turns and the wire diameter. Therefore, it is recommended to calculate in this order. Based on geometric considerations, determine the size of the coil, the diameter and the length of the wound, and then counting the number of turns by the formula:

n=10*(5*L*(0.9*D+2*l))1/2/D

D= n=

10

L=

0.8

0

l=

(2)

20

turns.

After you have found the number of turns, determine the diameter of the wire with insulation according to the formula: d=l/n

l= d=

20

0

(3)

n=

14

mm.

Where d - diameter of the wire, mm; l - winding length, mm; n - number of turns. Example 2. We need to make a coil with a diameter of 10 mm and with a length the winding of 20 mm, the coil should have an inductance of 0,8 μH. The winding has one layer, turn to turn. Put the values in the formula 2, we get: n = 10*(5*0.8*(0.9*10+2*20))1/2/10 = 14 The diameter of the wire: d = 20/14 = 1.43 mm To wind the coil with a wire of smaller diameter, it is necessary to place obtained by calculation 14 turns across the entire length of the coil (20 mm) with equal intervals between the turns (the step of winding). The inductance of the coil will be 1-2% less than the nominal value, it should be considered in the manufacture of these coils. To wind the coil with a thicker wire than 1.43 mm, the new calculation should be done with the increased diameter or length of the coil winding. You may also need to increase both the diameter and the length at the same time, until get the desired dimensions of the coil for a given inductance. It should be noted that the above formulas is intended to calculate the coils with the length of winding l equal to or more than half of the diameter. If the length of winding is less than half the diameter of the winding D/2, the more accurate results can be obtained by using the formulas below: L = (D/10)2*n2/((4D+11l))

(4)

D= L=

18

n=

l=

0

20

μH

and n = (10L*(4D+11l))1/2/D D= n=

10

L=

0.8

l=

0

(5) 20

turns.

Hidden wire detector If you want to drill a hole in the wall then you must be sure that there's no electrical wiring. This simple device.shown on the figure 1, can detect electrical wiring in the walls or ceiling. Resistor R1 protects IC CD4011 against electrostatic. A rigid copper piece of wire (~ 18 AWG, with length of 5..15 cm) works as antenna. Sensitivity of the circuit depends on the length of the antenna. When the antenna is placed near the electrical wiring, then the circuit produces a sound with 50 or 60 Hz frequency.

Fig. 1. Simple Non- AC Mains Voltage Detector This device can detect broken wires in cables - near the broken wire the sound is off. The piezoelectric speaker HA1 is connected to the bridge circuit to achieve higher loudness. On the figure 2 is shown more complicated circuit, except audio it has visual indication based on LED VD1. Resistance of the resistor R1 should be more than 50 Megohms.

Fig. 2. LED VD1 doesn't have a resistor in series with it, because IC CD4011 can limit the current. Radioamateur, 1998, N9,

Non- AC Mains Voltage Detector Radio, 1997, N 3 The circuit diagram of the non- voltage detector is shown on the figure 1. It consist of two parts the AC amplifier and the audio oscillator, based on the Schmitt trigger DD1.1 of IC CD4093 with the network R7C2, which determines the frequency of the audio signal, generated by piezoelectric buzzer BF1.

Fig. 1.Circuit diagram of non- voltage detector. DA1 - UA776; DD1 - CD4093; C1 - 47 mF; C2 - 33nF; Op-amp UA776 can be replaced with a 741 (then you don't need R5), but it may reduce the sensitivity.

When the antenna WA1 is located near a power supply, the interference of 50 or 60 Hz is amplified by IC DA1 and as a result the LED HL1 will illuminate. The same output voltage of the op amp starts audio generator. The current consumption of the device is no more than 2 mA when using 9V battery, and current consumption goes up to 6..7 mA when the LED lights up. If disconnect the LED and use only audio generator then it reduces current consumption. The PCB is shown on the figure 2. Antenna WA1 is made of a foil strip with size of about 55x12 mm.

Fig. 2.PCB of the non- voltage detector. The PCB is enclosed in a suitable plastic box, the antenna must be as far as possible from a hands. The switch SA1, LED HL1 and piezoelectric buzzer BF1 is mounted on the front . The sensitivity of the device is adjusted by the potentiometer R2 To make it smoother the potentiometer R2 220K can be replaced with another one of 22K and a resistor of 200K, connected between the lower pin and the ground. Correctly assembled device doesn't any special adjustment. The modernization of a crystal radio Radio, 2001, №1 A crystal radio... For many decades, it is one of the first designs built by novice amateurs. The crystal radio is an interesting introduction to the world of radio receivers. It allows the young enthusiasts of Radio Engineering to carry out a variety of exciting experiments with the radio receiving the local radio stations. However, what can be improved in this long-known device? But, as the author of this articles says, the potential for growth has not yet been exhausted.

In the simplest receivers (Fig. 1 a) the resonant tank is overloaded by the detector impedance. Although the volume and the sensitivity are quite acceptable, the selectivity is insufficient. Because of the low quality factor Q of the tank circuit, it is often to listen simultaneously to two or three radio stations. Assume that the receiver is tuned to the middle of MW frequency range (1 MHz). The inductance of the coil L1 is 200 uH, the capacitance of the capacitor C1 is 120 pF (typical values). Its reactive resistance is about 1.2 Kilohms and the impedance of the resonant circuit in Q times more. With the quality factor of the coil (with no load) Q = 200 we get 240 Kilohms. For the frequency range of LW the resonant impedance of the circuit is close to 1 Megohm! At the same time, the input impedance of the detector is considered to be equal to half the load resistance, the load is a high-impedance headphones with an impedance at audio frequencies is only 10...15 Kilohms (the full impedance of the headphones is more than the value shown on their case because of the inductance of the headphones capsules). It is easy to see that the tank circuit L1C1 is shunted too much, and its real Q is less than 10 (the ratio of the load resistance to the reactance of the tank circuit). Making the coupling with the detector circuit weaker, you can improve the quality factor Q, and hence the selectivity increases. The volume will almost not change because a voltage of a signal across a resonant tank circuit with a higher quality factor Q is higher, this will compensate the decrease of the signal across the detector. The coupling is usually adjusted by connecting the detector to a selected tap of the coil (Fig. 1b).

Fig. 1. If we adjust the coupling, it is useful to optimize the resonant tank. In [1-3] it was shown that the maximum efficiency of the antenna circuit is achieved when the antenna circuit is directly connected to the upper end of the resonant coil L1 without a coupling capacitor. Tuning are provided by changing the

inductance of the coil, as the capacity of the resonant tank is used the capacity of the antenna. If the antenna is large and its capacity is significant, then it is necessary to include a tuning capacitor in series with the antenna (Fig. 1b). This receiver works better than the previous one and has a higher selectivity, but it isn't convenient to regulate the coupling between the detector circuit and the resonant tank, because it would require a multi-tap coil. Therefore, the process of adjustment is not smooth. There is a method of impedance matching using a capacitive coupling, where the capacitive resistance of the capacitor is equal to the geometric mean of both impedances. In our example (the impedances of 240 Kilohms and 6 Kilohms is matching), it will be about 40 Kilohms( R=(R1*R2)0.5 ), and the corresponding capacity is only 4 pF! (C=1/(2*Π*F*Rc)). It turns out that the coupling can be adjusted by an ordinary trimmer of KPK or KPM type.

Fig. 2. VD1, VD2 - D18 (an old USSR Germanium diode); C1 - 5..180 pF; C2 - 8..30 pF; C3 - 680 pF But the coupling capacitor breaks the DC current path of the detector circuit. To avoid this problem it is possible to add the second diode to the circuit (Fig. 2). It seems we get a detector with a voltage doubler. In fact, because of the small capacitance of the capacitor C2 there is no voltage doubling effect. During the negative half-cycle of the signal across the tank circuit L1C1, the capacitor C2 is charged through the diode VD1, and during the positive half-cycle the capacitor C2 discharges through the diode VD2 and the load. The headphones BF1, shunted by the by capacitor C3 to smooth out ripple, is the load of the detector. The smaller the capacity of the capacitor C2, the less the charge and the energy, respectively, taken from the tank circuit. The coupling network is adding to the tank circuit a small reactive (capacitive) resistance, which is automatically compensated while tuning of the tank circuit in resonance with the oscillations of the input signals. In this experimental design the coil L1 is wound on a 12 mm in diameter plastic pipe with one layer of 0.2 mm (AWG 32) copper enameled wire, the coil has 240 turns. The ferrite rod of 10 mm in diameter made of ferrite 400NN (μbeg=400, μmax=800) is used for adjustment. The tuning range is from 200 kHz (when the capacitance of C1 is maximum and the ferrite rod is fully retracted) to 1400 kHz (with the removal of the rod and decreasing the capacitance of the C1).

At the apartment with a small antenna (about 7 m) and a ground (a central heating system) the receiver showed excellent results, received all Moscow LW and MW radio stations. By adjusting the coupling with the trimmer C2, it was able to get sufficient selectivity at the normal volume level. There is another advantage of the receiver - because the detector is powered by a current going through a high impedance of the coupling capacitor C2, the "step" on the current-voltage characteristics of diodes is smoothed out. By the way, the usefulness of the detector powered by the current has been reported in [4]. In our receiver a silicon diodes (with a threshold of 0.5 V) works almost as well as germanium diodes (with a threshold of 0.15 V). Moreover, it was possible to connect to the receiver a low-resistance (50-70 ohms) headphones, it is absolutely unacceptable in the traditional version. But in this case the bigger capacitance of the coupling capacitor is required - up to 40...50 pF. The sound volume will be less because of the significant losses in the direct resistance of the diodes.

Fig. 3 The high sensitivity of the detector described above to weak signals came to the idea to try a simple resonant tank-free version of the receiver (Fig. 3). It was easy to build - all components have been soldered to the terminals of the headphones, and a 1.5 meter of insulated hookup wire with the clamp "Crocodile" at the end worked as antenna. With the "Crocodile" the antenna can be attached to the trees or other high objects. The headphones cord has some stray capacity Cstray to the operator and further to the ground was used as the counterweight (instead of ground). Even with such a primitive version it was able to listen to some of the most powerful radio stations. This receiver almost does not perceive low frequency interferences, for example, from the mains power line because the small capacitance of the coupling capacitor C1 prevents it. The audio frequency current is completely shorted in the isolated network of the headphones BF1 and the diodes VD1, VD2. I cannot say that the circuit diagram of this receiver is something new. Half-bridge rectifier that's used in it, was known long ago - it was used in the indicator of electrical field [5]. By the way, nothing prevents to use a full-bridge circuit based on four diodes, connect it to the tank circuit or antenna by a capacitor of small capacity.

Fig. 4 A similar circuit has been described in [6], but, unfortunately, the author incorrectly interpreted the principle of operation of the receiver. The correct receiver circuit is shown in Figure 4. It differs from the author's circuit only in the presence of a stray capacitance Cstray between the headphones and the the earth, the stray capacitance acts as a coupling capacitor and matches the tank circuit with the detector circuit. By a happy coincidence, the capacitance Cstray was close to optimal. But the author didn't took it into ! As experimental results, it is proved to be excellent, as it follows from the publication [6]. At the end, let's go back to the circuit, shown in Figure 2 and bring it to the attention of the radio amateurs. This crystal radio set has shown excellent results. Experiments with it not less interesting and attractive than with the more complex electronic devices.

Loudspeaking radio receiver with a bridge amplifier powered by "free energy" Radio 2001, 12 Receivers without power supply are interested for radio amateurs. This article describes an improved radio receiver powered by radio waves. While experimenting with different receivers and amplifiers powered by "free energy", it was found that it is more convenient to connect the audio amplifier to the receiver by using only two wires for audio signals and supply voltage. This would allow to use the radio receiver with no switches, just connecting headphones to the output of the receiver. In general, this receiver reminds the previously described version of "crystal" radio receiver, but it has some interesting features.

Fig. 1 VT1, VT3 - MP37 (Germainum, hFE = 15...30, ft = 1MHz); VT2, VT4 - MP41 (Germainum, hFE = 30...60, ft = 1MHz); VD1-VD4 - D18 (Germainum); T1 - transformer with ratio 30:1; L1 - LW loopstick ferrite antenna.; * - tweak the value (see text). The schematic diagram of the receiver is shown in Figure 1. From the detector bridge the circuit is completely symmetric, the detector is connected to the amplifier by two wires (the terminals A and B) and the output of the amplifier is connected to the loudspeaker (the terminals C and D) by two wires. The resonant circuit of the receiver comprised the antenna capacitance and inductance of the coil L1. This solution provides a maximum power of the signal in the resonant tank circuit. The switch SA1 and the neon lamp HL1 are used to protect the receiver during thunderstorms. The static charge doesn't build up in the antenna because the antenna connected to the ground through the coil L1. A bridge detector circuit (VD1 - VD4) is used in this receiver, it works very well for the inductive load. The detector connected to the antenna through the capacitor C1, this capacitor is matching impedances between them. Once adjusted for maximum voltage across the amplifier, the capacitor C1 may be replaced with a constant capacitor with proper value. The optimal capacitance of the capacitor C1 is about 47 pF for LW band. The output voltage of the detector is symmetric with respect to ground. Through the wires A and B the voltage es from the detector to the input of the audio amplifier. At the input of the amplifier the voltage decomposes into AC and DC parts. The AC part feeds through the coupling capacitors C3 and C4 to the transistor bases of the bridge amplifier. The DC part charges through the low-frequency chokes the capacitor C6. The DC part is used for power supply. The receiver doesn't have a common wire. The arms of the amplifier balances automatically, because the bases of the complementary transistors are connected together. But transistors in this type of amplifiers don't have a bias, they does not work in the class "B" but rather in the class "C". This leads to crossover distortion of the signal waveform, as shown in Figure 2(A).

Fig. 2 The graph shows the dependence of the output current in one arm of the amplifier (for example, VT1, VT2) on the input voltage. We see a distorted output current for a sinusoidal input voltage. These distortions are especially noticeable with silicon transistors that have higher junction drop voltage of about 0.5 V. Germanium transistors has lower junction drop voltage of about 0.15 V, so they are used in the audio amplifier. Crossover distortion is related to the moments when voltage crosses zero point, that is very unpleasant to the ear. Crossover distortions can be reduced by using a slight forward bias Ubias, as shown in Fig. 2(B). The distortions disappear but some initial current i0 appears, it makes the amplifier less efficient. The same result can be obtained by other means. If mixing the audio signal with a high frequency signal, as shown in Fig. 2(C). This method is used in tape recorders with AC bias, because the magnetization curve of the type is very similar to the amplifier transfer characteristic of a push-pull stage without bias. By adjusting the amplitude of the "high frequency bias" the desired initial current (quiescent current) can be set, this current should not be too high, but sufficient to eliminate the distortion. But we already have high frequency bias, we got the detected RF voltage ripple. In the bridge detector circuit the ripple has twice the frequency of the carrier signal. We just need to tweak the value of the smoothing capacitor C2 (Fig. 1) to obtain the desired quiescent current. It's better tweak the capacitor C2 when there is no audio transmission (but there is a carrier frequency of the radio station) because if there is an audio signal then the current of the audio amplifier increases. At the output of the audio amplifier the ripple don't need anymore, so there is the smoothed capacitor C5. The coil L1 is wound with litzwire 7 x 0.07mm (7 wires x AWG 41) on a cardboard pipe with a ferrite slug of 8 mm in diameter and 160 mm long (the permeability of the slug is about 1000). The coil has about 200 turns. Actually, any other litzendraht may be used or any copper wire with silk insulation of 0.15...0,25 mm in diameter (AWG 35...AWG 30). A standard loopstick antenna with the LW band can be used as the coil L1. C1 is a ceramic or air trimmer capacitor.

In the detector circuit the best result was obtained by using diodes D18, diodes GD507 works not too bad, and the worst result was obtained by using diodes D311 (D18, GD507, D311 are germanium diodes). In the detector circuit may be used any germanium diodes. The transformer T1 has ratio 30:1. The primary winding has 2700 turns of the wire with diameter 0.12 mm (AWG 37) and the secondary winding has 90 turns of the wire with diameter 0.5 mm (AWG 24) wound on a former which is mounted on a core made of permalloy E-shape plates of 15 mm2. Any suitable output audio transformer can be used here. A primary winding of the same transformers can be used as chokes L2 and L3. The inductance of this chokes should be not less than 6..7 H. Any low frequency germanium transistors may be used in this circuit. If possible, match the transistors with similar hFE. The receiver can be adjusted in a few minutes. Disconnect the audio amplifier from the detector and connect a high-impedance headphones to the terminals A and B, check the detector part of the receiver, try to tune to a powerful radio station, if necessary change the number of turns of the coil L1. The tuning is performed by moving the ferrite rod in and out the coil L1. Next, connect the amplifier to the receiver and connect a high-impedance DC voltmeter across the capacitor C6 to monitor the voltage, tune the receiver to the frequency of a powerful radio station and adjust the capacitor C1 for the maximum reading of the voltmeter. Keep in mind that the voltage across C6 increases slowly because of the large capacitance of the capacitor C6. Connect across the capacitor C2 another capacitor with a value of a few thousand picofarads and wait for some seconds, read the voltmeter. Then tweak the capacitor C2 to get the voltage 20...30 % below the nominal value. In the author's version of this receiver the voltage was 5.5 V and 4 V. There is nothing more to adjust in this circuit. The receiver was tested in the city apartment located in the eastern outskirts of Moscow. An external antenna was used. The antenna has length 30 meters of copper enameled wire with a diameter of 0.7 mm (AWG 21). The maximum height of the antenna above the roof does not exceed 7 meters. Metal pipes of a central heating system was used for grounding. Even with this antenna it was possible to receive signals of a five radio stations with loud speaker volume. The loud speaker volume means that the volume is sufficient for normal listening in a small room when there is no ambient noise. The values of the detected voltages, currents and power, extracted from the air by the receiver of the above mentioned radio stations are shown in Table 1. The voltage was measured across the capacitor C6, and the current was measured in series with any of the wires A or B, while the receiver is working. Frequency, kHz Voltage, V Current, mA Power, W 198

4,2

0,3

1,25

261

3,5

0,25

0,9

549

2,5

0,17

0,42

873

3

0,2

0,6

918

1,2

0,1

0,12

It should be noted that the audio amplifier loads sufficiently the detector, because the value of the capacitor C2 that was chosen provides the best quality of the sound, so with this value the quiescent current of the amplifier is sufficient. The widespread opinion that the quality reception of long and middle wave signals is impossible especially in the night time are wrong, and this receiver disproved this mistaken opinion. This receiver does not suffer from interference because of its low sensitivity. The quality of the sound cannot even be compared to the sound quality of conventional portable receivers. Loudspeaking "crystal" radio receivers Radio 2000, 7 A lots of radio amateurs have an interest to power a simplest radio receivers with the "free energy", i.e. the energy, taken by the receiver antenna directly from the air. The circuits described here can provide a radio reception using a loudspeaker. The question of how much power can get out of a signal from an antenna, and how to build a loudspeaking crystal set, was already discussed in the author's articles [1,2]. However the questions "how much power we need for loudspeaking reception?" and "how to better use the power from the antenna?", still remain. According to the old reference books, to listening a voice of a broadcaster from the distance of 1 meter it takes the sound level of the loudspeaker about 60 dB. In this case, the radiated acoustic power is 12.6 μW. The necessary electrical power can be calculated by dividing the radiated acoustic power by the energy conversion efficiency of the loudspeaker. For the common loudspeakers the energy conversion efficiency is about 1%. Thus we get the electrical power about 1mW. It is interesting to calculate the required power for loudspeakers to get the sound level of 60 dB. The calculation results for the different loudspeakers are presented in table 1. Model

Power, mW

0,025GD-2

3,6

0,05GD-1

1,8

1GD-5, 1GD-28, 2GD-7

1

5GD-1, 6GD-1PP3, 6GD-30 0,25

8GD-1PP3

0,2

Table 1. From the table 1 we see that we need to use the high-efficiency loudspeakers. The acoustic design of the loudspeaker systems is very important, the bigger speaker cabinets is better. In the experiments the author used two loudspeakers type 4GT-2 in wooden cabinets with the enclosure volume of 50 liters. Horn loudspeakers has three times better efficiency because of the improved coupling efficiency between the speaker driver and the air and because of the directional characteristics of the produced sound waves. The simple and effective loudspeakers was built by radioamateurs, they used paper, cardboard and plywood [3]. Horn loudspeakers with a bass reflex system with U-shaped design provides with the loudspeaker 6GD-1 efficiency of about 2.3%, and at the low frequencies about 3.4%. So, we found that the audio signal of 0.2 mW is sufficient for the sensitive acoustic system. The second part of this "research" is related to electrical circuits of the loudspeaking detector radio. Analysis of the detector circuit leads to the conclusion that the current should be amplified, but not the voltage, because the voltage amplification could limit the peaks of the signal. Because of this it is wise to use in this circuit the push-pull emitter follower, based on the complementary pair of transistors working in class AB. This amplifier has good efficiency and low current consumption while the quiet sounds and pauses of the signal, this allows to store the energy of the carrier and use the energy at the peaks of the audio signals. The circuit diagram of the receiver with the amplifier based on the push-pull emitter follower is shown in Fig. 1. The AC component of the detected signal es through the coupling capacitors C3, C4 to the bases of transistor amplifier, and the DC component es through the choke L2 to the storage capacitor C5. This capacitor cannot be directly connected to the detector because in this case the audio signal will be smoothed and suppressed. The parameters of the choke are not critical, so you can use any choke or any transformer with a winding, containing not less than 2000 turns wound on the magnetic core with cross section not less than 1 cm2.

Fig. 1. VD1 - D311 (Germanium diode); VT1 - MP37 (Ge, hFE = 15..30, Ft = 1 MHz); VT2 - MP39 (Ge, hFE ≥ 12, Ft = 0.5 MHz); R1, R2 - 560K; C1 - 17..500 pF; C2 - 680 pF; C3, C4 - 0.68 μF; C5 - 68μF x 6.3 V; C6 - 22μF. L1 - magnetic loop antenna for MW with a moveable ferrite core; L2 - an audio choke; T1 - transformer with ratio 30:1; Loudspeaker BA1 - 4 ohms. The optimal transformation ratio of the transformer T1 is about 30 for the load of 4 ohms. It is convenient to use a small power supply transformers from transistor radios with the voltage ratio 220 V to 6.5..9 V. A suitable output audio transformer can be used too. The large size of the device (due to the heavy transformer and choke) is not an issue, because it uses the large antenna and a floor standing speaker system, so this is not a portable radio! The use of a voltage doubling rectifier allows to increase the supply voltage of the circuit. Distortion on peaks of signal will be decreased. A bridge amplifier loads the voltage doubling rectifier symmetrically and furthermore decreases the distortion. This allows to get rid of the capacitive coupling at the output. The circuit schematic of the receiver with the voltage doubling detector and the bridge power amplifier is shown in Fig. 2. The positive half-wave of the signal detected by the diode VD1, smoothed by capacitor C2 and filtered by low-frequency choke L2 and the capacitor C8, so it creates a positive supply voltage. Similarly, the components VD2, L3, C3 and C9 produce a negative supply voltage. The emitter followers based on the VT1, VT2 and VT3, VT4 are working in opposite phases, the signals to this emitter followers feeds from the different detectors. The emitter followers are loaded with the transformer T1. Just like in the previous circuit, the transformer ratio is about 30, but due to the bridge circuit the output power of the amplifier is higher than in the previous circuit. The purpose for the other components of the circuit shown in fig.2 is the same as the circuit shown in fig. 1, and the recommendation about the chokes is the same.

Fig. 2. Adjustment of the receivers powered by the "free" energy has some features. This receivers will not work until they are tuned to a powerful radio station, because there is no power supply. But after tuning it will take some time to charge the capacitors (C5 - in Fig. 1 and C8, C9 - in Fig. 2). Charge time is directly proportional to the capacity of this capacitors, so in the first experiments the capacity should not be too large. But in this case while receiving a long loud sounds (especially in the musical ages), the power supply voltage and the detected voltage drops significantly due to increasing current of the audio amplifier, the result of it is the limitation of dynamic range. This does not lead to issues, but even improves speech perception. When the receiver will be completely adjusted, the capacity of the smoothing capacitors can be increased even up to several thousand microfarads, it will improve the dynamics of the receiver and the audio amplifier would work out the peaks of the signals. In any case, all the capacitors should have a small leakage current (check it with an ohmmeter), to avoid the necessary load of the power supply. Tweaking of the bias resistors in the receivers are based on the next reasons: the greater the resistance, the less the current consumption (the current when there is no signal in the receivers - see fig. 1 and 2), and the less the gain of the transistors but higher the supply voltage! A compromise can only be found empirically for a particular antenna, by getting the maximum sound volume and quality of the radio reception. The bias resistors for the circuits shown in Fig. 1 and Fig. 2 may have different values, it depends on the parameters of the transistors. The voltage at the emitters of the transistors is half of the power supply voltage (Fig. 1) and zero (Fig. 2).

It's better to start the experiments without bias resistors, and then try to use this resistors with value from 2.7 MΩ to 1 MΩ; if there is a "powerful" antenna than use the bias resistors with value of hundreds of kilohms, because the power supply voltage can be dropped. If a pair of complementary transistors have a sufficient initial current, it can be reduced by inserting a resistor between the bases of the transistors, or the bases could be connected together, and one of the base capacitors could be removed from the circuit. It has not mush sense to use in this circuits a bias thermal stabilization networks due to the low power of the amplifiers (some milliwatts). This radio receivers has been tested in a country house (33 km to south-east from Moscow). The audio volume level has been enough for a small quiet room. The second circuit has been tested with especially good results. The antenna type the end fed half wave with the length of 12 m was used. The antenna was stretched from the window to the tree outside. The receivers was grounded to the water pipes of the well. The receivers was tuned to "Radio Rossii" 873 kHz, the radio stations "Radio 1" and "Radio Mayak" was received with loud volume too. The quality of the sound was excellent.

FM crystal radio receivers Optimization of a radio receiver powered by the energy of radio waves 7/8/2013 Dante Bianconi Vinci (Florence), ITALY This report shows the results of some experiments carried out on the basis of Mr. Vladimir Polyakov (RA3AAE) researches. The original circuit was modified by the use of a more efficient detection circuit and by the use of a simple amplifier, self-biased by the radio itself. An efficient antenna (5 elements FM Yagi) was used also to permit to explore overall the weak signals close to the range between 98 - 103 MHz. Earlier it was proposed by Mr. Polyakov, to use a simple dipole as an antenna. According to the original circuit, the high frequency germanium transistor GT311A with Ft = 300 MHz is used there, it provides high impedance at its output (the transistor was grounded to provide the matching with the low impedance of the resonant tank), so it was possible only to use ear speakers with at least 600 ohm impedance to listening the radio. A further analysis of the circuit came to the idea to use a simple amplifier based on a silicon transistor (BC109C with hFE = 700), it's finally allowed to get acceptable loudspeaking reception. With the use of a more efficient detection circuit, the voltage in unloaded condition reached 2.2 volt across the capacitor C8. With two loudspeakers (are both connected in parallel), the current in the high impedance circuit was also measured by a micro ammeter and sometimes the value of this current reaches more than 100 μA. The transistor of the simple amplifier was used in common emitter configuration to lower the impedance of the output of the high frequency transistor (AF239). The impedance transformer that gave

the best results had a 14 kΩ in input and 4 Ω in output. An active low filter was used to reduce the noise coming from the first stage of RF amplifier to the low frequency stage. With the directional antenna it was possible to receive three FM radio broadcast stations, two of them are located at 15 km, and the third - more than 30 km from the reception antenna. The current researches are focused on the circuit with a coaxial resonator, it would allow to reach high Q as well as the selectivity of the tuning.

Fig. 1. The circuit diagram of the FM crystal radio receiver T: Zin 14 kΩ → Zout 4,8 Ω (K ≈ 60:1) R1: 70+200 kΩ R2: 30 Ω Tr1: AF239 Tr2: BC109C

D1,D2: 1N82A L1: 5 turns (silver wire 1 mm, coil with internal diameter of 8 mm) L2: 7 turns (silver wire 1 mm, coil with internal diameter of 8 mm) C1: 8.5 pF (ceramic NP0 type) C2: 5-25 pF (KPV type) C3, C4: 4n7 (ceramic type) C5: 0.15 μF C6: 3-28 pF (KPV type) C7: 0.01 μF C8: 1 μF WA1: 5 elements YAGI antenna LS1: 3.5 Ω loudspeaker (diameter 200 mm) LS2: 3.5 Ω loudspeaker (diameter 100 mm)

Fig. 2.

Fig. 3.

Fig. 4. The FM crystal radio, called BIDA 1: the dimensions of this design are only 80x35x80. In the circuit is used the variable capacitors with the silver plated coils, it allows to obtain a better quality factor Q and the high frequency germanium transistor AF239. The taps on both coils are visible on the Fig. 3, it allows to reach the better matching between the antenna impedance and the main resonant tank L1 and the second coil L2. The signal amplified by the transistor AF239. The detection circuit was composed by using a Villard voltage doubler with the use of two very sensitive germanium diodes 1N82A. This sensitive diodes has been used for radar applications after the Second World War.

Fig. 5.

Fig. 6.

Fig. 7.

Fig. 8. 5 Elements Yagi Antenna

Fig. 9. 5 Elements Yagi Antenna - view from the side It's clearly visible on the circuit diagram (see Fig. 1) that the HF radio amplifier stage is self biased by the main radio circuit. The output AF stage is loaded with the high impedance transformer. In the Fig 8 and 9 you can see the 5 elements Yagi antenna, the heart of the FM crystal set (the central frequency is 100 MHz). See the folded dipoles and the indicated dimensions of the RG8 cable. The impedance of this antenna is close to 52 Ω as well as the impedance of RG8 cable. The cable's length was optimized, so it is only 6 meters long. While testing, the 5 elements Yagi antenna was pointed to the north-east direction (in the opposite site the field intensity is higher, but there is a medium voltage power line that probably interfered with receiving - unfortunately the most of the radio broadcasts are can be received in this south-east direction). The centre frequency of the Yagi antenna is 100 MHz, it allows to receive three radio stations: Radio Lady --> 98.2 MHz (20 km from the receiving antenna); Radio SeiSei --> 101.5 MHz (20 km from receiving antenna); RTL102.5 --> 101.2 MHz (35 km from receiving antenna). FM Crystal Radio Receivers

Radio, 2002, 7 The notion of "crystal radio" is strongly associated with huge antennas and radio broadcasting on long and medium bands, in this article, the author describes the experimentally tested detector circuits of VHF receivers designed to listening to a FM stations. The very possibility of receiving VHF FM detector was discovered accidentally. One day I was walking in the Terletskiypark in Moscow, Novogireevo, I decided to listen to the broadcast - I had a simple crystal set without resonant tank (this circuit is described in the "Radio", 2001, № 1, Fig. 3). The receiver had a telescopic antenna with length of about 1.4 m. Wonder whether it is possible to receive radio broadcast with this short antenna? It was possible to hear, but weakly, simultaneous operation of two stations. But what is surprised me is the volume of receiving was rise and fall periodically almost to zero after every 5...7 m, and it was different for each radio station! It is known that in the LW and MW bands, where the wavelengths are hundreds of meters, it is impossible. I had to stop at the point of receive with maximum volume of one of the stations and listen attentively. It turned out - this is "Radio Nostalgie", 100.5 MHz, broadcasting from the near city Balashikha. There were no line of sight between antennas. How does the FM transmission could be received by using the AM detector? Further calculations and experiments shows that it is quite possible and is not depends on the receiver. A simple portable FM crystal receiver is made exactly the same way as an indicator of the electric field, but instead of measuring device it is necessary to connect a high-impedance headphones. It makes sense to add an adjustment of coupling between the detector circuit and the resonant tank to adjust the maximum volume and quality of the receiving signal. The simplest Crystal radio The circuit diagram of the receiver suitable for these requirements is shown in Fig. 1. This circuit is very close to the circuit of the receiver mentioned above. Only the VHF resonant tank has been added to the circuit.

Fig. 1. VD1, VD2 - GD507A - an old USSR Germanium high-frequency diodes with the capacitance of 0.8 pF (at the reverse voltage of 5V), the recovery time of reverse resistance is no more than 0.1 uS (at the Idirect pulse=10 mA, Ureverse pulse=20 V, Icutoff=1 mA)

The device contains a telescopic antenna WA1, directly connected to the resonant tank L1C1. The antenna is also an element of the resonant tank, so to get the maximum power of the signal it must be adjust both the length of the antenna and the frequency of the tank circuit. In some cases, especially when the length of the antenna is about 1/4 of the wavelength, it is useful to connect the antenna to a tap of the tuning coil L1 (find the suitable tap of the coil by finding the maximum volume of the signal). The coupling with the detector can be adjust by trimmer C2. Actually the detector is made of two highfrequency germanium diodes VD1 and VD2. The circuit is completely identical to the voltage doubling rectifier circuit, but the detected voltage would be doubled if only the trimmer capacitor C2 value is high, but then the load of the resonant circuit L1C1 would be excessive, and its quality factor Q will be low. As a result, the signal voltage in the circuit tank L1C1 will be lower and the audio volume will be lower too. In our case, the capacitance of the coupling capacitor C2 is small enough and voltage doubling does not occur. For optimal matching the detector circuit with the tank circuit the impedance of the coupling capacitor must be equal to the geometric mean between the input resistance of the detector and the resonant resistance of the tank circuit L1C1. Under this condition, the detector is getting the maximum power of the high-frequency signal, and this is corresponding to the maximum audio volume. The capacitor C3 is shunting the higher frequencies at the output of the detector. The load of the detector is headphones with the dc resistance of not less than 4K ohms. The whole unit is assembled in a small metal or plastic housing. The telescopic antenna with the length not less then 1m is attached to the upper part of the housing, and the connector or the jack for the phones is attached th the bottom of the housing. Note that the phone cord is the second half of the dipole antenna (a counterweight). The coil L1 is frameless, it contains 5 turns of enameled copper wire with diameter of 0.6...1 mm wound on a mandrel with diameter of 7...8 mm. You can adjust the necessary inductance by stretching or compressing the turns of the coil L1. It's better use the variable capacitor C1 with an air dielectric, for example, type 1KPVM with two or three movable and one or two fixed plates. Its maximum capacity is small and can be in range of 7...15 pF. If the variable capacitor has more plates (the capacitance is higher), it is advisable to remove any of the plates, or connect the variable capacitor in series with a constant capacitor or a trimmer, it will reduce the maximum capacity. The capacitor C2 is ceramic trimmer capacitor, such as a KPK or KPK-M with the capacity of 2...7 pF. Other trimmers capacitors could be used too. The trimmer capacitor C2 can be replaced with a variable capacitor, similar to C1, and it could be used to adjust the coupling "on the fly" to optimize radio receiving capabilities. Diodes VD1 and VD2, can be GD507B, D18, D20 (it is old USSR Germanium high-frequency diodes. This diodes can be replaced with modern Schottky diodes). The shunting capacitor C3 is ceramic, its capacity is not critical and can have a value in range from 100 to 4700 pF. Adjustment of the receiver is simple. Tune the radio by turning the knob on the variable capacitor C1 and adjust the capacitor C2 to get the maximum audio volume. The tune of the resonant tank L1C1 will

be changed, so all operations must be repeated a few more times, and at the same time find the best place for the radio receiving. It is doesn't necessarily the same place where the electric field has maximum strength. This should be discussed in more detail and explain why this receiver can receive FM signals. Interference and conversion of FM into AM If the tank circuit L1C1 of our receiver (Fig. 1) will be set up so that the carrier frequency of FM signal falls on the slope of the resonance curve, the FM can be converted into AM. Let's find the value of Q of the tank circuit. Assuming that the bandwidth of the tank circuit L1C1 is equal to twice the frequency deviation, we obtain Q = F0 /Δ2f = 700 for both the upper and the lower VHF band. The actual Q of the tank circuit in a crystal radio probably will be less than 700 because of the low Qfactor of its own Q (About 150...200) and because the resonant tank is shunted by the antenna and by the input impedance of the detector. Nevertheless, a weak transformation of FM into AM is possible, thus, the receiver will barely work if its tank circuit detune a little up or down in frequency. However, there is much more powerful factor contributing to the transformation of FM into AM, - it is an interference. It's very rarely when the receiver is in the line of sight of radio station, in most cases the line of sight is obscured by buildings, hills, trees and other reflective objects. A few radio beams scattered by these objects comes to the antenna of the receiver. Even in the line of sight to the antenna comes some reflected signals (and of course, direct signal comes too). The total signal depends on both the amplitudes and phases of summing components. The two signals are summed if they are in phase, i.e., the difference of their ways is multiple of an integer of the wavelength, and the two signals are subtracted if they are in opposite phase, when the difference of their ways is the same number of wavelengths plus half wavelength. But the wavelength, as well as the frequency varies at FM! The difference of the beams and their relative phase shift will vary. If the difference of ways is large, then even a small change in frequency leads to significant shifts in the phases. An elementary geometric calculation leads to the relation: Δf/f0 = λ/4ΔC, or ΔC = f0/λ/4Δf, where ΔC - the difference of the ways of the , it's required for the phase shift ±Π/2, to get the full sum of AM signal, Δf - frequency deviation. The full AM is the total variation of the amplitude signal from the sum of the amplitudes of the two signals to their difference. The formula can be further simplified if we consider that the multiply of frequency by the wave length f0λ is equal to the speed of light c: ΔC = c/4Δf. Now it is easy to calculate that to get a full AM of the two-beam FM signal, the sufficient difference between the ways of beams is about a kilometer. If the difference of ways is smaller, the depth of AM proportionally decreases. Well, but if the difference of ways is more? Then, during one period of the modulating audio signal the total amplitude of the interfering signal will several times through the highs and lows, and distortion will be very strong when converting FM into AM, up to complete indistinct of the sound when you receive the FM by an AM detector. Interference with FM broadcast reception is an extremely harmful phenomenon. It is not only produces a concomitant parasitic AM of a signal, as it is described above, but it is produces the parasitic phase

modulation, what leads to distortion even if we got a good FM receiver. That's why it is so important to place the antenna in the right location, where the only one signal prevails. It is always better to use a directional antenna, because it increases the magnitude of the direct signal and reduces reflections coming from other directions. Only in this case with a very simple detector radio receiver the interference played a useful role and allowed us to listen to the radio broadcast, but the radio broadcast can be heard weakly or with significant distortions, and the radio broadcast can't be heard everywhere, but only in certain places. This explains the periodic changes in the volume of the radio broadcast in the Terletskiypark. Crystal Detector Radio Receiver with a frequency detector A radical way to improve reception is to use a frequency detector instead of an amplitude detector. In Figure 2 is shown a circuit of a portable detector radio receiver with a simple frequency detector, based on a single high-frequency germanium transistor VT1. The germanium transistors is used because it's junctions works at a low voltage about 0.15 Volts, this allows to detect very weak signals. The junctions of silicon transitions works at a voltage approximately 0.5 V, and the sensitivity of the receiver with a silicon transistor is much lower.

Fig. 2. VT1 - GT313A - an old USSR Germanium high-frequency transistor with hfe=10...230 (at DC: Uke=3 V, Ie=15 mA), hfe=3..10 (at f=100 mHz, Ukb=5 V, Ie=5 mA) As in the previous design, the antenna is connected to the input tank circuit L1C1, the variable capacitor C1 is used for the tuning function. The signal from the input tank circuit goes to the base of the transistor VT1. The other tank circuit, L2C2, is inductively coupled with the input tank circuit L1C1. The tank circuit L2C2 is tuneble with the variable capacitor C2. Because of the inductive coupling between this two tanks the oscillation in the resonant tank L2C2 is phase shifted by 90° relative to the signal across the input tnak circuit L1C1. From the tap of the coil L2 the signal goes to the emitter of the transistor VT1. A by capacitor C3 and high impedance headphones BF1 is connected to the collector of the transistor VT1. The transistor begins to turn on when its base and emitter has the positive half-wave of the signal, and the instantaneous voltage on the emitter is greater then its base voltage. At the same time the

smoothed detected current es through the headphone in the collector network. But the positive half-wave of the signal is only partially overlapping when the phase shift of the signal is 90° in the resonant tanks, so the detected current reaches the maximum value determined by the signal level. With frequency modulation, depending on the frequency deviation, the phase shift is also changing, corresponding to the phase-frequency response of the tank circuit L2C2. When the frequency deviates in one direction then the phase shift decreases and the half-waves of the signal at the base and emitter is overlapped more, as a result, the detected current increases. When the frequency deviation goes in the opposite direction, its decreases the overlap of half-waves of the signal and the current decreases. So the frequency detection of the signal occurs. The gain of the detector depends directly on the quality factor Q of the resonant tank L2C2, the quality factor Q should be as high as possible (in the limit of 700, as we calculated earlier), therefore the coupling with the emitter of the transistor is weak. Of course, such a simple detector does not suppress the AM of the received signal. In fact, its detected current is proportional to the signal level at the input, this is an obvious disadvantage. But anyway it's the very simple circuit. Just like the previous circuit, the receiver is built in a small housing, on the top of the housing a telescoping antenna is mounted, and the headphone socket in the bottom the housing. The knobs of the variable capacitors is located on the front . These variable capacitors should not be combined into one unit, because a louder volume and a better quality of reception can be obtained with separate tuning. The coils L1, L2 if frameless, they wound with the copper wire 0.7 mm (AWG 21) in diameter on the mandrel of diameter 8 mm. L1 contains 5 turns, L2 - 5+2 turns. If possible, the coil L2 wound with silver plated wire to improve the quality factor Q, the diameter of the wires is not critical. The inductance of the coils is adjusted by compressing or stretching of the coils L1 and L2 to get the FM radio stations in the middle of the variable capacitors tuning range. The distance between the coils L1 and L2 is in the range of 15...20 mm (the axis of the coils is parallel), the distance is adjusted by bending their terminals, soldered to the variable capacitors. With this receiver can be done a lot of interesting experiments, exploring the possibility of reception of VHF radio broadcasts with the detector receiver, exploring the propagation of radio waves in urban areas, etc.Can be done experiments to further improve the receiver. However, the sound quality in a high-impedance headphones with membranes is poor. Because of it a better receiver was developed, which provides better sound quality and allows you to use a different external antennas, connected to the receiver by feedline. Radio receiver powered by the energy of radio waves Experimenting with a simple crystal radio set, repeatedly had to make sure that the power of the detected signal is sufficiently enough (tens or hundreds of microwatts) to provide a very loud sound in the headphones. But the quality of reception is not good because there is no frequency detector. This problem is partially solved in the second receiver (Fig. 2), but the signal strength is also used inefficiently

because the transistor is powered by quadrature high-frequency signal. Therefore it was decided to use two detectors in the receiver: the envelope detector - to power the transistor, and the frequency detector - to improve signal detection.

Fig. 3. C1, C2 - 2.2...15 pF, C3 - 0.15 uF, C4 - 1 uF, C5 - 1 nF, R1 - 130 k The circuit diagram of the receiver is shown in Fig. 3. An external antenna (dipoles) connected to the receiver by a two-wire line, made of ribbon VHF cable with the impedance of 240...300 ohms. The impedance matching between the cable and the antenna is performed automatically, and the impedance matching of the input tank circuit L1C1 is performed by selecting a suitable tap of the coil L1. Generally speaking, unbalanced connection of the feeder to the input tank circuit reduces the noiseproofing of the antenna feeder system, but because the low sensitivity of the receiver, it doesn't matter. There is a well-known methods of balanced connections for a feeder with the use of a coupling coil or a balun. The author's folded dipole was made of a conventional isolated connecting wire, the dipole was placed on the balcony, in a place with a maximum field strength. The length of the feeder does not exceed 5 m. With such a small length the losses in the feeder is negligible, and therefore, the balanced line can be successfully used. The input tank circuit L1C1 is tuned to a frequency of a signal, and a high frequency voltage across L1C1 is rectified by an amplitude detector, based on the high-frequency diode VD1. Since the amplitude of FM signal has a constant value, there is practically no requirements for smoothing the rectified DC voltage. However, to remove possible parasitic amplitude modulation in case of multipath propagation of radio signals (see above story about the interference), the capacitance of the smoothing capacitor C4 is selected sufficiently large. A rectified DC voltage is used to power transistor VT1. For the control of the current consumption and for a signal level indication is used an analog current meter PA1. A quadrature frequency demodulator of the receiver is implemented with the transistor VT1 and phase shifter tank circuit L2C2. The high-frequency signal from the tap of the coil L1 is applied to the base of the transistor VT1 through the coupling capacitor C3, and it's signal is applied to the emitter of the

transistor VT1 from the tap of the coil L2 of the phase-shifting tank circuit L2C2. The work of the detector is exactly the same as in the previous design. To increase the gain of the frequency demodulator, on the base of the transistor VT1 is applied an offset voltage through the resistor R1, and because of it the coupling capacitor C3 is used. Note that the capacitor C3 has sufficient capacitance (0.15 uF) - this capacitance is chosen to shunt the low-frequency currents, i.e., for grounding the base of the transistor VT1 for the sound frequencies. This increases the gain of the transistor and increases the volume of reception. The primary winding of the output transformer T1 in the collector circuit of the transistor VT1 is used to match the high output impedance of the transistor to the low impedance of the headphones. A stereo headphones TDS-1 (8..16 ohms) or TDS-6 (8 ohms) can be used with this radio. Both the earpieces (left and right channels) are connected in parallel. The by capacitor C5 is used to filter the highfrequency currents in the collector circuit. The button SB1 is used to short the collector circuit of the transistor VT1 while tuning the input tank circuit and the search for a signal. The sound in the headphones at the same time disappears, but the sensitivity of the indicator PA1 is significantly increased. The design of the receiver can be very different, but anyway it needs the front with the knobs of the two variable capacitors C1 and C2 (each capacitor has individual knob) and the button SB1. To reduce hand effect on the tuning, it is desirable to make the front of a metal plate or a copper clad laminates. It can work also as a common wire of the receiver. Rotors of the variable capacitors should have good electrical with the . The antenna socket X1 and the phone jack X2 can be placed either on the front or on the side or back of the receiver. Its dimensions are dependent on the available components. So let's say a few words about them. The capacitors C1 and C2 is KPV type with a maximum capacity of 15...25 pF. The capacitors C3-C5 are ceramic. The coils L1 and L2 are frameless (see Figure 4), wound on a mandrel of diameter 8 mm, L1 contain 5, L2 contains 7 turns. The length of the winding is 10...15 mm (do some tuning by adjusting the length). The enameled copper wire of 0.6...0.8 mm (AWG 20..23) is used, but it is better to use a silver-plated wire, especially for the coil L2. The taps are made from 1 and 1.5 turns (L1) and from 1 turn (L2). The coils can be arranged coaxially or axis parallel to each other. The distance between the coils (10...20 mm) is adjusted. The receiver will work even in the absence of inductive coupling between the coils - the capacitive coupling through the junction capacitance of the transistor is enough. The audio transformer T1 is TAG-3, it has a winding ratio of 10:1 or 20:1.

Fig. 4. The transistor VT1 can be replaced by any germanium transistor with maximum operating frequency ft not lower than 400 MHz. A p-n-p transistor can be used too, for example, GT313A, in this case the polarity of the indicator PA1 and the diode VD1 should be reversed. The diode can be any germanium type, a high-frequency. As the indicator PA1 any ammeter with a current range of 50..150 mA can be used. Tune the tank circuits to the frequency of a radio station, adjust the taps of the coils and the distance between the coils to get the best result (maximum volume and best quality of the reception). It is useful to adjust the value of the resistor R1 for maximum volume. On the balcony the receiver with the antenna described above provided high quality reception of two stations with the strongest signal from the radio center at the distance not less than 4 km and with no direct line of sight (obscured by buildings). Collector current of the transistor was 30...50 mA. Of course, the possible design of VHF crystal radios is not limited to described above. On the contrary, this circuit should be considered only as the first experiments in this interesting field. When using an efficient antenna, placed on a roof and targeted at a radio station, it is possible to obtain sufficient signal strength, even at a considerable distance from the station. This provides a high-quality reception on a headphones, and in some cases, you can get loudspeaking reception. It is possible to improving this receivers by using a more efficient detection circuit and using a high-quality resonant tanks, in particular, spiral resonators as resonant circuits.

Rating: 0.00 / 10 (0 Vote cast) Category Free Energy Circuits / Power Supply Circuits Views 1186 Added on Sep 11, 2012 Comments 5

You need free power? This schematic transforms the surrounding radio waves to usable current. The levels of power can be stepped up by using more diodes. The critical points in this device are first the type of diodes and second the antenna construction. The more diodes the more power out! And guess what, no ground needed..

Lets begin with the antenna first. It has to be an Ferrite antenna Get a wire 18 feet long and wrap it around the the largest ferrite rod you can find! You can put many ferrite rods glued to make the length of the antenna you need. A 30 inch or more antenna, would be ideal. As for the diodes, try to find the lowest loss Germanium diodes, with the lowest breakdown junction voltage ~ 0.2 - 0.4 Volts. Keep in mind that in a heavy radio polluted area each diode will put out about 30mV. More details and testings results comming soon..

The above diagram is known to contain some flaws and it is recomended you take more notice of the follow diagram from the US patent as below

Tate Power from the air Tate Ambient Power Module. Joseph Tate Last Known Address 760 Waldo Point Sausalito Ca (4965) 415 331 8150 332 9918 U.S. Patent 4,628,299 This design converts radio frequency energy to power that can be usefully applied in power devices such as clocks,radios and smoke detectors. This design makes use of a doubler, splitter and rectifier. The device has been known to give 36 volt/9 watts. This devices does away for the need for batteries by extracting energy from natural and man made radio waves. THe device uses a coil made of the following design 479 turns of #22 wire on a 3" plastic tube, the coil should be loosely wound as a close wound coil tends to reduce power collection. One end should be connected to unit at antenna connection point and the other to an antenna of your choice . Other coil types could be used in your experiments such as sliding induction coils that is inside one another and find the best position by moving them in different positions.

Other coil types can be found by referencing any good book on radio fundamentals. Check out the Telsa type coils as well. Power could be also enhanced by placing a tin foil pyramid under the coil as this seems to also improve power. The power produced also seems to improve near bridges,ships and any thing containing a great deal of metal. A high antenna wire seems to improve the running of unit although a 7 foot whip antenna worked reasonably well too. A interesting side line is the inventor claims to be able to detect earthquakes by the rise and fall of energy levels.

Subject: Re: [nuenergy] Re: Tate power ambient circuit update Reply-To: [email protected] Geoff , the reason the power is so low is the capacitors being used. If larger capacitors such as used for power audio systems or the newer super capacitors such as the 50-500 Farad units You will begin to see real power. The schematics shows a great deal of naivety on the part of the Inventor and the issuing Patent Office. The basic idea is 1000% It relates to My Earth Electrical System II from 15 years ago. from an email recieved from Don L.Smith Introduction The Ambient Power Module (APM) is a simple electronic circuit which,when connected to antenna and earth ground, will deliver low voltage up to several milliwatts.

The amount of voltage and power will bedetermined by local radio noise levels and antenna dimensions.

JesAscanius Version of Nikola Tesla's Aerial System

Generally a long wire antenna about 100' long and elevated in a horizontalposition about 30' above ground works best. A longer antenna may be requiredin some locations. Any type copper wire, insulated or not, may be used for the antenna. More details about the antenna and ground will be discussedfurther on.

The actual circuit consists of two oppositely polarized voltage doublers (Figure 1). The DC output of each doubler is connected in series with the other to maximize voltage without using transformers. Single voltage doublers were often found in older TV sets for converting 120 VAC to 240 VDC. In the TV circuit the operating frequency is 60 Hz.

The APM operates at radio frequencies, receiving most of its power from below 1 MHz. The basic circuit may be combined with a variety of voltage regulation schemes, some of which are shown in Figure 2. Using the APM-2 to charge small NiCad batteries provides effective voltage regulation as well as convenient electrical storage. This is accomplished by connecting the APM-2 as shown in Figure 2B.

Charging lead acid batteries is not practical because their internal leakage is too high for the APM to keep up with. Similarly, this system will not provide enough power for incandescent lights except in areas of very high radio noise.

It can be used to power small electronic devices with CMOS circuitry, like clocks and calculators. Smoke alarms and low voltage LEDs also can be powered by the APM.

Figure 3 is a characteristic APM power curve measured using various loads from 0-19 kOhm. This unit was operating from a 100' horizontal wire about 25' high in Sausalito CA. As can be seen from the plot, power drops rapidly as the load resistance decrease from 2 kOhm. This means that low voltage, high impedance devices, like digital clocks, calculators and smoke alarms are the most likely applications for this power source. Some applications are shown in Figures 4 through 7.

Figure 4. A digital clock is shown powered by the APM-2. The 1.5 volt clock draws 28 microamps. Its position on the power envelope curve would be off the scale to the right and almost on the bottom line, dissipating only 42 microwatts. Figure 6 shows a clock which has the APM-2 built into it so it is only necessary to connect the antenna and ground wires directly to the clock. The antenna for this clock, which is a low frequency marine type, is shown in Figure 7. These antenna are expensive, not generally available, and usually don't work any better than the long wire mentioned above. But it may be necessary to use them in urban areas where space is limited and radio noise is high.

Building the Module The builder has a choice of wiring techniques which may be used to construct the module. It may be hand wired onto a terminal strip, laid out on a bread board, experiment board, or printed circuit. Figure 8 shows some of the different ways of constructing the APM-2. Figure 8A is constructed on a screw strip terminal; Figure 8B is constructed on a perforated breadboard; Figure 8C is built on a standard experiment board; Figures 8D, 8E, and 8F are all printed circuits; Figure 8F is made up on a solder strip terminal.

If you wish to make only one or two units, hand wiring will be most practical, either on a terminal strip or breadboard. Assembly on the terminal strip (Figure 8A) can be done easily and without soldering. It is important to get the polarity correct on the electrolytic capacitor. The arrow printed on the side of the capacitor points to negative. Figure 9 is a closer view of the terminal strip with an illustration of the components and how they are connected.

The breadboard unit is shown in Figure 10 with all components on one side and all connections on the other. All you need is a 2" x 2" piece of perforated breadboard (Radio Shack #276-1395) and the components on the parts list. Push component wires through the holes and twist them together on the other side. Just follow the pattern in the photo, making sure to observe the correct polarity on the electrolytic capacitors and the diodes. The ceramic capacitors may be inserted in either direction.

The experiment board unit is assembled by simply pushing the component leads into the board as shown in Figure 11. This unit is powering a small red LED indicated by the arrow.

The solder strip unit is made up on a five terminal strip. The antenna connection is made to the twisted ends of the ceramic capacitors. When soldering the leads of the 1N34 diodes, care must be taken to avoid overheating. Clip a heat sink onto the lead between the diode and the terminal as shown in Figure 12.

It is beyond the scope of this pamphlet to show how to make printed circuits, but the layout of the board is provided in Figure 13.

Figure 14 shows the front and back view of the completed printed circuit.

A small switch may be installed on the board to activate the zener regulator (Figure 15). This board was designed for use in clocks.

Antenna Requirements The antenna needs to be of sufficient size to supply the APM with enough RF current to cause conduction in the germanium diodes and charge the ground coupling capacitors. It has been found that a long horizontal wire works best. It will work better when raised higher.

Usually 20-30 feet is required. Lower elevations will work, but a longer wire may be necessary. In most location, possible ing structures already exist. The wire may be stretched between the top of a building and some nearby tree or telephone pole. If live wires are present on the building or pole, care should be taken to keep your antenna and body well clear of these hazards. To mount the wire, standard commercial insulators may be sued as well as homemade devices. Plastic pipe makes an excellent antenna insulator. Synthetic rope also works very well, and has the advantage of being secured simply by tying a knot. It is convenient to mount a pulley at some elevated point so the antenna wire may be pulled up to it using the rope which doubles as an insulator (Figure 16).

Figure 17 is an illustration of a horizontal wire antenna using a building and tree.

Grounding Usually a good ground can be established by connecting a wire to the water or gas pipes of a building. Solder or screw the wire to the APM-2 ground terminal. In buildings with plastic pipes or ts, some other hookup must be used. A metal rod or pipe may be driven into the ground in a shady location where the earth usually is damper. Special copper coated steel rods are made for grounds which have the advantage of good bonding to copper wire. A ground of this type usually is found within the electrical system of most buildings. Conduit is a convenient ground provided that the conduit is properly grounded. This may be checked with an ohmmeter by testing continuity between the conduit and system ground (ground rod). Just as with the antenna, keep the ground wire away form the hot wires. The APM's ground wire may through conduit with other wires but should only be installed by qualified personnel. Grounding in extremely dry ground can be enhanced by burying some salts around the rod. The slats will increase the conductivity of the ground and also help retain water. More information on this subject may be found in an antenna handbook. Good luck getting your Ambient Power Module working. It is our hope that experimenters will find new applications and improve the power capabilities of the APM.

Parts List for the APM-2 Four 1N34 germanium diodes ~ Figure 1, X1, X2, X3, & X4 Two 0.2 mfd 50 V ceramic capacitors ~ Figure 1, C1 & C2 Two 100 mfd 50V electrolytic capacitors ~ Figure 1, C3 & C4 Copper wire for antenna & ground connections

Nikola Tesla Free energy Circuit

More on A Practical Guide to Free-Energy Devices

A 6 V battery can provide 100-150 Vdc center-tapped at a high internal impedance (not dangerous though it can inflict an unpleasant jolt). A 6.3 V transformer is connected ' 'in reverse'' with a transistor used in a Hartley oscillator configuration. The frequency of operation may be controlled by varying the value of the 10 ohm resistor.

An FM Crystal Set

Updated 2010-11-28: Since this page was first posted in June 2010, I have made a number of improvements to the receiver described below. I've also received a few requests for additional information. In reviewing this page, I realized that it was long overdue for an update. The original information is unchanged. The updated information appears at the bottom of the page. Introduction Crystal sets–totally ive receivers without any amplification–have been popular for many years. The vast majority of them are designed and built for the Medium Wave band, 530 to 1700 kHz. However, there have also been some built for the shortwave bands, and occasionally some for the FM band (88 to 108 MHz). Having several local powerful FM stations, I'd occasionally noticed that I was picking up FM on a medium wave crystal set. Considering that there was a strong enough signal to receive FM when it wasn't wanted, it should be possible to build a receiver to pick up the signal intentionally. Most FM crystal sets use slope detection. That is, they are tuned slightly off the centre frequency of the station, and as the frequency modulated carrier moves in and out of the receiver's resonant frequency, an audio signal approximating the original modulation is produced. True FM detection requires a frequency discriminator circuit, and the complexity generally makes it unsuitable for a ive receiver. I'm only aware of one such project. It was designed and built by Edward Richley, and was written up in the Xtal Set Society Newsletter, January and March 1996. In order to achieve the high Q required, the circuit used a coaxial resonator constructed from 2 inch copper pipe. It was ingenious in that, while most FM discriminators have two tuned circuits, this one only required a single tuned circuit. This was an important feature, because trying to adjust two tuned circuits when tuning to different stations, would be extremely difficult, if not impossible. I've redrawn the Richley circuit here:

A very brief description of the circuit follows. For a more thorough discussion, please refer to the original newsletter articles. As described in the diagram notes, the resonator is constructed from a two foot piece of 2 inch diameter copper pipe, with a centre conductor of 1/2 inch copper pipe. The resonator is designed for an unloaded Q of 2000, and a loaded Q of about 500, which is required for acceptable selectivity. FM channel bandwidth is 200 kHz, or 0.2 MHz. Therefore, at a frequency of 100 MHz, required Q will be 100/0.2=500. Transformers T1, T2, and T3 are made from standard 300:75 ohm baluns which are included in the kit of parts that come with virtually every new television or radio. T1 is used unmodified. T2 is rewound as 1:3 autotransformer. T3 has a two turn primary and a two turn secondary. Transformer T1 converts the incoming unbalanced signal to a balanced signal which then feeds T2 and T3. The secondary of T3 is loosely coupled via the 8 pF capacitors to the resonator coupling loops, which develop a 90° phase shifted signal due to the normal behaviour of a coupled circuit at resonance. The secondary of T2 is connected to the common point between the diodes. This signal is 180° out of phase with the incoming RF. Hence, there are quadrature signals applied across the diodes which act as a synchronous detector. As the frequency modulated radio carrier moves above and below the resonant frequency, the phase

shift varies above and below 90°. The detector converts this phase shift into an audio frequency signal which matches the original audio source. Initially, I planned to build a copy of this receiver, but after doing a bit of research, I concluded that a helical resonator would be more practical at these frequencies. My new plan was to duplicate this circuit, except that I would substitute a helical resonator for the coaxial resonator. The first step then was to design a helical resonator with an unloaded Q of 2000 at a resonant frequency of 110 MHz, the top of the FM band.

Using standard helical resonator design formula, the resulting resonator has an inner coil constructed from 4.3 turns of 9.5mm (3/8") copper tubing. Coil outside diameter is 65mm (2.6"), and coil length is 84mm (3.3"). The shield is constructed from copper sheet, and is 84mm (3.3") square by 200mm (8") long. It is housed inside a wood enclosure for rigidity. The shield is about 2" longer than required to allow room for coupling loops at the bottom, and for a tuning assembly at the top.

The photo to the right shows the shield parts prior to assembly. The tuning mechanism is a small plate attached to a length of brass tubing which runs through the end cap. The rod is moved in and out by means of a screw. This changes the fringing capacitance at the open end of the coil. The closer the plate is to the coil, the lower the resonant frequency. The next photo shows the top cap with tuning mechanism assembled. The tubing has a square cross section to prevent the assembly from turning.

It slides inside a slightly larger piece of square tubing. This type of tubing is available at hobby shops and comes in a range of sizes suitable for close sliding fits. The coil and the inside of the shield were polished to remove oxidization, and hence maximize Q. The partially assembled resonator is shown in the next two photos. The photo on the left is of the top end, and the photo on the right is of the bottom end.

The next photo shows the top cap installed and the exterior part of the tuning mechanism:

I decided that for initial testing, it would be best to keep things simple and use a slope detector circuit. This is shown in the following schematic:

The 27uH choke and 150pF cap are a low filter to trap out a local high powered AM station that was causing some interference problems. For a diode, I used a 1N34A initially, but found another unknown type in my parts box which worked slightly better. It may in fact also be a 1N34A from a different manufacturer. The detector and coupling loops were built on a small circuit board and mounted on a block of wood so that the assembly could be repositioned easily to obtain the best performance. The assembly is shown below.

The RF and detector coupling loops are blue hook-up wire. The little postage stamp sized circuit board is a low power headphone amplifier which was used in conjunction with my relatively insensitive headphones to make testing a bit more painless. The initial working set-up is shown below:

Reception was fairly good. The signal was weak using my piezo phones without any amplification. So, I connected in the headphone amplifier, and did all the remaining testing with it. Selectivity was very good. However, I don't have any easy means to measure the Q. I found that I could easily pick either side of a station's centre frequency for detection, and if I tuned dead centre, the signal became very distorted. That leads me to believe that the Q is very high. I couldn't get that kind of precision with earlier experimental sets which used a coil/capacitor tuned circuit. So with that bit of success, I started thinking about making a true discriminator. As mentioned above, I had intended to duplicate Ed Richley's circuit except that the coaxial resonator would be replaced with the helical one. However, after reviewing the original circuit, I concluded that two of the transformers could be eliminated. The first transformer is a balun to convert from an unbalanced RF signal from the antenna to a balanced one. Since I intended to use a dipole antenna, this was unnecessary. Rather than excite the resonator via coupling loops through the small capacitors, I decided to drive the resonator directly at a tap near the ground end of the resonator coil. This eliminated the need for the balanced secondary winding on T3.

I decided that I could combine T2 and T3 into a single transformer. I had disassembled several baluns and found some to have two-hole cores and some were simple toroids. I used the toroid from one of these and wound a 4-turn primary, and 4-turn secondary. The secondary has a tap after the first turn which is grounded. The schematic follows:

The secondary arrangement still gives the same 3:1 power ratio between the diodes and the resonator excitation, but the circuit is much simpler. The revised coupler/detector board is shown below:

The black wire with the brass clip is for connecting to a tap on the resonator coil. I made it this way so that I could quickly try different positions. Optimum turns out to be about 40mm (1.5") from ground. The pickup loop is the single loop of blue wire with the audio output tap at the halfway position (black wire). Diodes are socket mounted so that different ones may be tried without fuss. I didn't put in an AM trap this time, because the circuit arrangement is not conducive to AM pickup, since the diodes are essentially shorted at MW frequencies by the pickup loop. Here's a brief description of how the circuit works: Input transformer T1 splits the incoming RF (untuned) into two signals 180 degrees out of phase with reference to ground. The low level side goes to the resonator and the high level side goes to the common point of the diodes. The voltage induced by the resonator into the pickup loop will be 90 degrees out of phase with the the input signal. This is the nature of a loosely coupled tuned circuit at resonance. The phase of the signal on the pickup coil will vary on either side of 90 degrees as the carrier is frequency modulated. The "90 degree" signal is applied to the outside ends of the diodes while the

180 degree signal is applied to the diode common point which then act as a quadrature detector. Hence it is essentially the same mode of operation as Richley's original circuit. The following photo shows the discriminator assembly in position:

Now, with everything connected up, and after finding the best position for the pickup coil I found that the discriminator appears to be working correctly. Tuning is sharper, and there are no double slopes to select as I could previously. In fact tuning this receiver has very much the same feel as tuning a commercially made FM receiver. Audio level has increased significantly too. This is something I had been wondering about. I was concerned that a true discriminator might be less sensitive than a slope detector, but it appears not to be the case. 2010-11-28 Update: Since the original material first appeared on this page, I've continued to tinker with this circuit, resulting in several incremental improvements. Although the improvements came in many small increments, the cumulative result is that this is now a very good performing receiver. Improvements listed in order of importance (most important first) are: 1. •Changing the detector diodes to Avago HSMS-2850;

2. •Changing the turns ratio on the RF input transformer; 3. •Getting a good set of sound powered headphones; 4. •Permanently mounting the detector loop; 5. •Finding a good antenna location; 6. •Shortening the resonator helix. Diodes The Avago (formerly Agilent) HSMS-2850 diodes are rather odd ones. I had purchased these along with a number of other different types of detector diodes, more or less at random, to test their effectiveness. These particular diodes were described as "zero bias" which made them too intriguing to up. When they arrived, I set them aside and didn't think about them again until I ran across this page on Dick Kleijer's site. He had also built an FM crystal set and had tested a number of different diodes. Among them were the HSMS-2850's which turned out to be his best performers. I dug through my diode assortment and confirmed that they were the same ones I had. These are surface mount, and so I had to construct a small printed circuit adapter board. Otherwise, I would have tried them much sooner. When I tried them, there was a considerable improvement in performance. I mentioned that these diodes are odd ones. They have a very low R0 (zero crossing resistance) of about 5000 ohms (many thanks to Mike Tuggle who measured them for me). Coincidentally, this is about the same R0 as a galena detector. The low R0 would normally make them a bad choice as a crystal set detector (galena aficionados notwithstanding). However, it appears that this particular resonator circuit has a very low resonant RF output resistance, which turns out to be a good match to the diodes. For more info on this, see Article #4 on Ben Tongue's site. This low RF resistance has another consequence which is discussed below. In addition to the above mentioned diodes, I also recently tested Sanyo 1SS351 diodes. These are a surface mount type, with two diodes in a single package, and while not quite as sensitive as the HSMS2850's they are significantly better than any of the germaniums or other Schottkys that I've tested. I also tried paralleling two of these, and got a further improvement in performance. In a listening test (which was completely subjective) I found it difficult to tell much difference between the parallel 1SS351's and the singleHSMS-2850's. RF Input Transformer Getting the best turns ratio for the RF input transformer was an iterative procedure which also affected the optimum input tap position on the resonator. The schematic appearing earlier on this page shows a primary with 4 turns, and a secondary with 4 turns, with a grounded tap at 1 turn. After much experimentation (and wire breakage), I currently have a primary with 6 turns, and a secondary with 8 turns, with the grounded tap again at 1 turn. This is the current schematic:

I might have arrived at this turns ratio a bit sooner if I hadn't made a bit of a blunder, confusing a dipole antenna with a folded dipole antenna, and thinking my antenna impedance was 300 ohms when in fact it is 75 ohms. (You may pick up on this goof in the earlier text of this page, which I haven't corrected yet.) I'm not yet at the point where I will claim this transformer configuration to be optimum, but it is much improved over what I had before. Another addition is the 27µH choke connected between the centre tap on the detector loop and the phones. This seems to help isolate the audio wiring from the RF part the circuit, and vice versa. This improvement is subjective at this point (it seems to increase volume), but regardless, it doesn't appear to be a bad idea. There were a few other changes made to the circuit as well, but in the end, except for the diodes, the revised RF transformer windings, and the addition of the 27µH choke, the other changes resulted in negligible improvements. Hence, they were dropped. While on the subject of the schematic diagram, alert readers will note the apparent short circuit at audio frequencies if the diodes happen to be conducting. The consequence of this is that when receiving a very strong signal, there is an upper limit to how much audio the detector will produce. My earlier circuit simulation verified this, but I haven't yet encountered a real life signal strong enough to hit this limit.

However, I have allowed for circuit modifications should this ever become a problem. It would involve the addition of a resistive or reactive component (Z1) between the diodes as shown in the diagram here. The circuit model used in the simulation does not yet have sufficiently accurate parameters for the diodes or other components, to give a good idea what values to use for Z1 or the coupling capacitors. My circuit board has several strategically located jumper plugs to allow for this future addition without the need for any soldering or de-soldering. Sound Powered Phones There's not much I can say about sound powered phones other than: If you're serious about crystal radios, eventually you'll have to get a pair. There is nothing else that compares in sensitivity. Piezo phones are not bad if you have nothing else available, but simply don't compare to sound powered phones . I was lucky enough to get a pair of Western Electric SP phones (thanks to Darryl Boyd), and with the other improvements that were made in the receiver, there is no longer any need for an amplifier, not even for testing purposes. The Western Electric phones (model D173014) have an impedance of about 600 ohms per element, and I have them wired in series for a total impedance of 1200 ohms. This turns out to be such a good match for the low impedance detector, that I have not found any advantage to using an audio matching transformer. Originally, I tested these with a Bogen T725 transformer wired as an autotransformer. The best transformer configurations were the 2:1 and 4:1 ratios. However, I couldn't honestly detect any improvement over the direct connection using no transformer at all. I did notice a slight difference in frequency response (better bass with the transformer), but no net improvement in volume using the transformer. I'm guessing that the optimum impedance would be somewhere around 2400 to 3000 ohms, but the transformer's insertion loss probably negated any benefits from the better impedance match. Detector Loop Mounting Originally, as shown in the early photos above, I mounted the diodes and detector loop on a separate moveable base. Once I'd determined the optimum position for the loop, I mounted it permanently

through the side of the enclosure. This eliminated quite a bit of excess wire which is never good to have when dealing with VHF. The RF transformer and diodes are now mounted on the side of the enclosure where the leads come through. Here is a detail of how the detector mounts:

The detector loop is 50 mm (2") diameter #14 AWG (1.6 mm) copper wire, which is stiff enough to maintain optimum shape and position, once those have been determined. It is a friction fit through the side of the enclosure. Both leads are insulated where they through the shield. The detector loop could mount through any of the four sides of the shield. However I chose the arrangement shown here, where both ends of the loop cross the ground end of the helix at 90°. It's my belief that this would minimize any possible unbalance in the loop due to ground capacitance effects. What isn't shown in the photo, is the separation between the detector loop and the bottom of resonator helix. The distance is approximately 16 mm (5/8"). The loop has a bit of a twist in it, to follow the pitch of the helix, so that separation is a fairly constant 16 mm at all points, except where it bends around the ground end of the helix. Audio is taken off at the middle of the loop through the 27µH choke. The input tap (black wire) is connected to the resonator helix about 75 mm (3") from the grounded end.

Antenna Location Unfortunately, antenna location is much more critical when dealing with VHF than when dealing with medium wave. A change in position of less than a metre can drastically affect reception. Initially, I found that to receive some stations, the antenna would have to be in one position, and to receive other stations, the antenna would have to be moved somewhere else. In all cases the antenna would have to be rotated to get the strongest signal. My antenna is a simple set of rabbit ears (dipole) that was supplied with some long forgotten television receiver. I have it mounted on a camera tripod with a pan head, making it easy to rotate. Currently, it sits in one corner of my house where I find that it gives good reception for all of my local FM stations (except one; more on that below). I have no explanation for why this particular location is best. No doubt, there are many complicated factors. However, I do note that this is in a far corner of my house which likely has the least amount of electrical wiring in the walls and ceiling. Resonator Helix Length Originally the helix was 4.3 turns of tubing. According to my design calculations this should have given a resonant frequency of about 110 MHz, just above the top of the FM band. In practice, it gave a resonant frequency of just a bit over 103 MHz. I expect this is mainly because I wasn't able to make the helix diameter exactly as designed. The resonant frequency is also affected by fringing capacitance between the ungrounded end of the helix and the outer shield. If the helix is not perfectly centred in the shield (particularly the ungrounded end) the fringing capacitance will be higher and the resonant frequency will be lower. The resonant frequency can often be increased slightly by adjusting the position of the helix (ie., making sure it's exactly centred). However I wasn't able to increase the resonance enough, using this method. So, I trimmed pieces off the end of the helix until the resonance increased to an acceptable value. After trimming, the helix was stretched out lengthwise to return the overall length to the design value of 84mm (3.3"). Currently, the the helix is 3.75 turns, and it tunes up to 106.3 MHz which covers all of the FM stations in my area, but not quite the whole FM band. Performance Shortly after building the phase discriminator version of the detector circuit, I was asked–quite legitimately–whether I was certain that the circuit was behaving as a proper phase detector, and not just an oddly configured slope detector. So, it was quite important that I its operation. Previously, I'd run a circuit simulation which indicated it would work. Then subsequently, I put a digital voltmeter on the audio output and verified that as an unmodulated carrier was slowly and manually swept across the resonant frequency of the receiver, the output went negative, crossed through zero, became positive

and then dropped to zero again. But that's a far cry from actually seeing a decent response curve on a scope. Unfortunately, I don't have a VHF signal generator which can be suitably frequency modulated. However, I do have a cheap FM modulator (intended for playing portable music players through a car's FM radio). It uses a Rohm BA1404 FM modulator IC. The RF output is tied back into the 12V power cable which plugs into the car's cigar lighter. This was not suitable for providing a reliable and consistent signal to the receiver. So, I had to perform some surgery. Thanks to an online data sheet for the BA1404, I was able to locate the RF output (pin 7, for anyone interested), and connect a separate RF output lead. Once I managed to separate the RF output, I got a nice clean signal. My audio sweep source is a sine wave from an audio generator. The only remaining complication was that the FM modulation was slightly out of phase relative to the input audio signal. So, I had to build an adjustable phase shift circuit to go between the audio generator and the scope X-input. Once it was adjusted properly, the waveform cleaned up very nicely, and I was quite thrilled to see a fairly good S-shaped discriminator response curve which is shown here:

This is the actual DC response of the detector with an input FM signal of about 100 MHz. Mid position on the graticule is 0 Volts DC. Some asymmetry can be seen in the response curve; the skirt on the right tapers off much more gradually than that on the left. At some point I will probably investigate why this is happening, though I'm not too concerned about it. The flat section on the left is an artifact of the FM modulator. All things considered, I'm quite happy with the results. Unfortunately, because the modulator is uncalibrated, I'm unable to say just what the actual bandwidth is. I hope to address this in the future.

As a side note, it can be seen that the trace is somewhat fuzzy. This is because there is a superimposed 19 kHz stereo pilot tone which is produced by the FM modulator. This had me baffled for a while until it dawned on me what it was. By adjusting the audio generator frequency, the 19 kHz signal can be made almost stationary, but still moves too fast for the camera to catch it. The test set-up, used to generate the above response curve, is shown here:

Listening Test I have twelve local FM stations (within approx. 30 km). With this receiver I can clearly hear all except for two of them. Of those two, the first (91.7 MHz) seems to be temporarily off the air (although I can pick up its unmodulated carrier on other receivers). The second (100.3 MHz) is so weak that I can barely pick it up on a regular FM receiver, and even then, it is buried in noise and is barely intelligible. So, I wasn't too upset about not picking it up on the crystal set. The ten local FM stations, which I can hear clearly, range in power from 1300 Watts to 100,000 Watts. All of them can be heard at pleasant volume in a reasonably quiet room. The 1300 Watt station is one of the loudest. However, its transmitter is probably closer to my listening location than the others. The receiver is selective enough that there is absolutely no overlap in stations. Audio quality is very good with the sound powered phones. Four of the stations are strong enough that they can be heard fairly well with Sony MDR-W08 Sport Walkman phones when properly matched with a Bogen T725 transformer. Although the volume was considerably lower than with the sound powered phones, it was still quite intelligible.

Here is an audio clip tuning through the ten local stations starting at 88.7 MHz and ending at 105.5 MHz, which gives a general idea of the audio quality. File size is 1.6 MB. It was created by connecting the crystal set audio output through a simple RC de-emphasis filter, into a microphone preamp and then into the audio input of my computer running Audacity 1.25 software. The audio is exactly as recorded with no editing or other processing. You will notice that the sound quality changes rather erratically during tuning. This is due to backlash in the tuning mechanism (particularly troublesome at the bottom of the band), and simultaneous adjustment of antenna direction. More to Come While this project is close to being complete, a few things remain to be done. Firstly, the tuning mechanism is very non linear. Tuning at the low end of the band is very touchy, and backlash in the adjusting screw makes tuning these stations a bit frustrating. I'm currently looking at an alternative tuning method which should result in more linear tuning. Secondly, I would like to test the receiver with a good quality outdoor FM antenna. Finally, I need to do some final clean-up of the construction, and make the case a bit more presentable. Construction Data Summary (as of 2010-11-06) Since a number of the circuit parameters have changed over the course experimentation, and the numbers are scattered around this page, I've consolidated the pertinent information into the following summary: Resonator 1. •Helix conductor: 9.5 mm (3/8") diameter copper tubing; 2. •Helix outer diameter: 65 mm (2.6") 3. •Helix length: 84 mm (3.3") 4. •Helix number of turns: 3.75 5. •Helix input tap position: 75 mm (3") from ground end of helix 6. •Shield material: 16 oz. copper flashing; 0.56 mm (0.022") actual thickness 7. •Shield width: 84x84 mm (3.3" x 3.3") 8. •Shield length: 200 mm (8") 9. •Distance - helix mounting hole centre to bottom edge of shield: 50 mm (2") Detector Loop

1. •Material: #14 AWG (1.6 mm) bare copper wire 2. •Diameter: 50 mm (2") 3. •Spacing from helix: 16 mm (5/8") 4. •Estimated detector output impedance: 2400 to 3000 ohms RF Input Transformer 1. •Core Type: Ferrite toroid 2. •Core Material: Unknown ferrite mix, but presumably suitable for VHF broadband transformer use 3. •Core outer diameter: 8 mm (5/16") 4. •Core inner diameter: 4 mm (5/32") 5. •Core length: 4 mm (5/32") 6. •Primary winding: 6 turns, #30 AWG, Kynar insulation (AKA: wire-wrap wire) 7. •Secondary winding: 1+7 turns, #30 AWG, Kynar insulation, grounded tap at 1 turn Miscellaneous Components 1. •Detector diodes: Avago HSMS-2850 Alternate type: Sanyo 1SS351 2. •Detector RF choke: 27 µH, JW Miller 79F270K (Note, due to the low self-resonant freq. of 14 MHz, this choke may not the best choice)

Back to: Projects Home This page last updated: March 15, 2012 Copyright 2010, 2012, Robert Weaver

Crystal Set Analysis byBerthold Bosch, DK6YY (First published in German in 1993/94; see references at end. Updated 12/03/2002) (See some of Berthold Bosch´s realized xtal sets here) Contents: 1.Voltages and Powers in Set, 2. Antenna/Earth as Signal Source, 3. Set with Parallel- Tuned Circuit, 4. Diode Properties, 5. RF Matching, 6. AF Matching, 7. Computer Simu- lations, 8. Series-Tuned Circuits . Since my schoolboy days I have been fascinated by crystal radio reception: radio in its most basic form. However, in many cases I was not really satisfied with what I read on the subject in the literature. In the treatises I came across, the descriptions often remained rather vague, presenting little convincing foundations. Partly they were rather speculative and even presented contradicting conclusions. For this reason I found it advisable to carry out my own investigations. My intention was to obtain more quantitative results, for example as regards the best diode and the understanding of the obvious interdependence between the radio-frequency (RF), audio-frequency (AF), and DC subcircuits, what it meant for an optimum design. In the following I present results obtained over the years. Only mediumwave reception is considered. 1.Typical Values of Voltages and Powers in Set Let us first see of what order of magnitude the RF and AF voltages and powers are which we have to deal with. According to amplitude-modulation theory, the AF power contained in the total AM signal of power PRF is given by m2 PRF / (2+m2) where m is the modulation factor. If we assume m=0.5 we thus have 11 percent of AF power in the AM signal. Sometimes broadcasting stations use modulation factors of up to m=1 (100 percent) which then causes a correspondingly higher AF power component. At my urban location, in the West of (Ruhr District), the strongest station (15 km away) produces an electric field strength of 0.18 V/m and, with my antenna and earth arrangement, an RF power of about 3 mW is available in the crystal set. Hence 330 µW of AF are contained in the RF if we assume m=0.5. A practical (linear) diode detector coupled to a tuned circuit delivers 70 to 80 percent of this to the AF load. This means that ideally I can expect about 240 µW of AF being available from my local station, sufficient for moderate operation of a loudspeaker. In the crystal set that I am going to investigate (Fig. 2 below) I measured the following RF voltages across the tuned circuit when RF and AF matching existed (Secs. 2, 5 & 6): a) Tuned to the local station (WDR 2, 720 kHz, 200 kW, 15 km away):8.9 V b) From of my "district station" (DLF, 549 kHz, 100 kW, 35 km, field strength 40 mV/m, 0.2 mW of RF power):2.3 V.

c) At night - with a wave trap for the local station - more than a dozen stations appear from all over Europe with 1 to 5 mV/m, producing 130 mV across the circuit as a mean value (0.5 µW of RF). Such low voltages will move the working point over only a rather limited part of the diode characteristic where the relative curvature is low. Consequently, the detector efficiency now drops to below one percent. d) Good headphones produce an audible signal down to 10 pW of applied AF power. Employing a signal generator and using a sensitive diode (see below) I found that an RF power of about 10 nW is required to generate this 10 pW of lower-limit AF. The detector efficiency has at this very low RF level thus fallen to a mere one per mille. Obtaining an RF power of 10 nW in my set requires a field strength of about 0.3 mV/m. According to estimates based on groundwave propagation theory, a 1000 kW transmitter operating near 1500 kHz should generated 0.3 mV/m at a distance of 190 to 200 km; the electric field strength is roughly proportional to Radicle (PTX)/(f2d2), where PTX = transmitter power, f = frequency, and d = distance. The particular example is chosen because at 1440 kHz I can during the day just hear the signal of RTL Luxembourg, being 195 km away and reported to radiate 1200 kW. The voltage measured across the tuned circuit was 40 mV in this case. To be able to receive RTL I carefully have to suppress the local as well as the district station. e) When I connect an AF amplifier to the crystal set, a number of stations located about 150 to 250 km away can additionally be heard via groundwave propagation in the daytime. The diode thus provides (some) detector action at RF levels even lower than 10 nW. But the AF power generated is then too small to produce an audible signal in the phones directly. The above numbers show the considerable variations in the RF voltage generated across the tuned circuit. Hence it is not surprising that a crystal diode found to be best suited for DX reception is not necessarily the optimum choice for achieving best loudspeaker operation from the local station. But before I present results on diode behaviour I am going to describe the circuit I employed. Let us start with antenna and earth as an integral part of the total circuit. 2. Antenna and Earth as Signal Source

(Fig. 1: Equivalent circuit of antenna/earth combination.) I use an inverted L-type antenna of 43 m length, about 10 m above ground. The earth connection is provided by three metal rods of 2 m length each, driven into marly, i.e. a not particularly well conducting, soil. The antenna/earth combination can be represented by the equivalent circuit shown in Fig. 1, giving measured values for the various elements. The antenna capacitance is denoted by CA, the inductance by LA, RE is the earth loss resistance, RR the radiation resistance, and VA,O the antenna source voltage. The two last elements increase in value with the antenna's height and length. For the source voltage I measured a value of 1.6 V, using a selective RF voltmeter. The knowledge of this quantity, which is produced by the strong local station, permits to easily determine the earth resistance. For it 210 ohms were obtained, a relatively high value. Reducing it by installing a better ground system would pay high dividend. Note added in 2002: Meanwhile I installed an extensive counterpoise net in the garden as earth terminal. This reduced the earth resistance to about 25 ohms, with an associated marked increase in available RF power. Maximum power is transferred to the load, i.e. from the antenna to the crystal set connected to A-E, when we arrange for impedance matching and for resonance in the resultant antenna/earth series circuit. The set will presents, in general, an inductance which is too small for achieving resonance in the antenna circuit. Therefore, an additional coil must be inserted (Fig. 2). The resistance of about 400 ohms in the series-tuned circuit (200 ohms source resistance plus 200 ohms resistance of set when matched) yields a Q-factor of only 4. But this is still helpful regarding sensitivity, but also selectivity (sharpness of tuning), since the delivered current (voltage) is increased by this factor of 4, meaning 16 times in power. 3. Crystal Set with Parallel-Tuned Circuit As a sort of "standard set" I used and investigated the popular arrangement shown in Fig. 2 which employs two tuned circuits. The inductance LC couples the antenna to the coil of the tuned circuit, the degree of the variable coupling chosen so that matching is achieved. The fixed L1 has a somewhat larger value than required for tuning the antenna/earth circuit of Fig. 1. The variable capacitor C1 is then used for tuning to resonance. The numbers for L1 and C1 apply to my particular case. C3 serves as AF storage capacitor for obtaining a maximum of AF amplitude at the phones, and it additionally provides a short for the RF. In practice, however, it often can be omitted without audible drop in AF. - To be able to properly match the diode detector to the parallel-tuned circuit, the detector branch is hooked up either to the top of coil L2 (wound with Litz wire onto a suitable ferrite rod) or to one of 11 taps provided on it. In this way the diode can be connected, via a switch, to 12 resistance values along the tuned circuit. Such a fine adjustment was required for the investigations reported in Sec. 5. The unloaded tuned circuit has a resonance resistance of 105 k ohms (at 1000 kHz), which drops to 52 k ohms when matched to the antenna/earth. (These are not particularly high resistance values because of the many leads from the taps to the switch.) According to the switch position chosen the diode can so be connected to 12 resistance values that vary between 52 k ohms and 100 ohms. When the diode is set to the tap that provides matching the total resonance resistance then drops to 26 k ohms. - On the right in Fig. 2 the equivalent circuit of the headphones is given which we require later on.

(Fig 2: Wiring diagram of crystal set with parallel-tuned circuit and tuned antenna/earth.) We have now the task to match (1) on the RF side the diode branch, via the tuned circuit and the antenna coupling, to the antenna source, and (2), regarding the AF, the phones or the loudspeaker to the diode. Both procedures are strongly interrelated. But first we must find out more about the diode and its dynamic resistance. 4. Crystal-Diode Properties All semiconductor diodes principally show a dependence of current I on applied voltage V (static characteristic) like I=IR,0*exp (V/nVT) - IR,0, where IR,O denotes the reverse saturation current at high negative voltages, VT the temperature voltage of 26 mV at room temperature, and n an ideality factor between 1 and 2.

(Fig. 3: Theoretical current-voltage characteristics of various p-n diodes; equal diode areas assumed.) Reverse breakdown is neglected. The relationship applies to both to p-n diodes and to Schottky diodes. However, the composition and physical meaning of IR,O differs completely for these two diode types (diffusion current resp. field current). Important is the fact that the exponential rise of the diode current occurs the faster, i.e. the turn-on voltage (as a kind of threshold) becomes the lower, the higher the saturation current IR,O is. Calculated characteristics for p-n diodes made from germanium, silicon, two modern compound semiconductors and from galena (PbS) are given in Fig. 3, assuming equal diode areas. The associated reverse currents are stated. The difference in diode behaviour is caused by differing electronic material properties (band gap). Real curves, in particular of the natural crystals, are flatter because of parasitic elements, mainly of diode series resistance. Regarding low turn-on voltage a galena p-n crystal diode is theoretically even slightly better than one made from germanium. For Schottky diodes principal curves like those in Fig. 3, relating to a particular crystal material, cannot be given. In their case the characteristics strongly depend on the kind of the metal electrode and on the processing parameters. But measured curves for Schottky as well as for p-n diodes are given in Fig. 4, including fairly good examples of galena (PbS) and iron pyrite (FeS) detectors; see also the Table below for identifying the diode types. Again one notices the relatively low turn-on voltage of the two natural crystals which equal or even are below that of the germanium diodes 1N34 and OA5, thus being well suited for low-level detection.

(Fig. 4: Measured current-voltage characteristics of various semiconductor diodes (reverse currents indicated)) Very interesting is the performance of modern low-barrier Schottky diodes made from silicon, like the NEC 1SS16 (almost identical: 1SS99, BAT32, BAT63), which show turn-ons at 0.15 to 0.18 V. And, indeed, they show superb performance at low levels. One should expect that the InAsSchottky diode (which was specially made for my experiments) and the TU 300, a backward diode made by Siemens, would according to the curves shown be even more sensitive detectors. But this is not the case. As mentioned, a low turn-on voltage is inevitably associated with a high reverse current. This current reaches values of a few hundred µA for the InAs diode, as also for the TU300 and the Schottky diode BAT33. If the reverse current, i.e. an unwanted back current, reaches such high values we have strong counteracting effects, and ultimately the detector action disappears completely. Anticipating the results of computer simulations described in Sec. 7 one can state that diodes like the 1SS16 show the optimum relation between low turn-on and still acceptable reverse current, thus making them the best choice of presently available diodes as regards detector sensitivity.

To show and compare the capability of various diodes the Table summarizes values of measured AF voltages and of rectified currents, for 1 and 100 µW of available RF power. A power of 1 µW is in my set typical for DX stations at night, and 100 µW for stations 30 to 50 km away. As is seen the 1SS16 leads the field. - For 3 mW of RF (my local station) I obtained with a 1SS16 a DC current of 715 µA, which increased to 1.85 mA in the short-circuit case (AF/DC load = 0), and to 2.95 mA when under these conditions the set was retuned.

(Table: Measured values of AF voltage (across phones of 4 k at DC) and of rectified DC current for various diodes and two levels of RF power) Sometimes a DC bias from a battery is applied for shifting the operating point of the diode closer to the turn-on voltage and so improving the detection efficiency. By this method the AF voltage obtained can be increased, for example when using the 1N5711 and the 1N914 at low RF levels. The 1SS16 group of diodes, however, hardly gains from a DC bias. Only at RF powers below about 200 nW I was able to measure a certain rise in AF voltage. At the lowest detectable RF level of 50 nW (Sec. 1), the AF voltage increased by 20 percent (i.e. power by 45 percent) when the optimum bias was applied. But this effect was measurable only, being still too small to be noticed by the ear. Note added in Jan. 2002: Backward diodes (BWD), like the TU300, are good detectors at extremely low RF signal levels, below about 1 nW with associated voltages of only a few mV. This is due to the relatively sharp bend in the BWD characteristic at zero volts. The generated AF signal is, however, too small for operating phones directly and calls for an AF amplifier. Then stations can be copied which are not heard when in such a set-up with AF amplifier a "normal" sensitive diode, like the 1SS16, is used instead of the BWD.

5. RF Diode Resistances and RF Matching For achieving best performance it is required to RF match the diode to the tuned circuit. The dynamic resistance of the diode depends on the amplitude of the RF voltage applied to it, and on the kind of AF load impedance. In AM tube radios the detector diode operates at a high level (linear detection) and has a load consisting of a large (ohmic) resistor shunted by a small capacitor. Calculations show that in this case the RF diode resistance, as presented to the tuned circuit, is roughly half of the ohmic load resistance. In a crystal set the calculation is somewhat more complicated since there the RF voltage on the diode is generally lower and the diode load is more complex (see equivalent circuit of phones in Fig. 2). Hence I preferred to measure the RF diode resistance RD. The measurements were carried out under actual working conditions using a signal generator. Figs. 5(a) and (b) show the results obtained for high-impedance phones with 4 k ohms DC resistance and for low-impedance ones with 120 ohms, respectively. The RF frequency used in these measurements was 1000 kHz, the modulation frequency 1 kHz with a modulation factor of 0.4 (given by the signal generator). Figs. 5 give the measured diode resistances, as a function of the RF power applied, for an 1SS16 (also some in parallel), a silicon p-n diode 1N914, and for natural galena as well as carborundum (silicon carbide; SiC) crystals. The diode circuit was in turn connected to the various taps on the coil L2 .When the RF voltage measured across the tuned circuit dropped by a factor of radicle (2)=1.41 compared to its value without diode, matching was achieved. Then the RF diode resistance equalled the RF resistance of the tuned circuit at the tap point. To avoid an error one must readjust the coupling to the antenna when the diode is connected to the first found (V/1.4) tap point and then repeat the search for the now somewhat altered (V/1.4) tap. A second iteration further improves the result, but not much. As in principle to be expected from the characteristics, the diode resistances vary rather widely, from some 100 ohms to some 10 k ohms, with lower values obtained when the DC resistance of the phones is low. The galena detector shows values only moderately higher than those of a single 1SS16. The silicon

diode 1N914 presents high values due to its high turn-on, which even more applies to carborundum.

(Fig. 5:Measured RF diode resistances versus available RF power: (a) for high-impedance phones (4 k ohms at DC), (b) for low-impedance phones (120 ohms at DC) The data obtained then indicate that the optimum tap position on coil L2 (for matching) depends on the diode type, the strength of the received station, and on the DC resistance of the phones. The larger the value of the diode resistance is, the higher must the tap position be up the coil. Sometimes it was suggested in the literature to have a fixed tap at a point of about 1/4 to 1/3 of the windings counting from the earth point. In the present case the tuned circuit has a resistance of approximately 6 k at the 1/3 tap point. As Fig. 5a shows, this indeed is a rather good choice for a galena detector when highimpedance phones are used and weak stations received. Impedance matching requires that the reactances of source and load cancel out. But in our case the resistance of the tuned circuit has no reactive part at resonance, and the reactance of the diode, caused mainly by the diode junction capacitance of at most a few pF, can be neglected. Connected to a particular tap, the diode resistance is (auto-)transformed up and appears in parallel to the resonance resistance of the tuned circuit. This means that not all of the available RF power reaches

the diode since a reasonable fraction of it is dissipated in the resistance of the tuned circuit. In order to really transfer the maximum of power from the antenna to the diode branch, the diode (of generally low resistance compared to that of the tuned circuit) should be connected untapped to the top of the coil L2. This, however, strongly reduces the selectivity of the set and requires a readjustment of the coupling of the tuned circuit to the antenna. With high incident high RF power (and/or low impedance of the phones) the tuned circuit can, under these conditions, become loaded to such an extend that variations of the capacitor C2 have no tuning effect any longer, which means that C2 is obsolete and can be omitted. The diode circuit is then aperiodically coupled to the (tuned) antenna circuit, while the coil L2 merely acts as the secondary winding of the transformer which matches the diode to the antenna. 6. AF Matching If a crystal ear phone is used or the diode detector is followed by an amplifier (generally of high input impedance) one has to design for maximum voltage at the detector output. Here, we rather have to deliver a maximum of power to the phones. Hence the impedance of the phones (or the speaker) as the AF load should have such a value that a maximum of AF power is transferred to it. The AF source resistance RG is at low RF levels (square-law detection) approximately given by the reciprocal of the slope of the diode characteristic at the operating point. At higher RF levels (linear peak detection) it is determined by the current spikes flowing through the diode. In so far, RG nearly equals the diode resistances as shown in Figs. 5. The tuned circuit presents an AF short. I determined the equivalent circuit of a pair of high-impedance Telefunken phones (4 k ohms at DC) at 1 kHz by using a measuring bridge and obtained the quantities given in Fig. 2. REA is caused by the electroacoustical transducing process. The AF source has to provide the real power for REA as well as, necessarily, for the DC coil resistance, and foremost the reactive power for the phone coils (2.5 H) that are to move the membranes. In order to obtain the maximum of power transfer the magnitude (amount) of the overall phones´ impedance ZAF ( 16 k ohms for my phones) must match the AF source. Again I preferred to experimentally find the optimum AF load: I connected in turn 14 phones and speakers of different impedance to the set, partly connecting two of them in series or parallel, which in total provided 20 load impedance values between 80 ohms and 75 k ohms in magnitude. From the AF voltage measured across these load impedances I determined the AF power. The coupling to the RF signal generator was readjusted to retain RF matching each time the AF load was changed. Fig. 6 shows the obtained results when using a) a diode 1SS16 at low RF power (1 µW) and b) with a 1N914 at higher power (1 mW). The optimum AF load impedance turned out to be, resp., 1.2 and 3 k ohms. In order to simplify matters the diode was in this experiment fixed to the 1/3 tap at the tuned circuit. This meant a compromise as regards match-ing and generally did not produce quite the maximum of achievable AF power. The dashed curves of higher AF power in Fig. 6 were obtained when the diode was connected to

the top of the tuning coil (as discussed above).

(Fig. 6: measured AF power versus AF load impedance.) In order to present my measurement results in a more general form, Fig. 7 shows the AF power obtained as a function of the AF impedance now divided by the respective occurring source (= diode) resistance. The curves indicate that the maximum is reached when ZAF has a value of 50 to 70 percent of the diode resistance. The simplifications introduced above, like choosing the fixed 1/3 tap, are probably the reason for not reaching a higher percentage. But we can say to be roughly correct with our predictions. - Sometimes it is suggested to match just available phones (speaker) to the diode by using a suitable transformer. I found this only helpful if the mismatch was extremely high. In the other cases the winding and iron losses of the transformer, as well as the inductive shunt, tend to dissipate more AF power than is gained by providing the right transformation ratio. One also has to consider that the human ear cannot small changes in acoustical power. Alterations like those shown above the dashed line in Fig. 7 will hardly be noticed by the ear. Thus, it seems that the exact value of the AF load impedance is not of paramount importance as regards noticeable output power. But the general principle holds that a number of small improvements in matching, each of which will not produce any audible effect for itself, might in sum indeed be noticed by the ear. So it turns out as an interesting and important feature that a high-impedance AF load, which is associated with a high DC resistance, will produce a high diode resistance (= AF source resistance), and vice versa. This means that the circuit has a self-optimizing tendency towards the matched condition. Regarding RF selectivity of the set, as another important quantity, a high AF impedance - leading to a high diode resistance - is of advantage. But the influence of the AF impedance in this respect is not

particularly pronounced. I measured an increase in -3db RF bandwidth by a factor of 2.5 when the AF load was decreased from 100 k ohms. At 720 kHz (local station) this bandwidth was 20 kHz in my set, which yields a total loaded Q factor of 36 - leaving room for improvement (see Secs. 2 & 3).

(Fig. 7: Measured AF power versus AF load impedance normalized to AF source resistance.)

7. Computer Simulation of Crystal-Set Behaviour As a summarising investigation I simulated the overall performance of the circuit shown in Fig. 2, using the analysis program SPICE. An RF frequency of 1 MHz, an AF of 1 kHz, and a modulation factor of m=0.4 were assumed. The tap on the coil L2 was held fixed at 1/3 of the windings from earth. The I(V) equation given in Sec. 4 served to describe the diode behaviour.

(Fig. 8: Simulated AF voltage across high impedance phones reverse saturation current of diode with available RF power as parameter (m=0.4). Measured values for various diodes included.) The simulation results are presented in Fig. 8, which shows the AF voltage VAF AF obtained across phones of 16 k ohms AF impedance (4 k ohms DC) as a function of the diode reverse saturation current IR,O. Parameter is the available RF power at the tuned circuit, ranging from 10 nW (lowest sensible level; see Sec. 1) to 1 mW (about local-station level). Measured values pertaining to various diodes are entered for comparison. These measurement values lie in part slightly below, partly somewhat above the simulated curves, but agree in general. In practical diodes, particularly Schottky diodes, the effective reverse current increases noticeably with reverse voltage (i.e. with increased RF power). This had to be considered when inserting the measured values in Fig. 8, and s for the slight bending of the vertical lines to the right. The dotted curve in Fig. 8 shows the simulated DC voltage across the phones for 1 µW of RF power.

The simulated curves for VAF drop at the right side for diodes with high IR,O because of the adverse effect of reverse current as mentioned in Sec. 4. The decrease at the left of Fig. 8 for low RF powers results from the high turn-on voltage shown by diodes having a low IR,O. In these cases the associated smaller RF voltages increasingly fail to reach the turn-on of the diodes. The curves make clear how important it is to choose a diode which has the right value of reverse current. A definite maximum in sensitivity, especially pronounced at low RF powers, is found for diodes having a reverse saturation current of a few µA. Particularly the 1SS16 diode class is in this range, but also the OA5 and 1N34 perform not too badly, and good specimen of galena (PbS) crystals behave still satisfactorily. Hence this result is quite in agreement with what we already have found in Sec. 4. - When the AF voltages in Fig. 8 are used for calculating the AF power, the AF/RF detection efficiency can be worked out. It is found that the efficiency drops drastically for low RF levels, with one per mille being reached at 10 nW of RF. This agrees with the observations described in Sec. 1. In the simulation the reverse breakdown voltage, at which in practical diodes the current starts to rise rapidly, was not included. For 1 mW, the highest RF power considered, the diode resistances have dropped to around 3 k ohms (Fig. 5a), calling for a low tap position on the coil. There the RF voltage is relatively low. My local station, with 3 mW of RF, produces 2.4 V (i.e. a peak-to-peak value of 6.7 V) at the required tap point, so that the negative peak only just reaches the breakdown voltage of -6 V for a 1SS16. In consequence, reverse breakdown seems not to be a particular limiting factor, even if the voltage across the tuned circuit might be somewhat higher in case there is a better Q factor. Assuming a constant available RF power, the RF voltage is proportional to the square root of the Q (resonance resistance). The low-barrier silicon Schottky diodes, which show a reverse breakdown in the range of -5 to -8 V, are thus well suited for use from the lowest to the highest levels of RF power generally occurring in crystal sets. 8. Some Remarks on Series-Tuned Circuits Historically the first crystal sets, in the pre-broadcasting days, were of the kind shown in Fig. 9a. There the values of L1 and C2 pertain to my particular antenna/earth situation. For maximum power transfer in the circuit of Fig. 9a the combination of crystal plus load in parallel should match the impedance of the antenna source. In the latter the earth resistance represents the main resistive part which in the then primarily commercial stations had values of only 10 to 50 ohms. On the other hand, the crystaldiode resistances were around a few k ohms so that a considerable mismatch existed. For this reason one soon changed to the arrangement of Fig. 9b with diode and load now in series, and with the possibility to match the diode branch to the antenna by choosing the right tap on the coil L1. To increase

the selectivity of the set a second tuned circuit was eventually introduced, as e.g. shown by Fig. 2.

(Fig 9: Series-tuned circuits: a) Diode directly in tuned antenna circuit, b) Diode across tuning inductance (preferably tapped), c) Diode in separate series-tuned circuit, coupled to the series-tuned antenna/earth circuit.) Using modern low turn-on diodes (1SS16 etc., Sec. 4) and having in general a higher earth resistance than in commercial stations, the circuit of Fig. 9a is however quite effective. Possibly paralleling of diodes is of advantage, depending on the actual source and load resistances. With two 1SS16 and employing a moving-coil speaker via a suitable transformer as the load I obtained an AF power of 180 µW from my local station. With ten 1SS16 and two moving coils of 16- ohms speakers in series as load, the obtained AF power of 210 µW approached the maximum possible after Sec. 1. Ideally the diode resistance should, in my case, about equal the 210 ohms of the antenna source (Fig. 1). Reverse diode current is not harmful, nor a possibly low reverse breakdown voltage. We have here a “currentcontrolled” case where voltages across the diode remain low with associated high currents, a few tens of a mV and some mA when I used the ten diodes. In contrast, voltage control is - more or less experienced when parallel-tuned circuits are employed where high(er) voltages and low(er) currents exist. A set according to Fig. 9a, then, is a most simple hook-up for effectively receiving the nearest station. Substantially higher selectivity, approaching that of the set of Fig. 2, is offered by the arrangement with two series-tuned circuits shown in Fig. 9c. Since there any resistance (loss) in the circuit made up by L2 and C2 should be kept low for achieving a high Q factor, the diode(s) - preferably paralleled again - and the phones/speaker should be of low impedance. The RF choke might help to improve performance. The diodes found in the left of Fig. 4, particularly the backward diode TU300 (which is of little use on parallel-tuned circuits), operate excellently in the arrangement of Fig. 9c. Modern 8/16- ohms headphones or, for stronger stations, directly the moving coil of a speaker are effective AF loads. - Backward diodes, being scarce these days, are tunnel diodes which have the typical

current hump reduced to a flat region at about 200 µA height. The I(V) curve of the TU300 shown in Fig. 4 has in reality reversed polarity. For reasons of comparison with the other diodes the polarity was changed in the graph. Other BWD types are: AEY17 /29, 1N3539 /3543, TU1B. In conclusion, the investigations sketched here have certainly enlarged my knowledge on crystal-set design, with the identification of the “best diode” and noticing the tendency of self-optimization which makes the set a sort of good-natured device. Other rewarding topics could not be covered, as there are, for example, more complex circuits for increased selectivity and for DX. Also short-wave crystal sets are fascinating since they provide DX from all over the world with simple designs. Based on: B. Bosch and M. Bussmann: ZurEmpfindlichkeit von Kristallgleichrichtern und HalbleiterdiodenbeimDetektorempfang.Funkgeschichte Nr. 93 (1993), pp. 275 - 285. B. Bosch: Anungs- und SchaltungsfragenbeimDetektorempfang.Funkgeschichte Nr.98 (1994), pp. 211 - 225. CRYSTAL SETS 2 Some Practical Designs MAKE YOUR OWN CRYSTAL SET !!

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CRYSTAL SETS 2: SOME PRACTICAL DESIGNS

I hope that you attempt building one or two of these crystal set designs and I really do recommend that the components are carefully connected up using soldered ts onto a piece of tag-strip for reliability. However if you are new to constructing such electronic circuits then some simple solderless techniques could be employed and these are suggested at the bottom of the page. Also see Crystal Sets Part 5 for more ideas on experimenting with crystal sets.

An early and very basic crystal set would have been nothing more than a coil of wire, perhaps 50 100 turns, wound around a cardboard tube about 3 inches (7cm) in diameter, a detector (or cats whisker) and a pair of special High Impedance headphones A very basic crystal set circuit. (as discussed in part 1). There would be a very large aerial strung up around the garden and the all important

connection to earth. The coil would have tapping points (connection points) at intervals of around 5 or 10 turns. See the circuit diagram on the right for details of who the set is wired together. The tapping points on the coil allow the set to be tuned to different frequencies by adjusting the position of tap B. Tap B would be connected to the coil at differently positions by way of a crocodile clip. The fewer turns between the

top (aerial end) of the coil and tap B, the shorter the wavelength received (ie the higher the frequency). Tap A would allow the detector to be connected at different positions to vary performance . There is an additional component drawn in the above diagram, the capacitor (value 1000pF), this is included in crystal sets that used the High Impedance magnetic headphones, and byed any remaining radio frequencies

(RF) to earth.

I have not built the set described above as it is so basic. Such a crystal set above would probably have been adequate in 1920 - 1923 when there would have been only one local transmitter receivable.

The Standard Crystal Set

When the BBC expanded transmissions and it became possible to hear more than a single station it would have became necessary to include a more convenient means of tuning the set. This was achieved by including a Variable Tuning Capacitor, of about 500pF (0.0005uF) connected in parallel with the tuning coil

forming a tuned circuit. The tuning capacitor would have a Bakelite knob on the spindle to aid tuning.

Because of the simplicity of crystal sets, it is often difficult to separate stations. When tuned into one station it is often possible to hear another close by station in the background, this is due to lack of selectivity. This can be reduced somewhat by adjusting the positions of the Aerial Tap and Detector Tap. Moving them closer to the bottom of the coil, the earthy end, reduces the load on the tuned circuit and this improves selectivity, however it does also reduce sensitivity which can make the station quieter. Headphones will often swamp a tuned circuit and reduce its selectivity (Q factor), so moving the tapping point lower down improves this situation. Every circumstance is bound to be different though so the best balance has to be found by experimentation. My crystal set has both the diode and the aerial connected to the same tapping point on the coil, about a quarter of the way down. The modern 'standard crystal set' shown above uses a Crystal Earphone, since suitable high impedance magnetic headphones (of 2000 to 4000 ohms) are no longer widely available. When using a crystal earpiece the 1000pF capacitor shown in the first diagram can usually be omitted an in its place a 47k ohm resistor is connected, this ensures that the Crystal Earphone will work at its most efficient i.e. the sounds will be as loud as possible. The resistor allows DC current to flow through the circuit efficiently this would otherwise be blocked when using a crystal earphone. In a modern crystal set the detector used is a Diode. Suitable diodes include OA80, OA81, OA90 OA91 and IN94 which are usually available from component stockists.

A Better Diode For Increased Efficiency The OA47 will be of particular interest since it has the lowest forward bias voltage of any of these diodes which will make the crystal set somewhat more sensitive and therefore louder. The US equivalent of the British OA47 is the IN34. On the right you will see my real working example of a crystal set A real working crystal set. Radio as if by magic with no battery or The large plastic knob on the front mains power. turns the variable tuning capacitor. This set receives the three UK national stations and also three local radio stations very well at my location. There is a small 3.5mm jack socket mounted on the front of the plastic case (MB5 from Maplin Electronics) that the crystal earphone plugs into. The coil can be seen inside the case, it is 70 turns of 30 gauge enamelled copper wire wound around the centre of a toilet roll and tapped every 10 turns, by scraping off the enamel insulation and making a small twist. The croc' clips can be seen clipped on to these twists to connect to the aerial and detector tap points.

THE MEDIUM WAVE COIL - MORE DETAILS

Medium Wave Coil

PHOTO SHOWING THE INSIDE OF THE COMPLETED CRYSTAL SET

The number of turns of wire required on the coil will vary depending on the size of the former (in this case the inside toilet roll) and the thickness of the wire. So to obtain the correct coverage of the medium wave band may need a little experimentation.

I usually find that between 50 to 90 turns is right and I generally use enamelled copper wire that is between 30 s.w.g. and 26 s.w.g (i.e. 0.315mm and 0.45mm diameter), so it's best to start with too many turns and then work down.

The more turns that you use the lower the frequency range will be, i.e. too many and the coverage of the top end of medium wave around 1500 1600 kHz will be lost, while too few and the coverage down to 500 kHz will be lost. It is also important that the coil former is non conducting, i.e. not metallic. It could be wood or cardboard or a short piece of PVC piping and with a diameter of between 1½ and 4 inches (4 to 15 cm) are common sizes. You could try using a ferrite rod too, see below. This particular set has a coil wound onto a toilet roll tube which consists of 70 turns of 30 s.w.g. (0.315mm dia)enamelled copper wire tapped at every 10 turns. It also has the additional small trimmer capacitor that helps match the aerial to the tuned circuit thereby improving selectivity, see below.

USING A FERRITE ROD AS THE COIL FORMER

The aerial coil could be wound onto a ferrite rod. A piece of 10mm diameter ferrite rod of between 3 and 6 inches long (80 to 150mm) will be most suitable and will require between 50 and 90 turns of enamelled copper wire to provide coverage of the medium wave band: First make a paper tube that is held together with sticky tape that will easily slide up and down the ferrite rod. Then wind the coil over this with the windings neatly side by side. Make tapping points every 10 or 15 turns so that the aerial and diode tapping points can be adjusted. Adjustments to the tuning range can be made by removing some wire from the coil so it is best to start off with too many turns and then work down. Fine adjustments can be made to the completed coil by sliding it up and down the ferrite rod.

AN IMPROVEMENT TO THE DESIGN

The crystal set above also has one small, but significant, improveme nt over the standard crystal set and that is an Aerial Trimmer. A trimmer is a variable capacitor, very similar to the tuning capacitor,

except smaller and adjusted with a screwdriver.

The value of the trimmer is usually around 10 50pF, but if a small tuning capacitor is available that will probably be just as effective. In the absence of such a Improved Crystal Set design, with good selectivity variable capacitor, individual fixed ceramic capacitors of e.g. 10pF, 50pF and 100pF can be tried in this position to judge which gives the best results with the particular aerial being

used. The trimmer capacitor adjusts the coupling to the tuned circuit, reducing the load of the aerial on the tuned circuit will improve the selectivity (Q), and it will be easier to separate stations. Again tapping points are used and I find this to be an excellent arrangemen t.

The picture on the right shows the general layout of the crystal set above. The coil is of approximately 70 turns is wound on the centre of a toilet roll, and has tapping points at 10 turn intervals.

Layout Of The Crystal Set - Although this is soldered together an alternative to tagstrip would be a 5amp mains connector block so that components can be trapped in place with screws. See article below.

The trimmer is soldered between the Aerial terminal and the piece of 5way tag strip, and a wire goes from there to a croc' clip which is clipped onto a tap on the coil. The Diode is also soldered onto the tag strip, one end connected to a piece of wire going to a second croc' clip & connected to a tapping point on the coil, the other end of the diode is connected to the 3.5mm jack socket that the Crystal Earphone plugs into. The 47k resistor is also connected to the earphone end of the diode and

goes to earth, the earth terminal wire is soldered to the tag strip at this point too. The tuning capacitor has two terminals, one connected to each end of the coil, and one of them is also connected to earth as shown. [Where the wires cross over in the diagram, they do not touch and are not connected together].

LONG WAVES

In most areas around Europe and certainly around much of the UK you will be able to hear a Long Wave station. To receive Long Wave on a crystal set will require an aerial coil with a greater number of turns to increase its inductance. As a good general guide a coil wound on a piece of 10mm diameter ferrite rod will require about 250 turns of enamelled copper wire: First make a paper tube that is held together with sticky tape that will slide up and down the ferrite rod. Then wind the 250 turn coil over this, the windings will have to be made over the top of each other. Make tapping points at, say, 50, 75 and 100 turns to tap the aerial and diode to. As with the medium wave ferrite rod aerial, adjustments to the tuning range can be made by adding or removing some wire from the coil, and fine adjustments can be made to the completed coil by sliding it up and down the ferrite rod. The longer the ferrite rod the better and anything between 3 and 6 inches long (80 to 150mm) will be very good.

SHORT WAVES

If you like experimenting, then reducing the number of turns on the coil to say 10 to 30 will allow reception of the higher frequencies, the Short Waves. I have found that winding the coil around a 'ferrite rod' often works even better with short wave reception. Obtain a ferrite rod about 7 to 15 cm long and about 1cm in diameter. Make a couple of small tubes of card, about 4cm long, that will fit tightly over the rod. On one tube wind two coils using 0.5mm diameter enamelled copper wire - one coil of about 30 turns and a second one of 2 or 3 turns wound over the top of the first. Secure the windings in place with Sellotape.

Example of a layout using a connector block to wire up a crystal set

On the other card tube wind a similar coil, but use about 15 turns for the first coil and for the second coil wind about 3 to 4 turns over the top, and secure with Sellotape tape.

These coils will provide coverage of short wave in two bands using the first coil for the longer wavelengths, typically 60 to 31 metre bands and the second coil for the shorter wavelengths typically 25 to 19 metre band. Wire up the circuit as shown in the circuit diagram below. USING A TOROID INDUCTOR FOR SHORT WAVES

Even better selectivity performance can be achieved by winding the inductors (coils) on a ferrite toroids (T50-2 yellow, or green will do). The aerial trimmer need not be used if selectivity and sensitivity is found to be adequate. It's all about experimenting, and I find it best to use a trimmer or small coupling capacitor to obtain the best selectivity. Up to 30 turns of 0.5mmenamelled copper wire can be used for the longer short waves below 10 MHz, while a winding of around 15 turns will provide coverage of the shorter short waves above 10MHz.

The circuit diagram of the Short Wave Crystal Set A completed SW Crystal Set using a toroid inductor. Note: the main winding has a tap to allow the switch to short part of the winding and thereby give two ranges.

AUSTRALIAN DESIGN

Moving back to the Medium Waves, here is a circuit for a very interesting Australian design that promises extremely good station separation (selectivity), and having built it I can vouch for that claim, it's really excellent. I receive three national stations and three local stations at my location with excellent clarity using a modest antenna and

standard crystal earphone. The coil is different to the other crystal sets described above, it is much bigger at 3¼ inches (8cm) diameter and 5inches (12cm) long. I made my coil former out of the cardboard from a breakfast cereal box - just like Blue Peter! The design is often referred to as The Mystery Crystal Set, by Proton.

The front of the Australian Crystal Set

Two distinct coils are wound on it, the first one consists of about 50 turns of 24 s.w.g (approx) enamelled copper wire. The second coil is 25 turns, very close wound right over the top of the first coil using 30 s.w.g. (approx) wire, try to get this second coil wound in between the windings of the first, for better inductive coupling. Then carefully wire up the set according to the diagram. Notice that the tuned circuit is not connected to earth and has no direct connection to the detector circuit. The detector circuit is connected to earth however. The two aerial terminals offer alternative selectivity performance, terminal A gives very good selectivity while B is very wide. I never bother with B.

Make the coil carefully and wire up this crystal set according to the circuit diagram opposite and you will be rewarded with a really high performance crystal set of a type that was used in the very early days of broadcasting in 1930's in Australia. This is probably my favourite crystal set!

THE DENCO PCC1 COIL

The PPC1 coil was a commercially manufactured by DencoClacton Ltd and was popular among hobbyists not keen on going to the bother of winding their own fiddly little coils. As a child I wanted try one of these coils and sent away for one by mail order. It arrived a few days later in a little cloth bag, like a miniature pump bag, with protective wrapping inside. The coil windings are entirely enclosed in what I can only describe as a cylindrical ferrite 'shell', the four very thin connecting wires exiting, two either side, from small apertures in the 'shell'. The performance of the circuit shown below I seem to was quite pleasing. Unfortunately I cannot find the set or the PPC1 coil at the moment, but here is a reproduction of the circuit diagram and data:

THE REPANCO DRR2 COIL

I recently rediscovere d an old Repanco DRR2 Longwave/

Mediumwav e coil that must have been kicking around in my junk box since the 1970's. The DRR2 coil was made by Repanco in Coventry. It came with a page of suggested circuit The circuit diagram of the crystal set using the Repanco DRR2 coil diagrams which I thought had been lost to the mists of time, but it recently came to light again, so I have now copied it below. Once again I included an aerial trimmer which can be adjusted to improve selectivity.

Repanco Ltd was formed by two ex-army signals engineers and from the earliest days of radio supplied crystal set kits and coils to radio construction enthusiasts. The Repanco DRR2 coil was for medium wave and long wave intended for use in when building simple crystal set and valve radio circuits.

A 'lash-up' of the Repanco crystal set

It consists of three coils; a Medium Wave coil at the top that includes a tapping point (for the aerial); a coupling coil or tickler in the middle; a lower coil which can be connected in series with to top coil to provide Long Wave reception. I have built a quick crystal set with the coil and it provides good reception with excellent selectivity, so it must have a very good Q factor.

The Repanco DDR2 coil was provided with a simple Foolscap size information sheet that showed four different radio circuits. Sadly the sheet does not give a huge amount of information and my copy is now rather tatty and faded - it is copied below:

Making The Spider's Web

(NB. TRANSLATED FROM DUTCH so I must apologise for any small errors) See Chris Dorna's WebsiteHERE!

Generally coils are cylindrical. We have chosen a spiders web coil because these are simple to wind. You must however pay attention that you lay the windings nice and tight when winding the coil. The material chosen is 3.4 mm MDF as is easy to work with. If you make a former then you could also use an old CD perhaps. On the former shown on the left 7 segments have been created. For the crystal radio in the article below, the coil has been made with 9 segments at 40° intervals. The inner diameter is 2.5 cm and the outer diameter is 12 cm. The former is 4.75 mm thick and there are about 60-70 turns on the winding. The wire should be reasonably thick as very thin wire can very easily break when winding. For calculating the coil we used Professor Coyle's Spiderweb Coil Calculator

The Completed Coil The Scouts "Jamboree On The Air" Below are a couple of photographs of the completed crystal radio that the Vught North Scouts constructed during the their "Jamboree On The Air". The spiders web coil is a very interesting feature of the set and was chosen because it was thought to be easier to wind for the younger scouts. The scouts were provided with a package that contained all the required components including the former for the coil, suitable wire to wind the coil and a base onto which they could build the spiders web crystal set on their Jamboree. They had to ensure, however, that great care was taken when putting up the wire for the large antenna for the wire which was a minimum of 10 meters long. The crystal set also had to be connected to an earth point and a water pipe was used for this.

Photo showing the completed crystal set consisting of a germanium diode, a tuning capacitor and the

spider web coil. The red wire is the aerial and the brown wire is connected to earth. The crystal earphone is shown in the foreground.

The germanium diode that converts the received radio frequency signal to audio that can be heard in the crystal earphone.

PARTS THAT WERE REQUIRED BY THE SCOUTS WERE PROVIDED IN THE THE PACKAGE:-

1

Base

1

Coil Former

1

Tuning capacitor

1

Reel Of Coil Wire

1

Germanium Diode e.g. OA47, OA90, OA91, IN34 etc

1

Crystal Earphone

All

Screws and fixings required, tools, solder and soldering iron

1

Constructional Description with photograph Click HERE to instructions. (7MB

PDF File)

Despite the clear instructions provided there was soldering involved in the construction of the crystal set and the scouts were guided in this process by experienced (such as radio amateurs) to ensure safety at all times.

You can get in with regard to this project at: [email protected]

Sincere thanks to Chris Dorna for bringing this exciting project to our attention. Excellent work one and all!

Kenneth Rankin has also sent in the details of the crystal set that he has built and you can see his photographs and notes in CRYSTAL SETS 4 I hope you try building one, it's easy and great fun!

Crystal Sets Part 4 >

No AM radio stations or transmitters in your locality or country?

Has your local medium wave broadcast station closed or been moved to VHF/FM or Digital? Don't worry. You can still build and experiment with crystal sets and TRF radios by also buying or even building a simple low power AM transmitter. So, not only can you use your crystal sets but you can also run your own radio station that can be heard in and around your home - playing the music or programmes that you want to hear!

SSTRAN AMT3000 : Superb high fidelity medium wave AM transmitter kits from SSTRAN. Versions available for 10kHz spacing in the Americas (AMT3000 or AMT3000-SM) and 9kHz spacing in Europe and other areas (AMT3000-9 and AMT3000-9SM). Superb audio quality and a great and well designed little kit to build: http://www.sstran.com/pages/products.html

http://www.sstran.com/ Other AM transmitters available: Spitfire &Metzo Complete, high quality ready built medium wave AM Transmitters from Vintage Components: http://www.vcomp.co.uk/index.htm Vintage Components offer a choice of the high quality Spitfire and Metzo transmitters:

SPITFIRE AM Medium Wave Transmitter with 100 milliwatt RF output power:

http://www.vcomp.co.uk/spitfire/spitfire.htm

METZO AM Medium Wave Transmitter with built in compressor:

http://www.vcomp.co.uk/metzo/metzo.htm

AM88 LP A basic AM transmitter kit from North County Radio. http://www.northcountryradio.com/Kitpages/am88.htm

LINKS:

BOWOOD ELECTRONICS - A friendly, helpful and very speedy source for many of your electronic components at prices that won't frighten your wallet! THE FOXHOLE and P.O.W RADIOS - Simple crystal set receivers used by soldiers during the war and by prisoners of war (P.O.W.'s). VINTAGE COMPONENTS - A great resource for crystal sets, components, valve radio kits and medium wave AM transmitters! 6V6 - Electronic Nostalgia and Vintage Components

Crystal Sets (Part1) | Build Your Own Crystal Set (Part 2) Crystal Set by Kenneth Rankin (Part 4) | Experimental Crystal Sets (Part 5) | Crystal Radio Links .

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CRYSTAL SETS 5: EXPERIMENTAL CRYSTAL SETS .

Picture 1 - The Complete Experimental Crystal Set THE POPULARITY of the crystal radio arises from its simplicity, and the fact that it needs no power supply. The circuit here allows for easy experiments with tuning, aerial and diode coupling, and frequency coverage. Wrong connections can cause no damage to any components. A Crystal Set is more often than not used for the reception of medium and long wave radio, but short wave reception is also quite feasible. It will normally be possible to receive some of the stronger international radio stations. This is adapted from an article that appeared in the 1970's in Everyday Electronics, and gave me almost endless hours of fun! BASIC CIRCUIT The basic circuit is shown in Picture 2 below. The coil L1 can be air cored, or have a ferrite rod placed in its winding. The variable capacitor C1, in conjunction with aerial-earth capacitance, tunes the circuit to resonate with the wanted radio station frequency. The diode D1 "detects" or demodulates the radio signal so that the programme is heard in the earpiece. This basic circuit can be modified in various ways to obtain better performance.

EARPHONE As most constructors will be using a Crystal Earpice to listen to the crystal set it is essential that a 47k

Ohm resistor is connected across the earphone terminals (TB1/1 and TB1/2 in the diagram), i.e. in parallel with the earphone, otherwise results will be very quiet. A High Impedance headset of 20k Ohms (20,000 Ohms) may give even better results, but these are very difficult to obtain , so unless you happen to already own such a headset the Crystal Earphone with 47k resistor will be the only option. An ordinary magnetic earpiece or Walkman headphones will not work with a crystal set.

ASSEMBLY Construction is of a 'breadboard' type using a wooden board of about 165 x 130 mm. A 12-way block connector, TB1, is used to connected together the components and this is screwed onto the wooden board. The use of a block connector provides an easy method of connecting the components together and then subsequently rearranging them as the experiments progress. Tuning capacitor C1 is screwed to a bracket made of some scrap metal which is then also screwed firmly down to the baseboard, see Picture 1 above. Thin plywood screwed to the front edge of the baseboard would also provide a suitable method of fixing the tuning capacitor to the base. A knob with pointer is fitted to C1, and a scale is drawn and fitted behind this. Except for C1, all connections are made by the terminals of the 12-way terminal block as shown in Picture 4. Loosen the screws with a small screwdriver, insert the bared ends of the wires, and tighten the screws. The various locations on the terminal block, TB1, are also shown in the circuit diagram, Picture 2.

AERIAL AND EARTH Crystal receivers need a long wire aerial preferably strung outside and about 25m long, or as long as is possible to install. If this is outside it should be high and clear of earthed objects as this will improve performance. An earth is absolutely essential for a crystal set to work properly. The earth lead can be run to an earth rod or spike that is buried to a depth of about 1 meter into damp soil. Or it may be soldered to a bare metal can which is buried in damp soil. It is feasible, though not recommended, that the earth lead can be connected to the earthing terminal of a hi-fi system or even to the bare metal case of a personal computer that is plugged into an earthed mains outlet, but is switched OFF.

Stranded, insulated wire, or purpose made aerial wire can be used for the aerial and earth leads.

Picture 2 - The Basic Circuit

Picture 3 - Photo Of The General Layout

INDUCTORS (The Tuning Coils) The following four coils are suggested for initial use as L1 : Coil 1: Make a thin card tube to slide on a 10mm diameter ferrite rod, and on this tube wind about 105 turns of 32 s.w.g. enamelled copper wire, side by side. Secure ends with sticky tape. Coil 2: Make a similar coil to to coil 1 having about 15 turns of 24 s.w.g. enamelled wire on the card tube. Loops of cotton will help hold the ends in place. Coil 3: Wind 9 turns of 20 s.w.g. bare tinned copper wire on an object about 20mm in diameter. Remove and stretch to separate the turns, to obtain a coil about 25mm long. Coil 4: Make a similar coil to coil 3, but with 5 turns. The Ferrite Rod It will be necessary to have a ferrite rod of about 60mm to 75mm long available. Coils 1 and 2 will provide reception of medium wave and the longer short wave bands. Coil 3 should cover about 3 10MHz shortwave with the ferrite placed in it, or about 6 - 18MHz with the ferrite rod removed. Coil 4 should cover about 6 -13MHz with the rod in, and about 9 - 20MHz without the' rod. It will be noted that as the ferrite rod is inserted, any particular signal has to be re-tuned by opening Cl. This arises because the ferrite increases the inductance of the winding, so less parallel capacitance is

needed for the same resonant frequency.

EFFICIENCY CHECKS Tune in am.w. transmission using coil 1 which gives good headphone volume. Place a microammeter or multi-range meter on a sensitive range in series with the headphones. A reading of 50-100uA or more may be obtained, depending on aerial, earth, earphone resistance and resistor value, coil and detector efficiency and strength of signals at your locality. Placing the ferrite rod in the coil and re-tuning should boost the meter reading to some extent. Surplus or other detector diodes can be tried by substituting them in turn and noting the meter reading. Improvements to the aerial (or earth) will also show up as a rise in meter reading. If experimenting with a crystal earpiece, which gives no direct current circuit, the meter may be clipped across the phone leads, i.e. D1 cathode to earth.

Picture 4 - Baseboard Layout Of The Crystal Set AERIAL COUPLING

The aerial loads the tuned circuit heavily when connected directly to the top of the tuned circuit, as in Picture 2. This damps the tuning action and it can be found that stations spread out all over the dial, which is unsatisfactory. The series capacitor, C2 connected in Picture 5(a) reduces the loading and thus improves the sharpness of the tuning. A variable or pre-set capacitor of about 250pF maximum is most suitable. for this role, though it is possible to experiment with a variety of fixed value capacitors in this range also. Connecting the aerial to a tapping on the coil, as in Picture 5 (b) also sharpens tuning. It may also increase volume. Try about 2 turns from earth for coil 4, or 4 turns from earth for coil 3. Another method is to have a coupling primary, as in Picture 5 (c). This consists of a second coil, with about one third the turns of the original wound on top of the existing coil. You can even combine these methods to find what arrangement best suits the aerial in use. The diode can be disconnected from the end of L1 and taken to a spare position on TB1 for example location TB1/9. You can then run a flying-lead fitted with a crocodile clip from this position, connecting it to various tappings on the coil as required as in Picture 5 (d). This method also reduces loading on the tuned circuit. Coils with spaced turns of bare wire are readily tapped. For other coils, small loops can be made every ten turns or so, and crocodile clips can be attached to these when selecting tapping points.

Picture 5 - Alternative Methods Of Aerial Coupling SHORT WAVES

For shortwave reception, a good efficient outdoor aerial is certainly recommended. Evening listening in the region around 5 - 9MHz in often proves to be the most fruitful. Since there is no amplification, as with a valve or transistor receiver, certain frequencies will seem to be completely dead at particular times of day. So if the crystal receiver works satisfactorily on medium wave and longwave, but no shortwave signals are heard, check again in the evening, or after dark, when conditions are different.

PARTS REQUIRED

C1

365pF or 500pF Air Spaced Tuning Capacitor

D1*

OA47, IN34, OA81, OA90, OA91, IN94 or similar point small signal Germanium Diode * The OA47 will be of particular interest since it has the lowest forward bias voltage of any of these diodes which will make the crystal set somewhat more sensitive and therefore louder. The US equivalent of the British OA47 is the IN34.

TL1

High Impedance Headphones (20,000 Ohms) or Crystal Earphone

TB1

12-Way Plastic Screw Block Terminal

Also Required:

47 k Ohm Resistor for Crystal Earphone: Enamelled Copper Wire: 32 and 24 s.w.g. for L1: 20 s.w.g. tinned wire for L1: Ferrite Rod 10mm diameter x 75 mm long: 25m of wire for aerial: Wire and rod or spike etc for earth: Wood for base e.g. 10mm x165mm x 130mm: Scrap of metal of thin plywood for C1 bracket/front : Knob: Crocodile clip(s)

Adapted from an article in Everyday Electronics magazine, November 1981, By F.G. Rayer.

HERE ARE A COUPLE OF VERY INTERESTING CRYSTAL SET DESIGNS SENT IN BY KRYSATEC - "THE RAT" - FROM THE CZECH REPUBLIC

.

1/ Using old coils from old bulb radio for MW and LW band. Though it would be straightforward to wind the coils - one for Long Wave, one for Medium Wave and a coupling coil. Variable capacitor is 2 x 500pF only one half is used: 500pF. For the crystal earphone a resistor of about 82k ohm in parallel is required. This set also uses two Ge diodes as a multiplier in the quest for for higher audio signal output.

2. If signals are not strong signal in your location, then the above circuit design can be considered. A simple transistor amplifier is used. A variable resistor M22 is used for better sensitivity which can be adjusted for poor signals. This crystal radio is aversion from cca 1960 - 1970 y.

Rat's finished Crystal Set with additional amplification - very neat!

'Minilabs' Crystal Radio BELOW: Ian Tomlinson kindly sent in a photograph of the box that contained the kit for his John Adams Toys 'Minilabs' Crystal Radio. It is a very simple circuit consisting of the coil (inductor) with a sliding that provides variable tapping points, a diode and crystal earphone. All that is added is the aerial and earth. There is no variable tuning capacitor for simplicity and to keep costs down. The coil provides the inductance required for tuning into a certain frequency (wavelength). These days a variable "tuning" capacitor is normally wired in parallel across the inductance (coil) in order to vary the resonance of the tuned circuit and therefore enable to easily tune into various transmitters on different frequencies. This crystal is tuned varying the number of turns on the coil (ie varying the inductance) by tapping off at different points using the sliding ("ball"). The crystal earpiece, or high Z headphone, is connected between the output of the detector diode (the

other end from the coil) and earth. The volume from a crystal earpiece may be considerably improved by connecting a resistor of - somewhere between - 4.7 k and 47k ohms in parallel with the earpiece. A crystal earpiece cannot directly allow current to flow through it and the parallel resistor therefore allows current to better flow through the circuit.

'Minilabs' crystal set by John Adams Toys

A discussion on configurations for Crystal Sets by Felix Scerri VK4FUQ

This discussion, by Felix Scerri VK4FUQ, was posted at this address which no longer appears on the web www.tarc.org.au/techinfo2.htm (error 404) so here it is reproduced:

Crystal Set design is one of my ions closely allied with my obsession for audio and high fidelity.

My main interest in crystal sets, apart from the wonder of a radio receiver that does not require a power source, is the potential excellence of the recovered audio quality from normal AM broadcast stations. Personally, it is one of my great laments that most people have never heard how good wideband AM can sound. A high performance crystal set or similar TRF approach is, in my opinion,the only way to do it. There are a few people around who have heard the audible results of my efforts,and can only agree. I have often wondered,given the ultimate simplicity of the crystal set, being essentially a tuned circuit,a diode detector and some form of output device, what it takes to achieve optimum performance. What follows are my thoughts on the matter. Crystal Set optimisation, is in my opinion, all about reduction of circuit losses. Essentially this means high "Q" tuned circuits and high quality detectors. Efficient output devices also help too. But as we will see, there are some trade-offs required as well. A high "Q" tuned circuit is always benefical, as a high "Q" tuned circuit has lowest RF losses,highest potential selectivity,and highest voltage at resonance, which is very useful for the diode being fed from the tuned circuit. Variable capacitors, even the "modern" miniature variable capacitors (although the older air dielectric units, as used in old valve receivers are more desirable) for various reasons,are generally quite efficient, and a higher "Q" coil will produce the most worthwhile improvements.The best (highest "Q") coils are wound with "Litz" wire, which is a multistranded woven wire with all strands insulated from each other. The performance of Litz wire wound coils is spectacular, unfortunately, although I know Litz wire is still being made, from personal experience, it is VERY rare in Australia. Efficient coil design can be quite complex and all my coils are wound on ferrite rods. There seems to be,at least for ordinary single wire windings (close wound), an optimum wire thickness for optimum coil "Q". I have determined .315 mm winding wire to be about optimum for simple (single wire) coils on ferrite rods. Thicker wire is NOT better, believe it or not. Lacking Litz wire, an interesting winding approach I have developed is to use two slightly thinner wires wound as a bifilar winding connected together at the beginning and end of the coil, yields considerably higher "Q" compared to a simple single wire winding. I have found 0.25 mm winding wire optimum in this application. Whilst high "Q" coils are beneficial from the RF point of view, there is a possible downside. If one is interested in maximum selectivity and sensitivity, there is no problem, but highest "Q" results in a narrowed audio band-width as a simple consequence of band-width. For high fidelity applications this could be a disadvantage under some circumstances, although there are clever ways around this.

Regardless of ultimate coil "Q", selectivity is a major issue with crystal sets generally. Here another trade-off is evident. For the maximum voltage into the diode, connecting the diode to the high impedance end of the coil (i.e. the top) yields the greatest voltage but the selectivity is usually terrible, because of severe "loading" by the diode circuit. For this reason, tapping well own the coil improves selectivity at the expense of signal volume (reduced voltage). Once again there are ways around this. As described in my "Double Tuned Crystal Set Tuner" article in "Amateur Radio" magazine, March 2002, the use of two separately tuned coupled resonant circuits allows top connection into the diode without compromising overall selectivity, thanks to the use of a second tuned circuit which is fed from the external antenna. The whole network forms a double tuned input band filter and in practice this approach works very well. For single coil crystal sets I recommend the use of an un-tuned "antenna" winding adjacent to the "hot" end of the main coil, preferably adjustable (old paper reels from sewing cotton threads are ideal). This allows the degree of coupling to be optimised under actual listening conditions. The double tuned set up is best, yielding superb selectivity, but the un-tuned antenna coil arrangement also works quite well, especially if the diode is tapped well down the main coil.Tapping halfway works well. The other method of performance improvement involves the use of the most effective detector system possible. Here things get very interesting. In fact the temptation is to use more complex circuitry, but that gets away from the charming simplicity of the crystal set. As an example, my own crystal set tuner has at times mutated into a TRF tuner complete with FET RF preamplifiers, active(powered) detectors and other enhancements. These modifications do work well, but loses the simplicity of a basic crystal set. In actuality, a simple diode detector can work extremely well, subject to some qualification. Diodes like to work with a reasonable level of RF input voltage. Audio distortion can result under conditions of low signal level, due to diode transfer curve non linearity and other factors, such as the widespread use of broadcast station "processing". The actual type of diode makes a difference. The 1N34A germanium diode is very popular for crystal set use, although in my experience just about ANY germanium diode will work, although it is worth trying different specimens. Some are definitely better than others. Even from a pack of twenty 1N34A's from the same source, some were definitely better than others. Measuring the average value of rectified output voltage across the diode load resistor will show which diodes are best. By the way, I regard a diode load resistor as being mandatory. I find a value of about 47K about right, especially if a crystal earpiece is being used or the crystal set is being used as a tuner feeding an audio pre-amplifier and following amplifier. If using high impedance magnetic type headphones, the headphones provide the diode DC load. Another type of diode that is very interesting, is the hot carrier diode. There seem to be a lot of different hot carrier diodes around these days. There are even hot carrier diodes now being sold as "germanium diode equivalents". I have tried them and they do work acceptably well, but they are not quite as good as genuine germanium diodes such as the 1N34A. Typical UHF mixer hot carrier diodes, such as the 1N5711 will not work well in crystal set service simply because their "turn on voltage"is too high, similar to silicon diodes such as the 1N4148/914 series, which require a lot of RF input to function adequately as RF detectors, however a simple technique can be used to turn hot carrier diodes such as

the 1N5711 into superlative detectors. I guess we are cheating a little, because the technique is to use a little voltage bias supplied via a 1.5v battery, through a simple potentiometer voltage divider arrangement, with capacitor (for DC isolation) fed into the diode from the tuned circuit. With applied adjustable bias, I find the 1N5711 diodes absolutely superlative detectors under ANY signal strength conditions. I find the detection quality also superlative, with a clarity and low noise profile unmatched by any other diode arrangement. In my opinion, hot carrier diodes, running bias,are the best detectors overall. Regarding other detector arrangements, the diode "voltage doubler" is often recommended, however my own experiments with the doubler arrangement have been inconclusive and slightly disappointing overall. I have found no real advantage in their use over a simple (one) diode detector, believe it or not. Yes, they do work, but they're nothing special, at least in my opinion. Any comments on this general subject of crystal set optimisation would be welcome.

73's Felix Scerri VK4FUQ. 22nd July 2002

Above: CRYSTAL SET BASED CIRCUIT PROVIDING A HIGH QUALITY PROGRAMME SOURCE IMPROVED VERSION OF THE ABOVE CONCEPT !! New update from Felix Scerri February 2010: New 'two FET infinite impedance AM detector' I've developed a new version of my old favourite FET 'infinite impedance' AM detector that I think sounds very nice. I include a short audio of one of our local AM stations. I picked this station as it is my reference 'torture test AM station' as they run very heavy 'processing' which normally sounds yuck with all my other (diode and non diode) detectors! However it's quite clean with this detector. What do you reckon? I'll do up a circuit if you'd like to feature it in your TRF radio section. A general draft article follows. 'A favourite non diode based AM detector that I've built and used many times over the years is the FET based infinite impedance detector, offering very good general AM detector performance, especially under weak RF signal conditions where diode based detectors do not perform well, especially in of audio distortion. However one of the slightly strange things I've noticed about the simple FET based infinite impedance

detector is the variable audio quality noted, even when using the same type of FET. Some I've built have sounded good and others slightly fuzzy when used with an audio preamp and fed into a high quality audio system. I've been giving this a considerable bit of thought of late and I've wondered if the audio distortion might be a result not necessarily of the detection process itself, but the FET stage in its guise as a 'source follower' audio stage which essentially, it is. I have long been aware that as a simple audio buffer stage, the FET based 'source follower' can exhibit a considerable amount of audio distortion, and a technique I've long used to greatly reduce this audio distortion is to use a second FET in the source lead of the first FET as a 'constant current source' which serves to 'linearise' and greatly reduce audio distortion in the buffer stage overall. So, to test the theory I built a simple one FET infinite impedance AM detector which worked well, but with just a hint of audio 'fuzziness' on received AM stations. So I added a second FET in the source lead of the first FET wired as a constant current source, taking the output from the source of the first RF detector FET and the source resistor and RF by capacitor off the source lead of the second FET 'constant current source'. The result, totally clean audio! The theory seems proved! I call this modified detector the 'Two FET infinite impedance detector' ))) Here's what is sounds like - click to play the audio file ((( Here is the circuit diagram:

This detector has been a real eye opener for me in of its excellent performance, especially considering its circuit simplicity. Indeed in the past I have designed other more complex FET based infinite impedance circuits that do not quite work as well in practical as this latest circuit, at least according to my well calibrated ears! I do not have access to any precise test equipment but my well calibrated ears tell me this 'two FET infinite impedance detector' is a beauty, suring practically every other AM detector I've built even at low RF input, and that's rather a impressive claim and the audio quality when used as an AM tuner feeding a high quality audio system is quite remarkable. Possibly the best thing about this detector is its excellent performance under weak signal conditions. Diode based detectors also work beautifully, but the use of an RF stage to ensure detection over a linear portion of the diode's curve is mandatory! This compound infinite impedance detector works beautifully on the sniff of a useable RF signal.

Just add a high Q tuned circuit and that's it! Felix Scerri VK4FUQ

A better FET for the 'basic' Infinite Impedance Detector: Quite recently by accident, I've realised the MPF102 FET that I've long used in my FET based infinite impedance detectors is possibly not the best FET to use. This was the reason why I developed the 'two FET' infinite impedance detector some time ago which works very well. However I've found the choice of a more suitable FET works beautifully in the basic FET based infinite impedance detector circuit, which has appeared for many years in many editions of the ARRL Handbook. I use the 2N5457 and others of the same 'family' may be equally suitable, but I haven't tried them! However with a 2N5457 in place of an MPF102, the basic infinite impedance detector has became my AM detector of choice. It works beautifully even at low signal input with lovely and clean low distortion audio along with a very high input impedance for good tuning selectivity. It's a beauty! The basic generic circuit is attached, courtesy of Rod Elliott's ESP website. 73 Felix VK4FUQ 10 / 02 / 2012.

The basic generic circuit is attached, courtesy of Rod Elliott's ESP website Felix Scerri VK4FUQ As often happens with me, my renewed interest in FET based 'infinite impedance detectors' of late has led to some interesting new research and I may have considerably improved the 'two FET infinite

impedance detector' as a result. My research suggests that although the use of a CCS (constant current source) reduces audio distortion in an audio stage, the value of the 'source resistor' in the CCS stage is somewhat critical for best results. By using a potentiometer in lieu of a fixed resistor I have found that a resistance value of around 470 kohms cleaned up all overall audio distortion. I used an MPF102 as the CCS in this circuit. An interesting and worthwhile little circuit refinement. 73 Felix VK4FUQ 21 / 02 / 2012.

A Minimum Component Count High Quality AM Detector I was generally messing around with various circuit ideas and I came up with this AM detector circuit, a simple diode detector along with a FET stage. It was an attempt to provide good performance along with minimum number of components. Actually I've been pleasantly surprised at the excellent level of general performance and the best of all, it sounds great! The circuit is quite conventional being a BAT 46 diode detector feeding an MPF102 FET buffer/ common source amplifier stage. I would ordinarily use a FET source buffer stage in this application, but opted to use a simple low gain FET 'common source' amplifier stage instead, with excellent results. I also used a BAT 46 Schottky diode instead of an ordinary germanium signal diode. This was done for several reasons. Firstly, germanium diodes are now very hard to find but in any case these 'germanium diode equivalent' Schottky diodes are actually a superior diode, having very low noise, almost zero back leakage and an essentially complete absence of carrier storage effects and very good weak signal sensitivity. I call these diodes high fidelity diodes as they sound wonderful as RF detectors.

Circuit Diagram of the Minimum Component Count AM Detector by Felix Scerri

Minimum Component Count AM Detector by Felix Scerri

A Closer View

The high impedance of the FET's gate circuit is perfect for optimal buffering of the diode detector, something very important for good low distortion diode detector performance. Apart from providing slight voltage gain, the use of the common source FET amplifier is a new idea, as this prevents the possibility of incidental RF rectification occurring in a FET source follower stage, which can happen. A 1 uf plastic film capacitor may be added in series with the audio 'hot' output lead to block the DC offset out of the FET drain, if required. Despite no additional RF stage ahead of the diode, audio quality on even relatively weak RF strength stations is actually very good, and of course the audio quality will be even better with increasing RF signal strength, something which will also increase the audio output level. Just on this, for a long time I was somewhat negative regarding diode detectors, as one AM station locally (the strongest one) was always distorted when using a diode detector. I strongly suspected a transmitter fault, but my complaints were ignored, until one day some time ago when all audio distortion suddenly disappeared! Nothing was ever 'said', but I realised that my suspicions of a long standing transmitter fault were correct, after all! 73 Felix VK4FUQ 02 March 2012

Voltage Doubler Detector This is an AM detector circuit that I've long known about and which worked ok, but never seemed to

work as well as it should have. However I spent some time late last night trying to optimise the circuit, with some success. It is a curious circuit being essentially a 'voltage doubler' originally developed for power supply applications, and its use as an AM detector is hard to analyse! It seems that the component values in the circuit are somewhat critical for good performance and if not, the performance is rather 'ordinary'. The circuit that I eventually came up with uses a 150 picofarad 'input capacitor' with a 150 kohm 'load' resistor and loaded into a FET common source voltage amplifier stage (as previously described) through a coupling capacitor with a 1 Mohm input resistance.

With these circuit values, it all works 'quite well'. Give it a go! It's an interesting AM detector with quite good 'sensitivity' and clean audio quality, and it seems to work well at low signal levels. 73 Felix vk4fuq. 16th April 2012.

Simple AM detectors: What works best? A practical experimenters viewpoint. I have written a lot about simple AM detectors for use as tuners for feeding into an audio amplifier, and it has been a long time interest. These days I use either diode based or 'infinite impedance' types of AM detectors. In this location our 'local' AM stations are quite distant and are therefore quite weak in of signal strength. As such I find infinite impedance detectors based on field effect transistors give consistently better results for tuner applications due to their lower apparent overall detector distortion. Diode based detectors are quite 'fussy' as they require both optimal output buffering (AC/DC ratio) and an 'adequate' (beyond the diode knee) level of RF signal injection. http://www.tonnesoftware.com/appnotes/demodulator/diodedemod.html Diode based detectors will happily 'detect' at very low signal levels, however the (inevitable) audio distortion that results, can be extremely irritating to the ear! Under these conditions I find infinite impedance detectors (even without additional RF preamplification and subject to individual FET characteristics), generally sound 'cleaner' and more pleasant to the ear. FET's of course require a power source for operation whereas diodes are ive (un-powered) detectors (most of the time), however this is of no real advantage in a tuner application as an 'active' audio amplifier stage will generally be required anyway for audio level boosting, buffering etc. In the end it will come down to a consideration of prevailing RF signal levels and other related circuit considerations at one's location. If local RF levels are strong, a well designed diode detector will give excellent results. If not, an 'infinite impedance' type of detector is most likely the better option unless one goes towards the option of additional RF preamplification prior to the diode detector. 73 Felix vk4fuq. 17th April 2012.

ANOTHER DETECTOR by Felix Scerri ! I’ve tried diode based ‘voltage doubler’ (or more correctly ‘diode integrator’) AM detectors before with indifferent results, however the other day, just trying a few ideas I came up with this version that works rather well, with low audio distortion, high audio output and really ‘nice’ audio quality and the best of all, it seems to work very at very low RF input level.

The two diode ‘voltage doubler’ detector using two BAT 46 silicon schottky diodes feed directly into a MPF102 source follower stage set at 1 Megohm input resistance. The ‘input’ capacitor feeding the diodes from a tuned circuit is 68 picofarads. I have the simple RF filter right on the output of the FET stage. In that respect this circuit is vaguely similar to the old ‘Selstead-Smith’ valve AM detector of the past. An interesting one! I am very happy with its general performance. Regards, Felix vk4fuq 11/04/2013

Felix Scerri VK4FUQ

UPDATE - JUNE 2013 G’day all, readers may recall the two FET infinite impedance detector I developed some time ago. That circuit worked well, but some samples of the MPF102 regretfully produced distorted output. However a recent discovery has resulted in an improved version that has truly exemplary performance. The MPF102 is in all honesty a device essentially designed for VHF applications, but is often used in simple audio applications albeit with occasionally indifferent results, due to general device parameter ‘spread’. More or less by empirical trial and error I have found that a simple change in source resistor value to 100 k (from a much lower value), pretty well fixes everything. In a FET source follower circuit this is interesting as the output impedance is actually a lot lower than 100 k due to the action of the FET’s transconductance. It is similar to the action of a bipolar transistor emitter follower circuit. In fact a simple infinite impedance detector with a 100 k source resistor actually works very well and will be good enough for many applications, however the addition of a second FET as a constant current source does markedly reduce overall audio distortion in the stage overall, and produces very clean and low distortion audio quality and also reduces the output impedance considerably (good for tuner applications), so take your pick! Regards, Felix vk4fuq 02/06/2013.

UPDATE - JANUARY 2014 A simple modification to the basic FET 'infinite impedance' AM detector that dramatically improves performance.

G’day all, over the Christmas break just messing around I worked out (mostly by accident), a simple modification for the simple FET based ‘infinite impedance’ detector circuit that dramatically improves weak signal performance and also greatly reduces audio distortion. Essentially by the addition of another FET ‘buffer’ stage, another source follower,capacitively coupled from the first detector stage. The circuit is actually a simplified version of the circuit that I described in this link, http://sound.westhost.com/articles/am-radio.htm (figure 6) and testing the two head to head, they both sound superb and the simpler version is actually somewhat easier to build. I cannot get over how low distortion and ‘nice’ the recovered audio sounds. It is a joy to listen to!

Regards, Felix vk4fuq 12/01/2014.

Diodes for ‘weak signal’ crystal set applications. As I am primarily interested in using crystal sets as AM tuners for feeding into a preamplifier/amplifier and loudspeakers, the actual type of diode can be relatively critical. In a strong signal area, not so much, but in a weak signal area such as where I reside, definitely. A diode with a good ‘square law’ performance (the area below the ‘knee’ in the diode curve), generally results in much cleaner and lower distortion than other good performing diodes, and believe me diode distortion under weak conditions, even with optimised diode ‘buffering’ used is very nasty sounding to the ear! Testing many, many diodes in actual working crystal sets and listening critically to the audio output, it seems to me that the best diodes to use under weak signal conditions are the so called ‘gold bonded’ germanium diodes. I have sampled many different gold bonded germanium diodes and they all work well in this specific application, although sometimes the rectified output voltage may not be as high as other germanium or silicon schottky diodes, however the audio quality is much cleaner and shows much less apparent distortion! I have tried OA5, OA47, IN141 and several ‘CG’ gold bonded germanium diodes with consistently excellent results. Other ‘ordinary’ germanium diodes may also work well under weak signal conditions, but they will need to be tested individually to check actual performance in a working circuit. One thing that I have noticed about germanium diodes is that due the ‘point ’ nature of their construction, even diodes of the same specific type can exhibit rather different levels of performance! The otherwise very good BAT46 silicon schottky diode works extremely well at good RF input level but not so well at weak RF input, especially when heavy broadcast ‘processing’ (commonplace these days), is used. New ‘gold bonded’ germanium diodes are probably no longer made although I am aware that they can be purchased through vendors over the internet. Apart from that, they may be found in old gear. As stated earlier, ordinary germanium diodes may be quite good but will need to be checked individually. Diodes are complex things! Felix (vk4fuq). 29/01/2014.

************** Just when I thought that I’ve tried everything I realised that I’ve not tried a voltage doubler (diode integrator) detector with an RF stage in front. It is an unfortunate fact that all diode detectors ‘need’ a good RF injection level (and buffering) or audio distortion becomes ‘bad’ (an understatement!)

Consider yourself very lucky if you live in a strong signal area! (unfortunately I don’t). Anyway I quickly built a couple of prototypes which worked ok but not as well as hoped. Initially I tried using a source follower buffer (no voltage gain), but converting it to a common source amplifier (with appreciable voltage gain), seemed no better, which was strange. After staring at my prototype for what seemed like forever with my magnifiers on, I (finally) realised my mistake. A common source stage takes its output off the ‘drain’ terminal of the FET, not the ‘source’ terminal as it does with the FET source follower buffer stage. I shifted one wire, and all worked as expected and it sounds fantastic! All diode detectors (one diode envelope detectors, doubler detectors etc), need good RF input and a simple untuned FET RF gain stage works very well. As to the sound, it sounds great. 73 vk4fuq 10/03/2014.

Felix Scerri's FET Crystal Set with Voltage Doubler ************** That's it for crystal sets. I hope you try building one, it's easy and great fun! See some useful links below.... 73's

My Crystal Radio Page As a youngster I made several crystal radios1 and one-tube radios, and various amplifiers and many transmitters (mostly illegal!). I even made a special five-tube radio designed to mount in the steering column of a 60 Series scooter. It worked swell, but the filaments all fell out of the tubes after about 5 miles on that rough scooter. Since then, both in business and as a radio amateur, I have designed and built a multitude of receivers, transmitters , and other electronic equipment, but I never forgot my early fascination with the crystal set. More information about crystal sets appears at the bottom of this page. This page is arranged with my latest Crystal Radios at the top, and working down through time to my first radios. I enjoy making these little radios and I usually spend several times as long in the design and layout stages as I do actually constructing the sets. All my sets are fully designed first on paper, then in a computerized drawing (CAD) program. All s and chassis holes are drilled using computer coordinates on a mill. At assembly time it is a lot like building a Heathkit™ product.

Coming Soon: My suggestions on how to make a very high performance crystal radio for minimum cost. If you have an area you would like me to cover send me an email. The email address below is a .jpg image to avoid spam harvesters, so you will have to type it into your browser. Please use the subject line "crystal radio".

My Beacon Receiver This radio is special. Built in April, 2013, it is designed to receive the frequencies below the broadcast band. Between 198 kHz and 530 kHz are a multitude of low power aircraft beacon stations. Most run about 15 watts of

power with very small antennas. But, despite their low power they really get out because of the low frequencies they use. Also, there is instant gratification because they all send their call letters continuously and you immediately know who you are receiving. A list of the beacon stations is available at http://www.dxinfocentre.com/ndb.htm and at other sites. Europe has a broadcast band around 170 kHz, but it probably would not be receivable on a crystal set. Perhaps it could be received by someone on the East Coast of the US. This radio has two bands, the first from 540 kHz down to 190 kHz and the second from about 450kHz down to 160 kHz. (It takes a lot of added capacity to go just a little farther!) I made it come up to 540 kHz because I have a local station there and I can use it for verification that all is OK. It has both variable antenna coupling and theHobbydyne™ coupling circuit.

I have received about 25 beacon stations to date on this set and the prototype set I was using for a couple years prior to building this one. My best DX is North Carolina in the middle of the afternoon. There was an article about my crystal set beacon reception in an issue of Monitoring Times last year. Monitoring Times is my favorite magazine. If you are not familiar with this magazine you should check it out.

I also listen to beacon stations with a Kenwood R-5000communications receiver. With it i have logged 82 different beacons, including several in Canada. The reception "down under" is extremely variable. One night only locals, the next night the whole southeastern US comes in. The time of the year also affects this band

The 80% Hobbydyne Crystal Set This set is an exact 80 percent size copy of the Hobbydyne™, with added refinements. I had two smaller Miller military style vernier dials I wanted to use, but they were too small to look good on the standard size Hobbydyne™ front , so I scaled the down to match the knobs. This one has variable coupling for the antenna, and a fully frequency compensated Hobbydyne™ circuit. These particular National dials have to be rear-mounted – they cannot be installed on the front side of the like the regular National dials. This presented a problem in construction since if the dials were mounted to the rear of the there was no way to mount the tuning capacitors. This was solved with a sub- about an inch behind the main and a sub-miniature shaft

coupling. The tuning capacitors are mounted to the sub-. The also serves as the mounting base for the choke, the compensating capacitor, plus the resistor and capacitor for the "Benny" Made in April, 2013.

The knobs on this set were homemade from black Delrin rod. They are knurled around the perimeter for a nice hand feel. The original knobs were the standard rounded ones used on the larger dials. My knobs have a nice military appearance which I like better.

The Second Generation Hobbydyne Kit We are selling the little Hobbydyne™ kit again. This is the prototype of the second generation kit. The dimension of the were chosen so that 3 inch National Velvet Vernier Drives can be used with it if desired. The kit is sold complete, with a drilled front and all components and hardware except for the knobs and the wood base. Suitable knobs can be found on eBay, or at Radio Shack. This is a very selective crystal radios with both variable antenna coil coupling and the Hobbydyne coupling circuit. The switch is to add capacity to resonate smaller antennas. This set was made in February, 2013. To inquire about purchasing this set send an email to me at the address listed above.

My Spider Coil Radio This set was built in November, 2012. It uses a very high Q Spider Coil. The coil is wound with 660/46 Litz and the Q is in excess of 1000, too high to measure on my HP 4342A Q Meter. The radio has only one tuned circuit, the spider coil using a "Holy Grail" silver and ceramic variable capacitor. The antenna is coupled to the spider coil with the smaller non-resonant spider coil. This coil does not resonate anywhere in the broadcast band, if it it did it would try to inject stations at its resonant point.

The selectivity of this set is very good, using the Hobbydyne™ coupling circuit to the detector. It is a pleasure to tune because there is only one adjustment.

My Tivoli Crystal Radio Model One This set was finished in late October, 2011. I have had a Tivoli AM/FM table radio in a very attractive walnut cabinet for several years. I kept looking at it, secretly wishing that the circuitry would fail so I could build an crystal radio

in its beautiful cabinet. Well, it happened - the polycon variable capacitor broke. I suppose I could have returned it for repair, but I decided to do what I had wanted to do for years - steal its cabinet, speaker and speaker grille. The circuitry of this set is identical to the antique set directly below, except that the fixed antenna coil setup worked so well in the Antique Set below that I did not install an antenna tuning capacitor in this one. I also used the Tivoli super high quality speaker and left its bass port in place. I called it the "Model One" in honor of Tivoli who also used that model number. I used a 6 dB bass boost circuit on the LM386 amplifier and it really sounds good. The bass port is mostly hidden in the picture by the shorter antenna coil. The printed circuit board is mounted atop a shelf and under the shelf is the 9 volt battery. It is accessible through a door on the rear . The chassis slides into the walnut cabinet and is secured when the rear is attached with the four mounting screws. The knobs are homemade from Delrin rod and are knurled for a nice feel.

This set also has adjacent channel selectivity. It is also extremely sensitive. Last night I received WLS in Chicago, WBT in Charlotte, NC. WWL in New Orleans, LA, WSM in Nashville, TN, WPTF in Raleigh, NC and WOR in New York City, plus a lot more not identified.

Incidentally, the Tivoli is an excellent little AM/FM radio and it has my highest recommendation. It is obviously monoral with the single speaker, but when used with earphones it provides full stereo. I have two others in addition to this one.

My Antique Set This set was made to look a lot like the old 1915 crystal radios that my Granddad made and sold to jewelers all over the northeast. Actually, it cheats a little because it has a FET transistor isolation stage and a LM386 amplifier so that I can power a speaker as well as earphones. The case is made of solid 1/2 and 3/4 inch cherry wood. The circuitry is all on a printed circuit board.

The radio, finished in early September, 2011 offers the a choice of a conventional two-coil crystal set, or it can be switched to series resonate the antenna coil with the antenna capacity to just below the broadcast band. Below the band is necessary because if it is resonant in the band any station on its resonant frequency tends to bleed over a portion of the band. Tuning the coil adds a little more sensitivity, but it is not really needed because it can receive down to the noise level with the fixed input. The single knob tuning with the fixed antenna coil simplifies using the radio, and as a bonus the detector dial can be calibrated accurately since it is no longer affected by the changing antenna coil. A small link coil wound on the cold end of the detector coil could also be used to connect the antenna in place of the fix tuned ferrite rod coil. A link does not give as good a sensitivity as the ferrite rod coil spaced about 1.5 inches away center to center, and the added capacity introduced between the link and the detector coil causes strong local stations to occupy too much of the dial.

The antenna coil is 350 uH wound with 100 strand #46 Litz on a 4.5 inch long .710 inch diameter Delrin form with a 4 inch by .5 inch ferrite rod inside. The form was threaded to give aa spacing of .010 between the turns. Its Q is 650. It is mounted on a sliding track so that the spacing between it and the detector coil can be changed easily. The detector coil is 183 uH, wound with 170 strand #46 Litz and the turns are spaced .015 inch apart. Its Q is 780. The tuning capacitor for the detector coil is a 500 pF military surplus ceramic insulated unit. The antenna tuning capacitor is a conventional two section 365 pF variable. The cathode of a theHobbydyne™ circuit diode is directly connected to the gate of the 2N5486 FET transistor. The output from the FET is taken from its Source lead. This accomplishes two things at once. First, the input coil is isolated for minimum loading because of the two megohm input impedance of the FET stage. Secondly, the Source output of the FET is a low impedance and it is capable of driving any earphones, 2000 ohm or sound powered low impedance phones without the use of a matching transformer. Except near a strong local daytime only station, it offers adjacent channel selectivity anywhere on the band. This radio was far more challenging to me for its cabinetry than for its electronics.

The Homodyne

The circuit this set uses, known as the "Homodyne", was invented in the early 1920's about the same time as the invention of the Superhetrodyne. It never became very popular because it was inferior to the Superhet, but nevertheless it is a fine performing circuit. A two part article on this modern two-transistor version was published by G.W. Short in The Radio Constructor, a UK magazine in 1972. The Homodyne is a regenerative receiver. It is unusual in that the is not obtained by the usual coil, but is obtained by taking the signal at the output of the second transistor (which is back in phase) and feeding it back in a controlled manner through a limiter and a potentiometer to the input of the first transistor.

Homodyne Front View I made several changes to improve the performance of the radio. First, I use a 20 turn winding adjacent to the tuning coil to feed the gate of the first transistor. This reduces the loading on the tuned circuit, improving the selectivity. Second, my antenna coil is on a separate ferrite rod spaced an inch away from the main coil. This provides the same sensitivity as a winding on the main ferrite rod, but eliminated the capacitive coupling which would exist with the coils close together. The true inductive coupling, with no capacitive coupling, provides additional discrimination against the strong local stations. I changed several values in the circuit because of the lower inductance of the 20 turn link. All of G.W.'s circuitry is on the left printed circuit board in the picture. The output of G.W.'s circuit is designed to power a pair of headphones. I added the board to the right, a 500 mW amplifier using a 386N, to drive the 3-inch speaker. I could not find either of

the transistors used in the article. I substituted a 2N5486 for the FET transistor and a 2N3638A for the PNP transistor. I built this set in February, 2011.

Homodyne Rear View The set is a more sensitive than the conventional regenerative sets shown below. With no antenna this set will receive quite a few stations, a couple over 50 miles away. When on an outside antenna there is little noticeable difference between the two versions because they both offer more than ample sensitivity. The Homodyne is much nicer to use however, since once the regeneration control is set it does not have to be changed. To read G.W's original article click here.

A One FET Regenerative Receiver with an Audio Amplifier and Speaker Here is a variation of my previous FET regenerative radios, pictured farther down this page. The detector portion is very similar, but this one has an IC audio amplifier and a front speaker all on the same PC board. It also has an earphone jack. Plugging in the earphones disconnects the speaker. Regeneration is very smooth like the previous FET set, and sensitivity is exceptional. A small amplifier using the LM386 chip drives the speaker to about 400 milliwatts, plenty for good room volume. An electronic bass-boost circuit is

employed and makes the 3-inch speaker sound pretty good. It runs on one 9 volt battery. This one was made in January, 2011. Note that the winding and the antenna winding is on a separate ferrite rod about an inch away from the main coil.

Regenerative Set with 3 Inch Speaker Front

Regenerative Set with 3-Inch Speaker Rear

Another One FET Regenerative Set with an Audio Amplifier and Speaker

This set is similar to the one above. It has a 2-inch speaker and its speaker amplifier is on a separate board. It was made in January, 2011.

Deluxe Crystal Set with Variable Selectivity This set, made in September, 2010, has its electronic components hidden inside a small walnut box. It has variable coupling between the coils and the Hobbydyne™ variable capacitive coupling capacitor located on the front . The and base is solid walnut. It has a military surplus silver plated tuning

capacitor on the detector side and a standard two gang 365pF variable on the antenna side.

Another small Crystal set with fixed selectivity This is another small double tuned crystal set made in 2010. It has my usual Hobbydyne™ capacitive coupling circuit and fixed primary and secondary coils. The Hobbydyne™ capacitor is on the PC board on this set, and is adjusted for optimum selectivity. This is another in my "simplicity" series. It works great, and has good selectivity due to the ™Hobbydyne coupling and the very high Q ferrite antenna and detector coils.

A One JFET Regenerative Radio Pictured here is a regenerative receiver using a MPF-102 JFET transistor. For a clean look only the tuning and the regeneration knobs were mounted on the front . The off-on switch, the phone jack, and an on indicator LED are located on the back.

The set uses two 1/2 inch by 4 inch #61 Ferrite rods separated one inch center to center for the coils. The main tuning wound on the top ferrite rod with 100/46 Litz. The unloaded Q of this coil is 760. The regeneration winding a antenna winding are at opposite ends of the lower rod and are wound with #24 copper. The windings on the lower rod wound on sleeves that allow moving them independently across the rod to vary the coupling. During testing the feedb winding was moved to the spot that provided the best overall . The antenna coil is not critical and it works we anywhere on the rod. The antenna coil was originally wound beside the main tuning coil on the top rod, but the set h more selectivity and gain with the winding relocated to the bottom rod. This set is unique because is entirely printed c construction It has no connecting wires - no wires at all except the leads from the coils to the PC board.

A variable resistor from the Source of the FET to ground provides a gain adjustment. Mostly it just increases or decreases the earphone volume; it has little to no effect on the sensitivity or selectivity and affects the only slightly. Best performance does require some resistance here, the set does not work well with the Source directly grounded. The LED is the biggest power a just over one milliamp. The FET current draws in the microampere range . The set is powered from an on-board 9-Volt battery which should run this set for a very long time. In the rear picture you can see the battery, the antenna posts at the left and the earphone jack at the right. It uses an antique UTC HVC-5 adjustable iron core choke to isolate the DC from the audio. I was amazed at the performance of this little set. It is very sensitive, very selective and the is very smooth. It works the pants off of my tube type set using a 1Q5GT that is pictured farther down this page. It gets all the local stations and a couple in Tampa, about 50 miles away, using only the ferrite rod. When using only the ferrite rod (no loading from an antenna) the coil Q is extremely high and the selectivity is so sharp that it is hard to tune in a station even with the National Velvet Vernier dial .

It is also extremely directional in this mode. With an outside antenna it pulled in over 60 stations last night. I have several other JFET's to try in this set. If any work better than the MPF-102 I will provide more information.

Posted March 18, 2009.

A Two Stage Tracking Crystal Set The crystal radio pictured below, built in November, 2008, is a very sensitive and selective two stage tracking set. It uses the conventional high performance antenna circuit (antenna, coil, and capacitor in series) for maximum performance. The Miller crystal set of the 1950's had two coils that track, but the Miller coils are parallel tuned. While the Miller is selective, it is not particularly sensitive due to the way the antenna has to be coupled to the first coil. I believe that this is the first tracking crystal radio to use

the standard high performance antenna configuration. The antenna circuit tracks with the detector circuit nicely all the way across the broadcast band.

It is a dream to use because only one knob is required for tuning, The usual "pulling" of the detector tuning when the antenna circuit is resonated is completely eliminated. This set is antenna specific - it will track properly only with the antenna that it was designed for. A different antenna requires a change in circuit values, and it may not be possible to accomplish tracking with some antennas, particularly short ones. For this reason this set includes an inductive antenna coil adjustment that allows it to be used with full performance with any antenna.

The ferrite core in the antenna coil is adjustable by means of a dial cord arrangement and a knob on the front . For my antenna with tracking, I simply set and forget this adjustment. This radio uses the Hobbydyne™ capacitive coupling circuit. A differential capacitor and a small piston trimmer eliminate any frequency change when the detector coupling is adjusted. All the electronics are built on the rear of the small circuit board.

My Smallest DX Hobbydyne™ Crystal Radio I made this radio in August, 2008. The design objective was to make a crystal set a little smaller than my previous ones, but with no compromises in performance. This set uses two antique 3.25 inch National Velvet Vernier drives. The antenna capacitor is a dual 365 pF obtained from the Xtal Set Society. The loaded Q in the antenna circuit is fairly low, and a quality ceramic insulated capacitor is overkill here. For the tuning stage I used a vintage silver/ceramic TRW capacitor that measures 500 pF.

Like all of my Crystal Radios, this set uses the Hobbydyne™ circuit to couple the detector coil to the diode. This time I located the Hobbydyne capacitor on the chassis rather than on the front . In practice I usually set the capacitor to the optimum value and leave it there anyway. The set has a ceramic trimmer across the main tuning capacitor so that the upper end of the broadcast band can be calibrated to exactly zero on the dial. The small toggle switch is used to select one or both sections of the antenna tuning capacitor. The two metal binding posts near the headphone jack are for the Schottky diode. These two terminals and the Hobbydyne™ capacitor are all insulated from the chassis using Nylon insulators. The red and black jacks are for the antenna and ground.

The chassis and front are both made of solid walnut. They were recycled from a Walnut award certificate I received for selling cellular telephones a few years ago. The front was cut down to approximately 3/8 inch thick and the chassis is 3/4 inch thick. To keep things neat all wires run in groves routed on the under side of the board. Both coils are wound on 1/2 inch by 3-inch ferrite cores. The antenna coil is wound with 100 strand #46 Litz and the detector coil is would with 170 strand #46 Litz. The approximate Q's are 550 for the antenna coil and 650 for the detector coil. The detector coil is affixed to the chassis, the antenna coil can be adjusted to provide optimum coupling. I found that inserting the ferrite rod into the wood base for caused a severe reduction in Q, so Delrin adaptors were made to elevate both coils slightly above the wood. The adaptors press-fit in the wood base; the coils press fit in the Delrin adaptors and are held in place with a drop of model airplane CA cement. All controls except the two tuning dials are on the chassis in order to keep the front as small as possible. The headphone jack was deeper than the 3/4 inch base so a Delrin extension was made to space it up slightly. The jack mounts in the Delrin adaptor, the adaptor press fits into a hole on the chassis.

The front and the chassis were first laid out in a CAD program on my computer. The coordinates provided by the CAD program were then used with a mill and digital readout to drill all holes within .001 inch. This accuracy allows the vernier drives to be hard-coupled to the tuning capacitors with no binding. No flexible couplings are necessary.

The HRO Dial Hobbydyne I finished this latest Hobbydyne™ in April, 2007. It features a HRO dial and gearbox that I picked up on eBay. It also uses an antique Pilot Centragrad 500 pF straight line frequency variable capacitor in the detector circuit. The detector coil is 192 uH wound with 660 strand #46 Litz. The combination provides a band spread of five divisions between each broadcast station, evenly spaced all the way across the broadcast band.

Two Styrene sewer couplings were staked together with a short piece of pipe and cement to provide enough length for the detector coil on one end of the form and the smaller 2.3 inch antenna coil inside the other end. The smaller coil rotates with a knob on the front to adjust the coupling. A small air variable is used to trim the high end of the band to exactly 1.7 mHz with the tuning capacitor plates fully open. It tunes to down to 530 kHz with the capacitor plates fully meshed. This set uses my Hobbydyne™ circuit with a tiny differential capacitor to enhance the selectivity.

In this particular set there is also a small piston trimmer between second set of stator plates and ground to null out any frequency change when the differential capacitor is rotated.

The front spacing is not as nice as some of my previous sets, but it was necessary for this particular dial/coil combination. This set equals my previous large coil sets in both sensitivity and selectivity. There is no measurable degradation from the use of the smaller size antenna coil. The proximity of the smaller coil to the detector coil causes only a very slight reduction of the larger coil's Q. The Q with the small coil in place tops out my HP Q meter at 1000. The antenna tuning capacitor is one of the tiny two gang Russian military surplus units with both sections in parallel. It requires a turn and a half to go from minimum to maximum capacity and I find this just right for quick, yet accurate trimming of the antenna coil. This set is an absolute dream to tune with all the band spread that it offers.

Here is a Close-up of the Circuit Board showing the trimmer capacitor (right), the hobbydyne™ capacitor (center top), and the piston trimmer (to the left of the trimmer capacitor). The insulated plate provides an interface between the HRO dial on the front and the Pilot Centragrad on the rear. The other components are two RF chokes in series, the "Benny", and an RF by capacitor. The Pilot capacitor is mounted upside down to keep it farther away from the detector coil.

The Hobbydyne™ Pro This Crystal Radio, made in June, 2006, I call the Hobbydyne™ Pro. It has two high Q ferrite coils wound with 100 strand #46 Litz wire that track using a dual 365pF tuning capacitor, plus a third independently tuned ferrite Litz wound coil that is variably coupled to the other two. It has better selectivity than any other set I have built. In place of my usual Hobbydyne variable capacitor for adding additional selectivity a two position knife switch, upper right on the , is used to select normal or high selectivity. A very small air trimmer is used as compensation to insure that a frequency change does not occur when the selectivity is changed. This allows very precise dial calibration. The large National Vernier dial is used to tune these circuits. Future plans call for a frequency calibration chart in the lid.

The two tracking tuned circuits use a modified version of the Negative Mutual Inductive Coupling that J. W. Miller used in their 1950's Hi-Fi Crystal Radio Tuner. In this set a ferrite cored mutual inductor is used to allow the coupling between the stages to be adjusted to the desired value. Individual trimmer capacitors are used on each tuned circuit, plus a padder capacitor on one of them to obtain near perfect tracking across the broadcast band. More information on the Miller tuner can be found at the bottom of this page.

The third independently tuned circuit used for the antenna input provides impedance matching and additional selectivity. The amount of coupling between this tuned circuit and the following circuits is fully adjustable by rotating the coil with the upper left knob on the front . This circuit is tuned by the dial at the lower left. Initially this was a vernier dial. After operating the radio for a while I decided that a vernier dial was not necessary for this stage because faster tuning was desirable, so it was modified to provide a direct drive. For casual reception the knife switch at the upper left can be thrown to by the antenna tuned circuit and connect the antenna directly to a link on the first mutually coupled coil. Both the sensitivity and selectivity are reduced some, but tuning is simplified since only the large National vernier dial is used for tuning.

The Hobbydyne™ Crystal Radio Kit This HobbydyneTMCrystal Radio kit uses the same circuit as our crystal radio that was featured in CQ Magazine, November 2005 pages 79-80. Like our other sets, it uses two tuned circuits with very high Q Ferrite coils. We are now selling this kit again. The kit comes with a drilled front and audio board and a preassembled rotary rod, but does not include the National Vernier dials. No knobs are included in the kit, but you can find some at Radio Shack or use the nice National dials shown in the picture. For more information email Jim at the address at the top of this page.

Dual Band Crystal Radio Here is a set that I built in 2005. Somehow it escaped the fate of appearing on this web page - until now. It features two large 4-inch antique National Velvet Vernier dials and ceramic insulated variable capacitors from Jackson Brothers in England.

It covers the broadcast band in two segments, 530 kHz to 970 kHz and 970 kHz to 1700 kHz to spread out the tuning on the dial. Each segment has a independent air variable capacitor that is used to set

the high end of the range. The coils are wound with 170 strand #46 Litz wire on 1/2 inch x 3 inch ferrite rods. The Q of the coils is over 600. The larger coil on the left is the lower range, the small coil on the right is the higher range. The antenna coil is in the middle and couples to both coils. The antenna coupling is adjusted using the lower left knob on the front . A dial cord is used to couple the shaft to the rotary rod. The lower center knob is the band switch and the lower right knob is the Hobbydyne™ variable capacitor . The lower three knobs are homemade, the coil s and the rotary rod are made from Delrin.

Antique Dual Bakelite Dial Crystal Radio This crystal radio was made in July, 2004.. It has a pair of 3.25 inch NOS National Velvet Dernier dials made ofbakelite that are over 70 years old. This radio was designed to have a "military" appearance and will be installed on my ham radio table. It has a straight line frequency silver plated ceramic tuning capacitor and the same Hobbydyne™ circuit as my other sets. The detector coil turns are space wound with a strand of silk thread between each turn and it has a Q of over 800 at 1 kHz as measured on a Hewlett Packard Q meter.

The rotary rod that turns the antenna coil is controlled from the center knob with a dial cord drive. The rear of the drive spool can be seen between the two coils in the second picture. It is very sensitive and selective. This set is featured on page 79-80 of CQ Magazine, November, 2005.

The Hobbydyne™ JR. The Son of Hobbydyne™ pictured below inspired this easy to build, easy to duplicate set with the same performance. I guess I can call it "HobbydyneJr". It uses the same circuit and the same parts as its Dad but it is built on a simple open chassis. Actually, by moving the audio board over it could be put in an antique looking case like the set below. This crystal radio uses a European Jackson Brothers detector tuning capacitor and a Xtal Set Society dual gang antenna tuning capacitor. This radio uses the Jackson capacitor for the detector because it has ceramic insulation. The less expensive and easier to obtain single gang capacitor sold by the Xtal Set Society will work fine. The Like its Dad, this set is super sensitive and super selective. I consider it a state of the art crystal radio. The ferrite coils used in this set and the one below give excellent performance. Substituting the very expensive 6.5 inch 660/46 Litz wound coils for the ferrite rods gives only a tiny improvement in selectivity. So little difference that it takes a calibrated

signal generator to tell if there really is a difference. This set also uses a Schottky diode and the small Hobbydyne variable capacitor for detector coupling. It is designed to work best with an earphone load of at least 100K ohms, although it will work pretty well with plain old 2000 ohm dc headphones. The operational adjustments are the same as the Hobbydyne™ and the Son of Hobbydyne pictured below.

The Son of Hobbydyne™ Pictured here is my Son of Hobbydyne™. This set was constructed using only parts that are readily available commercially, except the two National dials. It uses quality miniature tuning capacitors with ceramic insulation and 1/2 inch Ferrite rod coils wound with 100/46 Litz wire. The detector coil is wound on a 3-inch rod, the antenna coil on a 2-inch rod. The Q of the 3 inch rod coil is only about 10 percent less than the 6.5 inch Litz coils in the original Hobbydynes. The 2-inch rod is rotated with a knob on the front to vary the coupling between the two coils, and the shorter form was used to hold down the cabinet size. All other components are far enough away from the coils that they have little or no affect on their Q. The schematic of this set is essentially the same as the original Hobbydyne. It uses a Schottky diode as its detector. The detector diode, the by capacitor, the DC equalization resistor and capacitor, and the RF chokes are mounted on the small board at the lower left in the rear view picture. I use a pair of WW2 sound power phones through a 150K ohm matching transformer, but it also works well with a pair of quality 2000 ohm (DC) headphones. The extra hole is for a switch to add capacity to the antenna circuit which will be add later. The set is very sensitive and very selective. As the coil coupling is reduced the tuning of both coils becomes very sharp, and the signal will increase when the correct coupling is found. Then the differential detector coupling capacitor can be used for an additional increase in selectivity with very little loss of sensitivity. Using the coil coupling and detector coupling controls, while carefully tuning both the antenna and detector coils can really bring a weak station up out of the noise.

The Son of Hobbydyne™

The Hobbydyne™ II Here are two pictures of my Hobbydyne II crystal radio This set is very similar to the Hobbydyne™, but it uses a second Litz wound coil in the antenna circuit in place of the ferrite rod. This set is a little more sensitive and a lot more selective than its predecessor due to the higher Q antenna circuit, and

the ability to vary the coupling between the coils. Both capacitors are silver plated and ceramic insulated , both coils are space-wound on threaded forms using 660 strand #46 Litz wire. The detector coil is 6.5 inches in diameter and has a Q well in excess of 1000. The antenna coil is 4.5 inches in diameter with a Q of about 900. When the two coils are rotated to loosen the coupling the selectivity is really sharp. The presently unused control location is for a switch to add capacity to the antenna coil, but this is only required with a very short antenna. The differential capacitor circuit has been changed slightly, and has a second balancing capacitor. With the balancing capacitor there is no change at all in frequency as the detector coupling is adjusted. It uses four Aligent 5082-2835 Schottky diodes in parallel for the detector. A 200K to 1K transformer feeds a pair of WW2 sound powered earphones. Slightly more sensitive and selective than the Hobbydyne™, it receives about 30 stations clearly in the daytime and many more at night.

The Hobbydyne II One Tube Regenerative Radio This one-tube radio uses a 1Q5 beam power pentode with a screen potentiometer for regeneration control, just like the one I built in 1950. It follows the classic one-tube radio design from years ago, but has a modern feature or two. If you look in the middle of the coil you will see a 1/2 inch ferrite rod. The coil of Litz wire on this rod measures 250uH and it is used as an auxiliary antenna input coil. The set also has a six turn conventional antenna winding with a tap at one turn. A ground binding post plus three antenna binding posts on the front allow the selection of any of the three inputs. The ferrite rod input is superior to the conventional antenna input coils on most antennas, especially at the lower end of the broadcast band. The main tuning coil is wound on a 3 inch ABS sewer coupling from Genoa. It is wound with 100/46 Litz wire at 37 turns per inch. The form was threaded to allow a little spacing between the turns. The other windings are wound with #22 solid which is also 37 turns per inch when close wound. The winding was optimum with only five turns. The recess to the right of the tuning capacitor is for a custom made 36 volt battery. The plate circuit is choke coupled to avoid DC voltage across the

earphones. It works very well and the regeneration is very smooth.

The Regenerative This is the underside picture of the radio. Notice one more modern feature, the printed circuit board that is used as a common ground plane. It does not extend up under the coil to avoid lowering the coil's Q. Feed thru capacitors are staked into the board and used both as capacitors and as connection points. There are also several Teflon insulated posts staked in the board for connection points.

The Heathkit Crystal Radio

My copy of the Heathkit Crystal Radio of the 50's This crystal radio is a homemade copy of the Heathkit CR-1 Crystal Radio from the 1950's. It is built in a 6 x 3 x 2 inch deep plastic box, the same size as the original. It uses a double tuned circuit with two ferrite core coils and 100 strand Litz wire. The spacing between the coils is adjusted for best performance during assembly. The original CR-1 Heathkits sell for around $200.00 on eBay. That is ironic, because most Heathkits of that area, even the scopes, generators and the like, go for much less. This reproduction Heathkit is featured on page 80 of CQ Magazine, November 2005.

The Hobbydyne™

The Hobbydyne™ Pictured here is my crystal set, that I call the Hobbydyne™. It uses 700 strands of #46 Litz wire on a 4.375 inch diameter detector coil. Seven individual strands of 100/46 were paralleled and lightly twisted in the correct direction. The coil form was made from two Styrene pipe couplings. They were ed and lathe turned to 1/2 their former wall thickness. They are grooved lightly at 19 TPI to give the Litz wire a little space between adjacent turns. The unloaded Q of this coil is over 1000 at 1000 KHz, higher at the low end of the band, and a little lower at the high end. It has absolute single-channel selectivity across the band except directly adjacent to my two local "barn burners" where it has acceptable alternate channel selectivity. Here in Central Florida this set receives 27 stations clearly in the daytime, and almost every assigned 10kHz channel at night. The input circuit has individual tuning capacitors for series or parallel tuning, or any combination of the two. It is similar to the Benodyne. It has two selectivity adjustments. The first is the ferrite antenna tuning coil that slides in or out of the detector coil controlled by a knob on the front . The second is a miniature differentia l capacitor used to couple the detector coil to the diode, also controlled from the front . For general listening both of these selectivity adjustments can be pre-adjusted and left in position. For weak DX reception they can be tweaked as necessary. When using series antenna tuning with the antenna capacitor

left fully meshed the detector tuning works just like a "real" radio. All 27 daytime stations can be heard clearly. Any chosen station can be further enhanced by tuning the antenna capacitor to resonance at the station's frequency. The diagram below shows a 1N34 detector; it has been upgraded to an AligentSchotty Barrier diode which provided a nice increase in both selectivity and sensitivity. Using a capacitor to provide variable coupling to the detector is not a new idea. In fact, it was used in the 1950's in the Miller Tuner circuit, Set #5 below. For some reason it is seldom used by experimenters, even though it is far easier and works much better than tapping the coil. It does require the use of a good quality RF Choke to provide a DC return for the diode. Although it cannot be seen in the picture, (some parts are hidden in a milled recess inside the white Teflon capacitor base) it has a variable resistor to equalize the AC and DC load for the detector. I retrofitted capacitor diode coupling into my set #3 shown below to replace the taps and the tap switch, and it enhanced its performance. Thanks guys, I borrowed ideas for this set from many of you. Update 8/29/03: For maximum performance at the low end of the band the RF choke should be at least 5 mH, and preferably 7.5 or 10 mH. Two or three 2.5 mH chokes can be placed in series.

Top View

My First High Performance Crystal Set

Front View This Crystal Radio built in 2003 adds a few refinements such as ceramic insulated variable capacitors, twin National Velvet dials and ferrite core inductors wound with Litz wire. The unloaded Q of these coils measure well over 400. In this set the detector coil has selectable taps to connect the detector circuitry. The front knob provides variable antenna coupling between the two ferrite coils. By turning the antenna coil 90 degrees to the detector coil the coupling can be reduced to a very low value. Less coupling means increased selectivity and it helps separate the weak stations from the strong ones. One day I noticed that this set still received a few stations when the tap selector switch was on an unused position. I found that eliminating the taps and adding a tiny differential capacitor and an RF choke for the missing detector DC return made this set receive much better. The Hobbydyne™ circuit was born.

My Crystal Set using Ferrite cores with variable coupling

The Mystery Crystal Radio

This set was built using the circuit of the famous Mystery Crystal Set

This Crystal Radio was made strictly for nostalgia purposes and it is based upon the circuit of the Mystery Crystal Set which was made famous in Australia in 1932. It is the only single tuned set I have built, and because of this it does not have the selectivity of the other two models, although it does work well.

The unloaded Q of its coil is quite high because it is wound on a 4.25 inch Styrene form with Litz wire. The majority of the wiring is concealed in wire ages routed underneath the wood chassis. As an experiment I put an antenna tuner ahead of the Mystery Crystal Radio. The selectivity and sensitivity were both greatly improved, and I decided to built a dedicated tuner for it. The tuner features both series and parallel tuning at the flip of a switch . It has #19 wire space wound at 18 turn per inch on a threaded styrene coil form. Its Q is also extremely high. No connections are necessary between the Mystery set and the Tuner because the tuner was designed to provide correct coupling when the two chassis are sitting side-by-side. With the tuner the Mystery Crystal Radio is a real DX go-getter and has excellent selectivity. The tuner has it's wiring concealed in routed ages under the chassis the same as the Mystery radio. Extra concealed ages are in place so it can be converted to a stand-alone crystal set very easily.

This is a Custom Antenna Tuner for the Mystery Crystal Set

Rear View of the Antenna Tuner For more information on the Mystery Crystal Radio go to www.google.com and type in the words "Mystery Crystal Set" or visit Ben Tongue's explanation of the Mystery Crystal Set at http://www.bentongue.com/xtalset/19mstry/19mstry.html

My Original "Test Bed" Crystal Radio

#1 Crystal Radio is my experimental model built in August, 2002. It has had several types of coils including a 2 inch cylinder, a rook which is a variation on the cylinder, a honeycomb and the present radial arm spider coils. I tried coils wound with solid wire and wire with 100 strands of #47 Litz wire. The Litz is far superior, providing a Q (quality) factor almost twice as high on some of the coils. The empty hole in the center of the front was the location for a switch that was used to select ten different detector coil taps. One particular tap was almost always gave the best performance. To eliminate any possible loss caused by the large number of coil taps, I rewound the coil with only that one tap. The present coils in this set are radial arm spiders wound using Litz on Teflon hubs with Polystyrene radials. These coils have an unloaded Q of about 285. Note that spacing between the coils can be adjusted. Best overall operation is with the two spider coils spaced three inches apart. When I want to try something new this is the set that gets picked on.

#1 My first Crystal Set. It has been a test-bed for many experiments

My Grandfather's 1915 Crystal Radio

This Crystal Radio was built by my Grandfather, E.E. Frederick in 1915 It is ironic that the basic circuit I am using is just about the same that my Grandfather, Edwin E. Frederick used in the crystal radios he manufactured and sold in 1914 and 1915. This was long before any broadcast stations existed. His radios were used to receive the Arlington, VA time signal that was operated by the Government. The Arlington time station ran a power of several hundred thousand watts2 on a very low frequency, and it could be heard for hundreds of miles. Jewelers were his customers, and they used the sets to receive the time signal and set their clocks. Jewelers often had crystal radio displays in their show windows and they drew many visitors. Some jewelers even had a ball in their show window that they dropped at noon. Radio was very new and people were fascinated with it. People would gather each day and watch the ball drop. We still have one of his sets, and it is pictured above. The little hinge-out door on the right side is for access to the crystal. I also have several newspaper articles, some customer testimonials, and about 20 handdrawn diagrams for his crystal radios. Each radio is slightly different from the preceding one. I also have the name of each person who purchased one of his radios, the date of purchase, and the price paid. To see two 1915 newspaper articles and a typical diagram of one of his radios click here.

Eddie's Crystal Radio

Here is a nice single-tuned Crystal Radio built by Eddie, the great grandson of Edwin Frederick who made the set directly above. Eddie is off to a running start with a 3 inch coil using Litz wire.

The J.W. Miller Crystal Hi-Fi Tuner

This J.W. MilllerHi-FI AM Tuner from the 1050's is a Crystal Set This is a J.W. Mller commercial High Fidelity Band AM Tuner from the 1950's. It uses two ferrite coils isolated from each other and coupled by a special "mutual coupling" coil. The result is a set that offers superior band selectivity. The two tuned circuits track precisely and it tunes just like any other AM The trade-off is reduced sensitivity, but it still receives stations from all over the southeastern US in the evening from my Florida location using only earphones. When connected to a high fidelity amplifier like Miller intended the amplifier more than makes up for the slight lack of sensitivity.

So, what can I receive with my crystal radios? I live near Lakeland, Florida and during daylight hours I can hear all of the locals and most of the 5000 watt and higher power stations from Tampa and Orlando, both about 50 miles distant. I have also heard WIOD in Miami in the afternoon. Some of the stations, like WDAE, a 5,000 watt station in St. Petersburg are so loud that I need a volume control. WIOD is on 610 KHz and WDAE is on 620 HKz, so you can see that it is pretty selective. In the evening I regularly hear WSM Nashville, WWL New Orleans, WBT Charlotte, WSB Atlanta, a station in Indianapolis, IN, and many more not yet identified, including a large number of Spanish stations.

So, the statement that a crystal radio with a decent outside antenna can receive as well as a 5 tube table radios is true. In fact, the crystal set receives better because the table radio picks up so much more electrical noise than the crystal radio's outside antenna does. How this foolishness all started Recently I came across an article about Crystal Radios on the Internet. The person who wrote the article said that a good crystal set with a reasonable outside antenna could pick up almost any station that a five tube table could hear. Well, I figured he was smoking something that I did not have access to. But it did get me interested. I discovered that there are hundreds of circuits and plans for building the little crystal radios, all of which are variations of and improvements to crystal radios made before 1920. The plans range from old copies of Popular Electronics and other hobbyist magazines, the work of Elmer G. Osterhoudt and his Modern Radio Laboratories, to the hobbyist (read Engineers) that are making high performance crystal sets today. If you think you are pretty sharp on electronics visit Ben Tongue's crystal radio site. I guarantee that you will learn the finer points at this site. In fact, I will not get too technical because it is all covered there. What did I learn? I discovered that deg and building a high performance crystal radio is far more demanding than building a multi-tube radio. With the multi-tube radio cutting a corner here and there will barely be noticed. Every part and parcel of a crystal radio must be perfectly designed if you expect it to work well, since all your operating power comes directly from the broadcast station you are receiving. The earphones used are also very important. Today's earphones are made to be used with powered radios and they are unsuitable for crystal sets. Most people who build crystal sets use vintage Baldwin, Brandes or Western Electric earphones made in the 20's and before. Personally, I find that of the regular earphones, the Western Electric models 509 and 509W are the best. Despite their age, these vintage earphones can still be found today. The most sensitive and best sounding earphones are made from World War II surplus sound powered elements and some models will work better than any regular earphones. These elements can be removed from military sound powered handsets or headset for use in custom made headsets

. The best sound powered elements by far were made by RCA and are often called "Big Cans". These sound powered elements are hard to find and very expensive. Here are a few Crystal Radio links. I will list more soon. Circuit Specialists carries an assortment of test equipment devices and tool that make do-it yourself projects quick and easy. Footnotes (1) I assumed that ever one knows what a Crystal Radio is. Perhaps I should explain just in case. A crystal radio uses only tuned circuits to select the stations, and a crystal detector to rectify the radio station's signal to enable it to be heard with a set of headphones. It is entirely ive, it has no active components like tubes or transistors and uses no electricity. When you listen to a broadcast station on crystal radio the sound in your earphones comes directly from the station your are listening to. Since there is no amplification of any kind, the circuits, especially the coils, must be very efficient so that sufficient power can be recovered (stolen) from the station to operate the earphones. (2) Many early pioneer radio stations ran powers of many hundred thousand watts. In some areas merchants could "steal" enough power from nearby stations to light their signs. Today the maximum power permitted in the United States for commercial broadcast is 50,000 watts. In those early days the frequencies above our current AM broadcast band were consider useless and were used mostly by amateurs. Visit Jim's Cushman Scooter Web Site www.hobbytech.com

(Please click on the image above to see a more detailed photo of the radio. It is well worth it!!)

By Mike Tuggle Written 05/21/04 One evening, while perusing MRL® Handbook-17, which I hadn't done in some time, the MRL® #39 circuit hit me between the eyes ! This circuit has some amazing similarities to, and differences from, the Australian Mystery Set. The original MRL® #39 had the antenna attached to point 3. Consider points 1 through 5 on the schematic: there are 20 possible ways to attach antenna and ground to these points -- 25, if you also consider antenna alone. Using a prototype set, I looked at all 25 and found that point 5 consistently was the best ground, and points 2 and 4 gave the best Selective and Broad tuning antenna connections. That's for my particular antenna and ground, anyway -- the point here is, experiment and see.

SCHEMATIC of the MODIFIED MRL® #39 SET Very little thought or science, on my part, went into the design: I just roughed out a primary which would match my 550 pF variable capacitor and followed the Australian design, making the secondary half that number of turns. Here are some parts specs. to start with -- feel free to adjust as needed: C1 two-gang, 550/500 pF per gang air variable capacitor, with vernier drive dial L1 "Mystery" type basket-wound coil: 36 turns 660/46 litz wire, wound in an 'over 2 posts / under 1 post' pattern on a 5-inch diameter, 13-point form. Inductance 184 uH; unloaded Q = 867 at 0.7 MHz. L2 18 turns 420/46 litz wire, inter-wound and centered with L1. Details below. The total length of L1L2 is about 1-7/8 in. D1 diode: 1N34 type, or galena Note on winding L2: The winding of L2 is carefully "synchronized" with the L1 winding so as to preclude any adjacent turn segments lying side-by-side. This reduces the L1-L2 coupling by some small amount. Whether this offers any performance advantage, I don't know, but this is good basketwinding practice. For the 'over 2 / under 1' pattern on a 13-point form, synchronized windings can be had by following exactly the pattern shown:

The elegance of the Mystery Sets, the MRL®# 39, and other like sets in the Telefunken genre (untuned secondary closely coupled to tuned primary) is what they accomplish with just single-dial control. The MRL® # 39 is the epitome: ONE coil (albeit a special one), ONE capacitor (albeit a special one), ONE diode -- that's it !

************************************************************************************* ******** New! Further Thoughts added 18 jun 2004 by Mike Tuggle Design/Construction My original intent was to design an Over-1/Under-1 basket coil with primary and secondary turns lying side-by-side for maximum capacitive coupling. But, the needed coil length came out way too long. I compromised and wound the Over-2/Under-1 coil described above. This coil has a near-perfect 2.5-to-1 diameter-to-length ratio, and its Qs show it: unloaded Qs are above 600 over all the band and above 800 over most of it.

Vernier Readout Scale I added a vernier readout scale to the dial to help with loggings. Vernier scales are best made by machine. Hand-scribing tick marks is exceedingly difficult, and results are usually so-so. While the idea is elegant, I'm finding that eyeballing fractions of a scale division with a fine hairline pointer is entirely adequate for crystal set work. Initial Results Performance is quite impressive, although -- no surprise -- it doesn't hold a candle to a double-tuned set. Using a trap on a 690 kHz local, I can pick up the 720 kHz 5 kW station on Kauai, 116 miles away, in the daytime. [I later found it could be pulled in without the trap.] DX conditions are way down with summer approaching, but in a few evening sessions I've heard a few of the strongest W.C. stations including KFI 640 L.A., KPNW 1120 Eugene and KXTA 1150 L.A. The last one is by far the most reliable west-coast station here. I'm looking forward to improved conditions in the fall. Antenna Connections I find the BROAD connection has sufficient selectivity for DXing, plus the advantage of more signal. While the SELECTIVE antenna connection might be useful for coping in the urban jungle, I was thinking of omitting it in future versions of the set, until ... I went through a systematic test of the 20 possible antenna-ground connections with this set. Qualitatively, behavior was similar to earlier versions: some connections work fine, others not at all. One surprise was, at the upper end of the band, the SELECTIVE connection was not only more selective, but it was also more sensitive than the BROAD connection. The BROAD/SELECTIVE connections might be better labeled LOW BAND/HIGH BAND. The crossover is in the 1300 to 1400 kHz region. Of course, all this may be specific to the particular antenna-ground system being used. SW QRM Susceptibility One potential drawback: the set is susceptible to lower-end short-wave QRM. Fortunately I only have to contend with 2.5- and 5 MHz WWVH out here. A small coil in the antenna lead may help to knock out this QRM. Telefunken and Other Precedents It appears that the precedent for this set and the 'Mystery'-type sets is the classic Telefunken circuit that has been around since the year one:

TELEFUNKEN CIRCUIT.

This particular circuit appears in the MRL® literature as the MRL® #4 set. If you make the dashed line connection you have the "Mystery Plus" set from the Brisbane Sunday Mail, April 16, 1933. My theory has it that 'Proton' "invented" the original Mystery circuit when he screwed up the antenna and ground attachments to his Telefunken set. Then, making further "improvements" to his newfound Mystery set, lo and behold, he arrived back at what is essentially the original Telefunken circuit ! Per the MRL® write-up (MRL® Handbook-17), it appears it was correspondent, Mr. R.B. Richardson, who came up with the two-gang capacitor (so-called "Tuggle") in the tuning circuit. Dan Petersen did some interesting, closely-related work, starting with the Mystery circuit: http://www.worldaccessnet.com/~petersen/Mystery-article.htm Basically, using a two-gang capacitor in the Mystery, Dan 'discovered' the MRL® #39 set ! He came up with Broad/Selective configurations different from mine -- possibly due to our antenna-ground differences.

Experimental Short-wave "Mystery" Crystal Radio by Ken Harthun Copyright January 2001 This is not the typical crystal set construction article to which you may be accustomed; rather, it is a description of a few workable circuits that leaves plenty of room for experimentation if you are so inclined. You can build a fine working short-wave crystal set based on what I give you here. These circuits perform very well. However, I’m still experimenting with them and I challenge you to do the same, to discover what works best for you. Tinkering with a circuit, trying different things to see what works – and what doesn’t – is the essence of our hobby and is what keeps this particular rock head interested! Because this set is experimental, I make no claims that this design – or any particular hookup that results from this article - is the ne plus ultra in short-wave crystal radios. We all know that the perfect crystal set does not exist. The "perfect" crystal set for my location is something that may or may not work for you (I have to deal with a 50Kw blowtorch that is located virtually in my backyard – you may be more fortunate). My antenna is about 150 feet of wire in an inverted V with the apex at about 35 feet. Some of it runs every-which-way across my roof and into my radio room. Though at BCB frequencies my antenna is much too short, it is actually much too long an antenna at short-wave frequencies. You can probably get by with a lot less, depending on your location.

With these things in mind, let’s begin and take a look at a project that I believe will give you plenty of enjoyment – and will probably raise many questions. In fact, I hope that this article will generate enough interest in short-wave crystal set reception that we see some real innovation in future designs. The radio is based on the Mystery Crystal Set designed by a fellow who called himself "Proton" and who first published details of the set in the Sunday Mail newspaper in Brisbane, Australia in 1932. I won’t rehash any of the details because the set is well covered in the articles posted at http://www.clarion.org.au/crystalset/mystery.html by Ray Creighton. I have simply adapted the circuit and modified it a bit for my set. Fig. 1 is the basic schematic.

Construction The coil form is a 2.5 inch long piece of 2" white PVC plumbing pipe with an outside diameter of 2 3/8 inches. This is the kind that has a thickness of about 3/16 inches and is known as "Schedule 40". Construction details are shown in Figure 2. L1 – 20 turns #22 close wound, approx. 35 uH L2 – 13 turns #18 or #20 wound at 20 tpi, approx. 12 uH L3 – 6 turns #22 bifilar wound in the center of L2 C1 – I’ve been using a 20 – 56 pF air variable for coverage of approx. 6 – 10 MHz and a 13 – 250 pF air variable for approx. 2.9 – 12.7 MHz coverage with the coils shown. Diode – 1N34A or crystal detector. (You may find as I did that a piece of pyrite and a cats whisker work much better than a diode. For some reason, I find that the pyrite detector is more sensitive and gives me much better volume than the diode. Also, with pyrite, you can eliminate the resistor across the crystal earplug.)

Resistor – 47K (*Note: you can eliminate this resistor if you are using high impedance magnetic headphones) Phones – crystal earplug or high impedance magnetic phones I have been using alligator leads to connect it up so I can rapidly change configurations. The photo shown at the beginning of this article is a completed homebrew version of the set that uses mostly nonelectronic parts (except for one Fahnestock clip hat was used to hold the detector rod). The capacitor is made from a plastic tube and some aluminum tape that you can find at auto parts stores.

In the original coil I wound, I used hot-melt glue to hold the windings in place. About six beads or "ribs" across each of the windings works very well. Also looks pretty good – better than tape! In the basic "mystery" circuit shown in Fig. 1, L1 is not used. The tuned primary (L2) has two antenna connection points, A and B. A is the least selective, but gives a louder signal. B is more selective, but the sensitivity drops notably. While I show a ground connection, I noticed that results vary. In most cases, I find that the set works better in this configuration without a ground and is very sensitive. Selectivity is enhanced with the ground connected, but some signals drop out while others get louder. I’m not sure why this is and I’m still experimenting. I have tried putting a small trimmer capacitor in the ground connection to vary the coupling to ground, but my results are inconclusive at this point. Overall, the basic configuration works very well and is a fine set in its own right. I easily pick up WWCR at 5070 kHz, WEWM on 9975 kHz, have heard Radio Havana (Cuba), the BBC and many other German,

Chinese, French and Spanish language broadcasts. My first night, I picked up a station that was running a program called "Let’s Learn Chinese". Bear in mind that propagation conditions vary considerably. You may or may not hear this much at first. Then again, you may hear even more. Some evenings I hear very little but I’m listening as I work on this article in late February and there seems to be stations everywhere as I tune. The audible stations fade in and out and back again, so I get a good variety without having to touch the tuning dial! As I mentioned earlier, in my location near Cincinnati, Ohio, I have a 50 kW blowtorch, WSAI on 1530 kHz, virtually in my backyard. I have to use a wave trap to tune this station out or it’s all I hear. As a matter of fact, I can hook a diode up to a crystal earplug, hold the diode in my hand and still hear this station loudly! My point is that your conditions will certainly be different than mine, so there is no way I can tell you what to expect to hear. But I can almost guarantee that you will hear something. Figure 3 shows another configuration I have tried.

In this configuration, L1 is used as a coupling coil for the antenna and one end is attached to either A or B on the primary. This is a very interesting hookup, more selective than the basic circuit in Fig. 1, but not quite as sensitive. With L1 hooked up to point A, the selectivity is quite sharp. When hooked to point B, the set is so selective that tuning can be tedious. Again, the ground is optional, but I find a little better sensitivity in this hookup with the ground connected and the selectivity hardly suffers at all. One thing to try would be to decrease the spacing between L1 and L2/L3. I think that the current spacing may result in coupling that is a bit too loose for this set. The configuration detailed in Figure 4 shows some promise for daytime reception in the upper shortwave bands and for further development as a set with variable coupling. With the existing design, the coupling between L1 and L2/L3 is somewhat loose at the ½ inch spacing. Making L1 moveable and possibly large enough to fit over L2/L3 are some things I am considering.

Here are some ideas for further experimentation. 1. Grounding connections A and B to see how this affects reception. (This would actually be defeating the mystery circuit and using L3 as just a detector-coupling coil) 2. Hook up a variable capacitor in series with the antenna. 3. Hook up a variable capacitor in parallel across L1 to tune the antenna. 4. Try the "Tuggle" circuit across L2. This would involve using a 2-gang variable capacitor and hooking one of the sections in parallel with L2 and the other section in series with the ground. 5. Try the Tuggle circuit across L1. 6. Use Tuggle tuning with main variable section across L1 and the second capacitor section in series with A/B connection. 7. Make L1 moveable to vary coupling. 8. Band switching and band spreading using various fixed and variable caps. 9. Use a smaller coil form to approximate a 1:1 length-diameter ratio. 10. Use a smaller inductance value (same as L2?) for L1. This is by no means a complete list of the things that can be tried with this circuit. In fact, I even redesigned the coil to the following dimensions: L2 – 18 turns #20 space wound to 1.3" long, 2.375" diameter for approx. 20 uH.

L3 – 7 turns 7/30* bunched cable bifilar wound in center of L2 L1 is not present in this configuration. *7/30 cable is 7 strands #30 enameled wire twisted into a cable of approx. size of #20. Individual wires are stripped at the ends then twisted together and tinned. I hope I have given you some food for thought and inspired you to try your hand at making this interesting circuit work. I intend to continue my own experiments and perhaps write another article at some future time. If you want to me, you can post a message on Rap-n-Tap, the Yahoo Crystal Set Radio Club, or email me at [email protected]. I welcome all comments and suggestions. Good luck and Happy Experimenting! 3/18/2001 Notes on further modifications L1 is now present again on the redesigned coil. It comprises 8 turns of # 22 AWG close wound, spaced ¼" from the bottom end of L2. The antenna and ground connections are made to this coil. L2 is now 14 turns instead of 18 turns to bring the coil more in range of the most active SW bands. L3 is unchanged. Experiments with a Perikon detector are very interesting. My Perikon detector consists of a piece of chalcopyrite in with a small cone-shaped piece of zincite. It is equally as sensitive as my pyrite detector – maybe more so under some conditions – and has an interesting property in that it appears to be somewhat "tunable". I’m not sure if this is due to signal strength or has more to do with the frequency to which the set is tuned, but I observe a distinct ability to peak signals differently as I tune across the bands. I seem to recall some mention of this property in early radio literature. One big advantage of the Perikon is that it is not as touchy as the typical catswhisker-and-mineral detector. It is easy to begin detecting a signal and once detection begins, it is a simple matter to peak the signal. I also observe that the Perikon requires a bit of pressure to work properly. It doesn’t require the light touch like other point detectors and adjustment is non-critical. Unlike the catswhisker detector, the Perikon doesn’t mind being bumped and tends to keep its setting. It is very stable. I like this detector a lot. I tried a zincite/pyrite combination, but you can forget that one. Just doesn’t work. Likewise, chalcopyrite with a catswhisker just isn’t very sensitive. It works, but it seems to been just a couple of steps up from a rusty razor blade. Yes, the razor blade detector actually works on this set on some of the stronger signals. And I have to say again that a 1N34 germanium diode just lacks sensitivity in this set. It doesn’t work as well as either the Perikon or the pyrite detectors.

Last night I cobbled together a simple FM crystal set. I figured that with all of the 100kW FM stations in town here, I should have no trouble picking something up. Coil is 7 turns of #14 wire, 3/4" diameter and 2" long. Capacitor is a 5-20 pF trimmer. Detector is 1N34A diode. The problem is that most FM crystal sets that I've seen, have the antenna built-in, and I want to use an external dipole or folded dipole (or Yagi if I get ambitious). So, I wasn't sure of the best way to couple the antenna to the tank. I tried using a small link winding, which didn't give me any signal at all. Then I connected the antenna directly across the tank, using a 5pF capacitor in series with the antenna to prevent the antenna from loading the tank. This worked, and I managed to pick up one station but thought I should be able to do much better. I'm just getting comfortable with MW and lower shortwave frequencies. No experience with VHF. So, if anyone has any suggestions about building an FM xtal set with external antenna, I'd like to hear them. BTW, I did find this one that uses a resonant cavity and takes an external antenna: http://www.vacuumtubesinc.com/TubeTidbi ... alSet.aspx but I'd rather stick with conventional coils for the time being.

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Post subject: KR1S Posted: Sun Apr 11, 2010 3:15 am Bob, Site

Don't see how you can get the required Q with LC. As you are doing slope detection on a wide signal I think you need much higher Q than for AM demodulation. I was impressed by the cavity method, as that's how I'd do it. For an LC circuit you'd be looking at about 1 pF and about 2.6 uH for mid-

band to get the impedance of a 240 uH coil at 1000 kHz. You could make the inductor but stray C will exceed 1 pF. And I'd want even higher impedance. You wouldn't have much Q at Z = 1500 ohms.

ed: Sat Jun 06, 2009 6:58 pm Posts: 3181 Location: South Florida

A cavity-tuned crystal set would be interesting! But to answer your question, the only way to couple an antenna would be inductively and as lightly as possible. Anything directly coupled to the LC is going to kill the Q. You could try a helical resonator with the inductor made of heavy tubing. It would need a pretty big box, though. http://www.rfcafe.com/references/electr ... onator.htm 73, 73,

_________________

http://kr1s.kearman.com/ http://qrp.kearman.com/

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Post subject: Bob Weaver Posted: Sun Apr 11, 2010 5:20 am I figured that the minimum practical capacitance would be around 10pF because of strays. So, the coil had to be 0.25µH. It appears that my coil could be optimized considerably though. Using an optimization spreadsheet, I see ed: Sun Apr 08, 2007 that I can wind a 3 turn coil on a 25mm form to use the minimum amount of 8:02 am wire, which should give the best Q. This seems more in line with the other

Posts: 1409 Location: Saskatoon

FM crystal sets I've seen online. I note that Broesel had good results with his: http://theradioboard.com/rb/viewtopic.php?t=1758 I guess I'll have to start by duplicating his, and then go from there. I'm not against using a resonant cavity. I like the idea, but I doubt that I could easily find a piece of 3" dia. copper pipe, and if I could, I doubt that I would want to pay the price. If there was a cheaper way, I'd be interested.

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Post subject: OErjan Posted: Sun Apr 11, 2010 8:06 am uhm, with some shopping 0.25mm sheet silver should be cheaper than regular copper, mainly because copper is close to 2.5mm thick at that diam, add some skill a torch and minute amount of flux... ed: Sun Nov 12, 2006 6:09 pm Posts: 1353 Location: N 64º 41' E 21º 14'

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Post subject: gusnaz Posted: Sun Apr 11, 2010 3:06 pm Bob,

Take a look at this site. Very good info on FM crystal radio. http://solomonsmusic.net/FM_CrystalRadio.html

ed: Wed Nov 08, 2006 4:15 am Posts: 782

Gus

_________________ Old age is 'when you still have something on the ball, but you are just too tired to bounce it.'

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Post subject: Broesel Posted: Sun Apr 11, 2010 5:41 pm Hi Bob, the FM set worked with me, but don't expect to much! As can be seen from the two pics I added, the transmitter is in sight with a distance of just 17km ed: Mon Apr 07, in a height of 2000m. As long as I can see the antenna, reception is good. 2008 6:03 pm Walking around in my flat, reception changes with place. Selectivity isn't that Posts: 393 great, but I can improve things changing length and direction of the antenna. Location: Villach, Austria Although I built-in fm-chokes, the headphone wire influences the reception. It is important to keep all wires in the tank circuit as short as possible because of additional capacitance/inductance. In my feeling, the diode is also improtant. Not that much in of impedance matching (the impedance of the tank is rather low), but having a type with lowest as possible junction capacitance is advantegous. The BAT 15 is quite well (0.3pF), a 5082-2835 worked also qood. Loudness is less than with LW/MW/SW sets due to the slope detection. Stronger signals do not result in more loudness but in more distorted sound. Anyway it is an interesting thing to play with!

Good luck! Brösel

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Post subject: Bob Weaver Posted: Sun Apr 11, 2010 9:54 pm gusnaz wrote: Take a look at this site. Very good info on FM crystal radio. ed: Sun Apr 08, 2007 http://solomonsmusic.net/FM_CrystalRadio.html 8:02 am Posts: 1409 Location: Saskatoon Hi Gus, Yes, I'd seen that. The circuit is similar to what I started with, except for the antenna. Hi Broesel, I've confirmed what you have said about location. By coincidence, the place where I was experimenting, happened to be a good spot to pick up one station. When I moved around the house, I found some other locations which brought in some other stations. There are definitely local peaks and nodes that you don't find with MW. It looks like a whip antenna would be the most practical way to go, so that I can wander around with it. I went to the page that you mentioned in your thread: http://www.jogis-roehrenbude.de/Detekto ... tektor.htm I tried the link coupling method, but didn't get as good results as when I just connected the antenna to a tap on the tank through a 5pF capacitor. I tried a few different methods. I had tried using the link to couple the antenna to the tank, and then take the detector off a tap on the tank. Later, I tried coupling

the antenna to a tap on the tank and then using the link winding for the detector. The link worked better for the detector, but not as good as when both the antenna and detector were both connected to a tap on the tank. For diodes, I tried a 1N34A and an FO-215, and a few different unmarked ones from my junk box. The 1N34A worked fairly well, but one of the unmarked mystery diodes worked best. My headphones are not ideal either. I'm using a set of homemade crystal phones, which aren't bad, but not as sensitive as they could be. I should mention, the main reason for this experiment is because I was playing with a MW crystal set a year ago, and was picking up a local FM station quite loudly. So, I thought that a purpose built FM crystal set should be able to pick up an even better signal.

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Post subject: Bob Weaver Posted: Mon Apr 12, 2010 3:58 am I decided to cheat. I inserted a small audio amp between the detector and the headphones. I wanted to see if my problems were on the tuned circuit end or on the detector/audio end of the circuit (or both). ed: Sun Apr 08, 2007 8:02 am I was pleasantly surprised to find that I could now pick up about five Posts: 1409 stations. So, the antenna, tank and detector are working, but my phones are Location: Saskatoon too insensitive or put too much load on the circuit. With the high impedance amp input, the load on the detector is considerably reduced, and the selectivity becomes surprisingly good. As I turn the tuning capacitor, the stations pop in and out fairly abruptly.

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Post subject: Broesel Posted: Mon Apr 12, 2010 7:01 am Quote:

ed: Mon Apr 07, 2008 6:03 pm Posts: 393 Location: Villach, Austria

I should mention, the main reason for this experiment is because I was playing with a MW crystal set a year ago, and was picking up a local FM station quite loudly. So, I thought that a purpose built FM crystal set should be able to pick up an even better signal.

This is the ugly side of radio-experimenting. One always gets the results if one doesn't need it I found a similar problem with my loop set. Here I use one of the so-called zero-voltage MOSFET as rectifier. The source is coupled via a two-turns winding. In the first tests I thought, that my selectivity is rather bad since there were some spurious stations in the background. Later I figured out that these were some FM stations and that I have to use some fm choke to get rid of it. I once tried to connect the FM crystal set to our house antenna system using inductively coupling since also some impedance matching is needed. Didn't work, maybe I should try again with a coupling capacitor on a tap of the tank inductance. I never tried connecting the phones directly to the diode. A Bogen T-725 for impedance matching worked well, currently I use another transformer having an input impedance of just 16 kOhms which is a bit low. Will replace it by one of the cheap and small mo transformers. Brösel

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Post subject: Bob Weaver Posted: Mon Apr 12, 2010 8:45 am The funny thing is, now that I'm getting reasonably good FM reception, I have an AM station on 650kHz coming in on top of everything else. So, Now I need to trap out the MW signals. ed: Sun Apr 08, 2007 8:02 am Hopefully this problem will resolve itself if I put it inside an enclosure. Posts: 1409 Location: Saskatoon Now that this is working reasonably well, I'll post a schematic and picture as soon as I have a chance.

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Post subject: Broesel Posted: Mon Apr 12, 2010 2:00 pm Hi, I just this one: http://www.crystal-radio.eu/enset6.htm ed: Mon Apr 07, There was an update too, it is interesting to see the behavior of different 2008 6:03 pm diodes. Seems, that here a diode is best with not only low junction Posts: 393 capacitance, but also low differential resistance (the HSMS2850 has 9k only, Location: Villach, Austria probably a bad choice for BCB). Probably I should use such a diode soon and compare with what I have currently. I think, such a design might be worth a try. Coming back to the antenna

coupling - here it is very weak, right? Brösel

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Post subject: homebrew Posted: Mon Apr 12, 2010 6:39 pm Quote - "I was pleasantly surprised to find that I could now pick up about five stations" What was the maximum station distance? Or better yet what is the ed: Tue Jan 15, 2008 maximum stastion distance on any FM crystal set? 4:26 pm My favorite FM station is 75 miles from home and so far I have not been able Posts: 987 to pull it in even on a very high Q - frequency specific set. Location: Dallas, Tx.

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Post subject: Bob Weaver Posted: Mon Apr 12, 2010 9:27 pm Broesel wrote: Hi, ed: Sun Apr 08, 2007 I just this one: http://www.crystal-radio.eu/enset6.htm 8:02 am There was an update too, it is interesting to see the behavior of different Posts: 1409 diodes. Seems, that here a diode is best with not only low junction Location: Saskatoon capacitance, but also low differential resistance (the HSMS2850 has 9k only,

probably a bad choice for BCB). Probably I should use such a diode soon and compare with what I have currently. I think, such a design might be worth a try. Coming back to the antenna coupling - here it is very weak, right? Brösel

Thanks for the link. I hadn't seen that one. It looks to be a little simpler and less expensive to build than the other resonant cavity set.

Homebrew, The stations are all locals. I would guess that they are all less than 10km away, and several are over 100kW. Mind you, one of the stations that comes in best is under 1kW. So, I have not accomplished anything special here. But, at least it's a beginning, and I can work at improving things. I would like to try a multi element outdoor antenna, and also get some better headphones to see what I can receive without the amplifier.

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Post subject: Bob Weaver Posted: Tue Apr 13, 2010 12:33 am Here is the receiver schematic:

ed: Sun Apr 08, 2007 8:02 am Posts: 1409

Location: Saskatoon

(Edit: Yes, I know those chokes are much bigger than necessary, but that's what I had on hand.) And you might be interested in the audio amplifier which I built using a CMOS hex inverter:

This gives a gain of about 300, and will drive most headphones. A suitable matching transformer may be beneficial. I used a Bogen T725 between the amp and a set of Sony Sport Walkman phones. Will post a photo of the set when I get things more presentable.

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Post subject: Bob Weaver Posted: Tue Jun 08, 2010 9:43 am Well, I took Jim's advice. I did a bit of research on helical resonators, and decided that one of these should perform as well as or better than the coaxial resonator in that other project that I linked to. Plus it would take up much less space. ed: Sun Apr 08, 2007 8:02 am I was involved in a little radio building competition this past week, and so I took Posts: 1409 the opportunity to put one together. Location: Saskatoon I built the resonator out of 3/8" diameter copper tubing and some copper flashing for shield. The coil is 4.3 turns 2.2" average diameter and 3.3" long.

Note the use of Brasso brand Q enhancement potion. The shield is 3.3" square by 8" long.

It's about 2" longer than necessary so that I would have room for coupling loops in the bottom, and could fit a capacitive tuning mechanism in the top. This is the tuning thing:

According to the design formulas, this resonator should give an unloaded Q of a little over 2000. So hopefully the loaded Q will be in the 600 range which is necessary for adjacent channel separation. Partially assembled:

Other end:

Tuner end:

The coupling loop and detector assembly is mounted on a separate board so that I could position it for best performance:

This is the initial setup for slope detection, using two loops, one for coupling from the antenna to the resonator, and one to couple from the resonator to the detector (both are blue hookup wire). The little postage stamp sized circuit board is the same CMOS headphone amp that I mentioned in my last post. Schematic:

The 27uH choke and 150pF cap are a low filter to trap out an AM flamethrower that was causing some problems. I got it all put together yesterday and gave it the big test. Here it setup for listening using a set of TV rabbit ears:

Reception was fairly good. The signal was weak using my piezo phones without any amplification. So, I connected in the headphone amplifier, and did all the remaining testing with it. Selectivity is very good. I don't know what the overall Q is though. I have a Q-meter, but it doesn't go up to these frequencies. I found that I could easily pick either side of a station's center frequency for detection, and if I tuned dead center, the signal became very distorted. That leads me to believe that the Q is very high. I couldn't get that kind of precision with the previous set using a coil/capacitor tuned circuit. So with that bit of success, I started thinking about making a true discriminator. Originally, I was just going to duplicate Ed Richley's circuit (XSS Newsletter Jan & Mar 1996): http://www.somerset.net/arm/reprints/richley_5.gif (I thought I'd posted a link to this previously, but apparently not. The whole article can be found at http://www.somerset.net/arm/fm_only_lowtech.html Scroll

down a about 1/4 page to "Phased FM Detection.") Anyway, my plan was to duplicate that circuit, except that I'd use a helical resonator rather than the coaxial resonator that Ed used. However, after giving it a bit of thought, I figured that I could eliminate two of the three broadband transformers. These transformers are just modified 300:75 ohm baluns. I have a pile of them, so there's no problem getting them, but I figured that one transformer would have less insertion loss than three. So, I modified the circuit. Here is the frequency discriminator circuit:

As you can see, for T1, I modified a balun so that the secondary has one turn then a grounded tap and another three turns. The primary has four turns as per original. The one turn end of the T1 secondary is connected directly to the resonator coil very close to the cold end. The three turn end goes to the diodes. I didn't put in an AM trap this time around because the circuit arrangement is not conducive to AM pickup as the diodes are essentially shorted at MW frequencies by the pickup loop. Here's a brief description of how the circuit works:

Input transformer T1 splits the incoming RF (untuned) into two signals 180 degrees out of phase with reference to ground. The low level side goes to the resonator and the high level side goes to the common point of the diodes. The voltage induced by the resonator into the pickup loop will be 90 degrees out of phase with the the input signal. This is the nature of a loosely coupled tuned circuit at resonance. The phase of the signal on the pickup coil will vary on either side of 90 degrees as the carrier is frequency modulated. The "90 degree" signal is applied to the outside ends of the diodes while the 180 degree signal is applied to the diode common point which then act as a quadrature detector. Here is the completed discriminator assembly:

The black wire with the brass clip is for connecting to a tap on the resonator coil. I made it this way so that I could find the best position. Optimum turns out to be about 1.5" from ground. The pickup loop is the single loop of blue wire with the audio output tap at the halfway position (black wire).

Here it is with everything connected and in position:

Now, with everything connected up, and after finding the best position for the pickup coil I found that the discriminator appears to be working correctly. Tuning is sharper, and there are no double slopes to select as I could previously. Audio level has increased significantly too. This is something I had been wondering about. I was concerned that a true discriminator would be less sensitive than a slope detector, but it appears not to be the case. I've very happy with the results so far. Tomorrow I intend to connect a signal generator to do some tests. I should be able to see a DC level go from negative to zero to positive as I tune through the resonant frequency. The only problem so far is that it tunes low. I'm only getting the bottom half of the FM band. So, I'll have to disassemble it and cut a bit off the resonator coil to bring

the frequency up. Once it's tuning the whole band, I'll be able to solder the one edge seam of the resonator shield which should bring the Q up a bit more. I'll also permanently mount the pickup loop through the side of the resonator. FM Crystal Radios? copyright © 2006 by Larry J Solomon

I have heard, even from a physicist, that it is impossible to build FM crystal radios. On the other hand some experimenters claim that they have built them. This argument intrigued me to try and build an FM crystal radio, which I have done successfully. To my surprise, the result is an astounding performer, pulling in four local stations in Tucson. When connected as a receiver to a good sound system the sound fidelity is as good or better than more expensive AM radios. In fact, it sounds "high-fidelity".

This picture shows the Solomon FM Crystal Set in an acrylic display case. I made the set specifically to fit inside this case (the case came first).

My definition of a crystal radio is one that is not powered, except by the radio transmission itself and employs a crystal detector. So, it should work without any batteries or AC power. An FM crystal receiver must be able to detect and receive FM signals well enough to be heard in earphones without any such extra power. This FM receiver is an amazing performer. It has crystal clear reception (pun intended), good sensitivity, but only fair selectivity. This set was a discovery for me. I started out by deg and building the normal AM sets. Then one day while testing the "Mystery" set (see my other web links), to my surprise, in addition to the expected panoply of AM stations, I heard a very faint signal that I could not tune out. At first, it seemed too weak to identify. When I tuned out all the AM stations, I was astonished to hear the announcement "KiiM FM, 99.5"! This is a country music FM station here in Tucson. It was all over the dial, untunable, but the much louder AM signals masked it when they were tuned in. I set myself the task of trying to improve the FM reception. I tried some simple circuit modifications that did not seem to improve anything. Then I connected a dipole antenna instead of the AM antenna I normally use. Suddenly, the FM signal was much clearer, although still weak. By using the audio output and sound system amplifier, I was even more amazed that four different FM stations came in loud (or rather medium) and clear. I found that changing the telescoping antenna length and position I could tune the stations in and out. They were KRQ, KLPX, KiiM, and KHYT all local FM stations with transmitters nearby. Their reception was also affected by the length and position of the audio output cable. After doing some research, I discovered that there was a physical theory that claimed that FM reception was possible and even probable using the same circuit as an AM receiver. The theory is called "slope detection". So, I set out to find circuit improvements. A web search yielded little, mostly theory. But there was enough information that I thought I could make some modifications to the AM circuits to make them more tunable to FM signals and less tunable to AM. Since FM operates at higher frequencies, all I had to do, I thought, was make the coil and caps smaller. After much "tinkering" I arrived at the current circuit. The circuit looks identical to a classic AM crystal circuit but is even simpler to build. The components were reduced in dimension to resonate at higher frequencies. This was done by experimenting with smaller and smaller coils and capacitors. The antenna is also much reduced in size (from that of AM) to resonate at higher frequencies (the antenna is crucial). The air variable capacitor I used has two trimmers in it which should be adjusted for best reception.Ihave found that a commonly available vernier dial and knob will fit the capacitor nicely. See end of article for a picture of the variable. C3 is a ceramic capacitor of 18 pf, but may be anywhere from 10 to 50pf. A detected FM signal is converted to AM due to an effect called slope detection that modulates amplitude. This FM Crystal Set works best near the transmitter (I have not tested it beyond about 10 miles). Secondly, the sound level is quiet, especially without an amplifier. A quiet room is needed for listening with earphones. One must be willing to move the set around to find a location for the best reception of

signals. However, in addition to listening with high impedance earphones (crystal or otherwise), the set can be connected directly to an audio amplifier's low level magnetic input which can then play amplified through a sound system at any volume -- sounds GREAT. In fact, I recommend starting tests with the FM crystal set by connecting it to the low-level phono inputs of a receiver or preamplifier. (Nowadays, many receivers don't even have a phono input!) That way you can crank up the volume, which makes it more likely to find the FM stations. If no signals are detected, I also recommend connecting an external "rabbit ear" antenna or hanging a short wire (12 inches or so) in various positions next to the internal antenna. The variable length of rabbitt ears can help to tune in stations. No additional wiring or antenna is necessary (the antenna is optimized in length for FM.)

L - 4 turns #18 copper or silver wire, 12mm inside diameter, tapped at 2.5 turns Ant - 7 inches of #18 bare copper wire C1 - 18 pf ceramic capacitor C2 - 50 pf air variable capacitor D - 1N34 diode or rock crystal R - 150K resistor The diode is tapped directly to the antenna. The vernier dial fits directly on the tuning capacitor. The antenna parallels the perimeter of the acrylic face plate. "Military style" #18 AWG wiring is used without any insulation. It is important to keep the components physically close together. The component specifications are the same as in circuit #2. The coil is silver rather than copper, but

copper does just as well. I think that the contrast of the silver and copper is beautiful. The coil was wrapped around a Sharpie Permanent Marker, then slipped off and expanded slightly. The wooden base is made from lacquered, polyurethane padouk. I consider this set a work of art as well as science and think it is the most elegant crystal receiver I have created. I love the contrast of the silver coil, the copper antenna, the clear acrylic faceplate, the black vernier dial, the white and transparent variable capacitor, and the subtle colorings on the resistor, the diode, and the lucite base. Yet the circuit is so ridiculously simple that some will not believe it is possible without building it themselves. No shielding is necessary, and there is no problem with hand capacitance. However, the output cable position may affect reception sensitivity. Photos of wired circuit

A hand is included in this photograph to show scale. Note the military style wiring, diode, and antenna. I wanted the wiring to create a modern design similar to a Mondrian painting. Not only is this set beautiful, it works! No power and no long antenna! It looks like a work of fiction.

Is this thing imaginary -- science fiction? Well, imagination did play a part, but it is definitely not science fiction. This shot shows the elegance of the FM set best, I think. There is only one resistor and one fixed capacitor.

The inside of the tuning capacitor and the phono jack/output can be seen here. Can you spot the fixed ceramic capacitor? Note the polished edge of the face plate and the reflection in the wooden base.

A quarter-inch piece of lucite was fitted under the tuning capacitor to anchor it. Note the two tiny trimmers on the back of the tuning capacitor. Brass screws were used to enhance appearance.

The vernier dial is large to accomodate ease of tuning, and the vernier makes it easy to separate stations. Two golden (brass) wood screws fix the face plate to the base. Holes for the face plate were made with special plastic drills, but ordinary drills may be used if drilled very SLOWLY. The knob is removable.

FM Crystal Circuit #2

L - 5 turns AWG#18 bare copper or silver wire, 12mm inside diameter, tapped at 2.5 turns D - 1N34 or rock crystal diode C1 - 82 pf capacitor C2 - 80 pf air variable capacitor C3 - 18 pf capacitor R - 150K resistor The following photographs show the circuit wired with the handmade Saturn Dial. and knob. It is perhaps not as visually striking as set No. 1, but it works just as well. In fact, this set was the original version. Notice that all the wiring and coil are copper.

The Saturn dial and knob were fashioned from a "doll's head" from Michael's Arts and Crafts, a piece of lucite cut with two circle cutters, and a brass paper fastener. The knob is fixed to the tuning capacitor with a small machine screw that fits in the hole below the brass fastener. The most difficult part of this was fashioning "Saturn's rings". This must be done very carefully and slowly. The inside edge should be cut slightly undersized and then sanded with a drum sander to fit snugly. The outside edges can be sanded with fine sandpaper and polished with a plastic polisher.

The air variable capacitor may be obtained from Electronix Express at http://www.elexp.com/. Part number 14VCRF10-280P. The 80 pf side is recommended for the second circuit, s 2-3. s 1 and 3 were used for the first circuit (50pf). 

OSC: 5-59 pf



ANT: 5-142 pf



OSC and ANT Trimmer 10pf range

Driver and 1 Sq. Meter Loop for 187 KHz A pair of bipolar transistors makes a 1 watt output stage that drives a 1 square meter loop antenna. Notice: Before operating a radio transmitter, find out what kind of transmitter operation, if any, is permitted in your locality. Radio transmitter operation is a serious legal matter. In the United States, operation of unlicensed intentional radiators is covered by Part 15 of Title 47 of the Code of Federal Regulations. This design can be readily adapted to different frequencies and different power levels. If you choose to build and operate the transmitter described here, you do so at your own risk. I'm only publishing this as an example of what can be done.

Overview

This page describes an antenna, a 1 watt final output stage, and a transformation network for radio experiments at 187 kHz. The Antenna

The Bangkok antenna was made with a frame of 1/2" PVC pipe and running a length of 0.5 mm square stranded copper wire (1 conductor of zip cord) through it. 90 Degree elbows are at three corners and a "T" is at the fourth corner (lower right-hand corner in this photograph), providing an exit for the 50 cm wire leads. A second antenna made in Arizona looked the same, including the slightly un-square look, but is white. I built a loop antenna that is 1 meter on each side. The reason for building this first antenna was to see if I could successfully load the transmitter into it. Why one meter on each side?) PVC pipe comes in 2 meter lengths (in Bangkok, anyway. In Arizona the lengths of 1/2 inch schedule F pipe were a little more than 3 meters in length). My interest in loop antennae comes from the fact that they are much simpler and more compact than capacitive antennas. No capacitive top hat, no loading coils, and no ground radial system to worry about. In the U.S., the Federal Communications Commission rules limit unlicensed transmitters in the 160 to 190 kHz band to a maximum input power to 1 watt and maximum antenna length of 15 meters. With such a small antenna compared to wavelength, the radiation resistance would be very low, and real (Ohmic) resistance in the antenna would tend to be a lot larger than the radiation resistance, making the antenna even less efficient. This puts a on getting the real resistance of the antenna as low as one practically can. Some have made antennae from copper water pipe and RG 8/U coax shield, and they would be more efficient for their size than this one, which used 0.5 square mm copper wire. This 1 square meter antenna has radiation resistance of about 10 nanohoms, making the 0.4 Ohm resistance (including skin effect, which about doubled the resistance from the DC value) the dominant loss in the antenna. The lower the resistance, the more efficient it would be - and from these numbers, efficiency is nearly directly proportional to 1/R; cut the resistance in half and double the efficiency. The antenna, which is a square loop of 0.5 square mm copper wire 1 meter on each side, measured 6.2 uH and under 0.2 ohms DC.

The Output Stage

The output stage can be recognized as basically the same as published by MurryGreeman, Lyle Kehler, and Bill Ashlock (possibly among many others). This output stage has a rich lineage. Murray Greeman, ZLBPU is credited as the originator of the circuit as an LF output stage. Bill Ashlock found that the output signal was cleaner when the transistors has separate base driver resistors. I chose to move the clamping diodes to the end of the base resistors connected to the transistor bases and eliminate the third resistor in the base circuit that was in common with both inputs. I encourage you to do an internet search on Murray Greeman ZLBPU, Bill Ashlock, and Lyle Kehler) for more information on Lowfer transmitter and antennas. Here are some URL's to get you started. Though these links worked on the day I created this page, I don't make any promises to keep them up-to-date. I added a 2N7000 to gate the NPN output transistor as a means to modulate the stage with either CW (A1) or tone bursts (quasi-A2). I damaged several 2N7000's with ESD in earlier experiments and have started adding gate protection to some circuits. Here, an MPSH34 (chosen for no other reason than because I have a lot of them) is used. Signals below ground are clamped to one diode drop below ground. Signals more positive than about +6 volts are clamped by avalanching emitter-base junction. On the breadboard, the avalanche voltage was measured at 5.8 volts when the emitter was biased with 1.8 milliamps. Raising the Antenna's Impedance so it can be driven The reactance of the antenna at 190 kHz is = 2 x Pi x 190 kHz x 6.2 uH = 7.4 Ohms. Counting the DC resistance of less than 0.2 Ohms and the skin effect about doubles the resistance. Since the resistive losses only increases the total impedance by 0.2% (Z = Sqrt(L^2+R^2), so I will keep it simple by ignoring it. The resistive component largely determines antenna efficency as it is very large with respect to the antenna's radiation efficiency.

The the transistors in the output stage are only capable of driving up to a few hundred millliamps into the antenna, and with a load of 7.4 Ohms, saturation losses in the output stage would eat up most of the 1 watt input power permitted under FCC rules. The challenge became one of how to transform the antenna's impedance to one high enough for the output stage to drive. This challenge is the main reason for this experiment. Note that there is no need to "match" the antenna to any particular impedance, only to raise its impedance to that which the small output transistors can drive comfortably. The overall objectives are to get as many amps of RF flowing in the antenna as possible given the constraint of 1 watt maximum into the output stage. I spent a lot of time on step-down transformers, but they all had high leakage inductance (that's uncoupled inductance between the primary and secondary) and they took a substantial amount of wire, so the Ohmic losses in wire were significant. Even exotic multifilar windings (up to 9 parallel windings to reduce leakage inducatance), as recommended by a knowledgeable friend, did not improve the leakage inductance enough. Finally, I decided to try a capacitive impedance transformation circuit as shown in the schematic. While I would like to say that I solved a system of equations and then ordered capacitors with the needed values,. In reality, I had to make do with parts in my junk box, and this driver produces square waves, not sine waves, so the formulae I worked out for only useful for the fundamental, and would completely miss the effects of the higher harmonics. For example, during the switching transitions, the output stage attempt to change the capacitance of the matching network instantaneously, which would result in very high charging current. That's what the 10 Ohm resistor is for, but the way, to limit current on those edges.

The method was to pick a pair of capacitances which, when put in parallel resonates with the antenna's inducance, resonates at 189 kHz. The larger the ratio of lower capacitor's value to that of the upper capacitor, the larger the ratio of amplifier current to the, and then try each combination out with SPICE until I saw something I liked.

I finally settled on the combination of mylar capacitors shown in the schematic - .092 uf directly across the antenna and .0165 uf and a 10 Ohm resistor in series with the driver. The .092 uf capacitance was made by placing two .047 uf capacitors in parallel and the .0165 uf capacitance was made by putting two .033 uf capacitors in series. The results I checked the circuits frequency response with a tunable oscillator and confirmed that the peak in voltage across the coil is at 187 kHz Then I connected the output stage to a 187.5 kHz signal source (See

"187 KHz RF Source" also on this site.). The voltage at the decoupling capacitor on the output stage was 9.01 VDC and the input current, as measured across the 10 ohm resistor in series with the power supply, was 82.6 milliamps, for an input power of 744 milliwatts.

Top trace: Voltage out of amplifier -5 volts per division, Lower trace: current out of amplifier - 200 ma per division. Timebase: 1 us per division. Note the current spike that corresponds to the transitions of the output voltage.

The waveforms above show the signal coming out of the amplifier. Looking at the edge of the voltage pulse at the top, you can see that the transistor was having a hard time charging the capacitance of the matching network during the transitions. this is apparent by the low slew rate. The lower trace shows peak currents of about 325 milliamps peak during the transitions. The output signal, of both voltage and current, are rich in harmonics. The Q of the antenna circuit is about 30, giving it a 6 kHz 3db bandwidth. Look at the antenna signals below and see how well the network cleaned up the signals.

Top trace: Voltage across antenna inductance -10 volts per division, Lower trace: through antenna inductance- 1 amp per division.

Timebase: 1 us per division. Note that the voltage (upper trace) leads the current (lower trace) byt 90 degrees. This is good because it shows that the coil's impedance is mostly inductive. It also confirms that the currentand voltage probes were in the right branch of the circuit.

The current in the antenna inductance (lower trace, above) was 2.5 amps and the voltage across the coil (upper trace, above) was about 19 volts. Ignoring the edges of the square wave driving signal, this makes the ratio of amplifier output current to antenna current approx = 2.5A/250 ma = 10:1. This allows the transistors to safely stay in saturation while driving 1.25 amps peak into the antenna. Overall, this system leaves a lot to be desired in of efficiency. Just looking at the output stage and impedance transformation network for a moment, the real power delivered to the antenna (the resistive part) is that power that is actually delivered to the resistance wire losses are about 0.4 ohms. The 10 nano-ohms radiation resistance (the transmitted power) will be ignored for the moment. Power is calculated using RMS current.

The formula above calculates the real power in the antenna. Here, Power is the power dissipated in the antenna's resistance of about 0.4 ohms and IPP is the peak-to-peak sine wave current in the antenna, which is about 2.4 amps. The power into the output stage, not counting the power lost in the decoupling resistor in series with the power supply, is 9.01 volts x 82.6 milliamps = 744 milliwatts. This allowed me to calculate the approximate efficiency of the output stage and impedance transformation circuit. See the calculation below.

Where is the other 430 milliwatts?

Take a look at the dissipation of the 10 ohm resistor in series with the output of the output stage.

Voltage across 10 ohm resistor that is in series with the output stage's output. Vertical: 5 volts per division. Timebase: 1 us per division.

This is a good application for a wide band true RMS voltmeter, but I don't have one, so I look at this as one 800 nanosecond per cycle sine wave that is 6 volts peak-to-peak and another 4 microsecond sine wave that is 4 volts peak-to-peak, that repeats every 5 microsecconds. The positive half cycle of the 800 ns signal is followed by the positive half cycle of the 4 microsecond signal, to you have to stare at the picture for a movement to get the idea. I can figure the RMS power dissipated by the resistor for each little sine wave and then add them together in proportion to their total time. Total power in the resistor is about 230 milliwatts. From the input voltage measurement and output voltage wave form, the transistors are loosing about 1 volt out of 9, or about 11% of the input power in saturation losses. This would amount to 74.4 milliwatts. A reasonable allocation of the input power would be as follows: Transistor saturation losses 11% 74 milliwatts Losses in 10 Ohm resistor 31% 230 milliwatts Power to the antenna resistance 42% 314 milliwatts Other losses 16% 119 milliwatts The total power doesn't add up to 744 milliwatts because of rounding. So, why is there so much power being sucked up by the 10 Ohm resistor and in the "Other losses" category? Its because only about 81% of the power in the square wave is at the fundamental frequency and the impedance transformation network, including the antenna inductance, is a band filter. If the network were driven with a sine wave, more of the power would end up in the antenna, and indeed the 10 Ohm resistor would not be needed. Click herefor a brief explaination of where the 81% figure came from.

Compare this with the radiated power. If the antenna's radiation resistance is 10 nano-ohms, then the radiated power is 7.9 nanowatts, or 100% x 7..9nanowatts / 744 milliwatts = 0.000001% overall efficiency. The output stage's efficiency can be improved by increasing the power supply voltage and also increasing the ratio of capacitances in the capacitive impedance transformation section. The net effect of these changes will be to reduce the collector currents and also reduce transistors' saturation voltages and to reduce the fraction of the total power supply voltage represented by the transistors' saturation voltages. I suspect that a large part of the "Other losses" is dissipation losses in the mylar capacitors. Changing to a high efficiency ceramic capacitor would likely help. One thing this experiment did was reinforce the belief that its worth experimenting with other topologies to improve efficiency. Of course, the largest possible jump in overall efficiency would come from moving up to a full-sized 15 meter antenna. What topologies would be good ones to investigate? A class B linear amplifier driving a sine wave into the antenna would give about the same efficiency as seen here, but with more power being dissipated in the transistors. A class D switching amplifier (basically pulse width modulation fed into a low filter) could give substantially better performance. I believe the approach that is the subject of this page - driving a square wave into a filter is quite well suited to small, low-power short range applications, where low cost components, low voltage operation, and small size are important factors.

Epilog: A cleaner, kinder modulator

The lower 2N2222, connected as a slew rate limited switch reduces key clicks.

I was uncomfortable with the keying method used in the first experiments -that of gating the drive to the NPN output transistor because I am sure that "keyclicks" extended beyond the 160 kHz to 190 kHz band. My objective is a beacon sending Morse code by cw keying, rather than tone modulated Morse code. Toward this end. Even though the duty cycle would be very low and probably would comply with the requirement that out-of-band emissions be 20 db below in-band emissions, I modified the output stage to eliminate these emissions. The modificaiton was to add a second 2N2222 as a switch to modulate the power to the output stage. Notice that I reduced the 100 uf decoupling capacitor for the output stage to 1 uf to enable the transmitter to switch on and off quickly enough, and that the antenna returns to the negative end of the decoupling capacitor rather than to "ground". The lower 2N2222 is connected as an integrator, which is a type of low- filter.. The closed loop gain is set by the input resistor and the capacitor so that the output will be reduced 3db at frequencies above 1/(2Pi R C) = 338 Hz, with the output reduced by 6 db every time the frequency doubles. It doesn't take too many octaves to be 20 db down, as the FCC requires out-of-band emissions to be, so this can operate safely within about 2 kHz of the band edge. The saturation voltage of the 2N2222 was about 80 millivolts, substantially less than the 300 millivolts given in the data sheet. This puts the modulator's losses at 100 x 80 millivolts / 11.5 volts = 0.7% of the input power. I can live with that. , read that Part 15! Read the Liability Disclaimer at the bottom of this page, too. MODULATION MONITOR Modulation Monitor is a simple and useful circuit for hams. You can listen to on air transmission from your rig and be sure about the quality of the transmission. It requires no direct connection with the transmitter.

In the prototype, headphones of 1K impedance are used. If you want to use ordinary low impedance head phones, connect a suitable length of wire from the junction between the diode and the coil through a 1000pf capacitor. L1 is easily available from junk box of hams since it the same as that used by hams in their QRP Transmitter. The entire setup can be housed inside a plastic box and placed near your transmitter adjust gang condenser C1 for maximum sound. The circuit is for 7 MHz ham band. Suitably changing the values of L1 and C1, it can be used for other ham bands also.

L1

- 23 Turns 24 SWG Wound over 1.5Cm

dia PVC tube C1

- 2J PVC Gang Capacitor

D1

- IN34 or IN4148

C2

- 0.01 ceramic disc

EP

- 1 K OHMS Headphone

Here's a pint-sized crystal radio with enough oomph to drive a 2 1/2" speaker. This units selectivity is far better than you would expect to find in a crystal receiver and volume is equal to that obtained with a transistor. No external power source is required. The unusual selectivity of this radio is due to its special double-tuned circuit. A pair of diodes connected as a voltage-doubler provides the extra kick to operate the small speaker. An output jack is provided for headphone listening and for connecting the set to an amplifier. Construction. The model was built on a 2 1/2" x 4 1/2" wooden chassis with a 3 1/2" x 4 1/2" metal front . However, size is not critical, and other materials may be substituted if desired. Two standard ferrite loopsticks, L2 and L3, are used. Both must be modified by the addition of a second winding, L1 and L4, respectivly. Each of the added windings consists of 22 turns of No. 24 cottoncovered wire wound on a small cardboard tube as shown on the pictorial. (Actually, any wire from No. 22 to No. 28 with cotton or enamel insulation will do the job.) The diameter of the cardboard tube should be slightly larger than L2 and L3 so that L1 and L4 will slide over L2 and L3 easily. Resistor R1 is used only for feeding the set into an amplifier; it should be omitted for both earphone and loudspeaker operation. Trimmer capacitor C2 should be soldered across the stator terminals of two-

gang variable capacitor C1a/C1b, as shown. The speaker and output transformer can be mounted wherever convenient. After all the parts have been mounted on the chassis, wire them together following the theschematic and pictorial diagrams. Be sure that the diodes D1 and D2 and capacitors C3 and C4 are correctly polarized. Alignment and Operation. To align the receiver, first connect it to an antenna and ground. (The optimum length of the antenna varies with location, but 50 feet will usually be suitable in areas serviced by several broadcast stations.) Next, plug in a high-impedence earphone at jack J1. Tune in a station near the high-frequency end of the broadcast band--say 1500kc.--and adjust the trimmer capacitors on variable capacitor C1a/C1b for the loudest signal. Trimmer capacitor C2 should then be adjusted for the best selectivity and volume over the entire broadcast band. Finally, coils L1 and L4 can be optimumly positioned by sliding them back and forth over coils L2 and L3. If a nearby station interferes with the reception of a weaker one, tune the slug on L2 for minimum interference. For loudspeaker operation, simply unplug the earphone from J1-- strong local stations should come in with fair volume. To operate the set as an AM tuner, wire R1 in place and connect J1 to crystal-phono input of a preamplifier or integrated amplifier. The set should give excellent results with a quality hi-fi system. -[30]How It Works The receiver employs a double-tuned circuit feeding a crystal-diode voltage-doubler/detector which drives a small speaker. In operation, r.f. signals picked up by the antenna system are induced into coil L2 from the coil L1. The desired signsl is selected by tuned circuit C1a-L2 and coupled through capacitor C2 to a second tuned circuit C1b-L3, which improves the selectivity by narrowing the r.f. band. The twice-tuned r.f. signal is then induced into coil L4 from coil L3. The positive half of the r.f. signal appearing across L4 es through the diode D2 to charge the capacitor C4; the negitive half of the signal es through the diode D1 to charge capacitor C3. Polarities of the charges on C3 and C4 are such that the effective voltage is doubled. This voltage appears across the primary of output transformer T1, which changes the high impedence at the output of diodes D1 and D2 to the low impedence required by the speaker. When the high-impedence earphones are plugged into closed circuit jack J1, the speaker is disconnected and the output from the diodes feeds directly into the earphones. Optional load resistor R1 is placed across the output of the diodes when the receiver is used with an amplifier. Variations on a theme: If you're so inclined to, I would install R1 inside the shell of the phone plug going to the amp, negating the need for its occasional removal from the radio circuit.

Air variables seem to be scarcer than hen's teeth, and ganged ones, more so. With that in mind, somewhere I have a similar schematic that does the same thing but independently tunes L2/L3. The advantage is the ability to sharply null out an intefering signal. Keep in mind the best laid plans..vs. Reality. Well, you know how that works. This version won't be appearing soon.... :) John P48 Phantom power from 9V d.c. By SmudgerDOn 16 February, 2013 · Leave a Comment · In Electronics Design, Project Here are a couple of options for generating proper 48V phantom power from a 9V supply. Now, before we go any further, I should point out that the quiescent current for the first option is about 27mA and for the second about 52mA, so don’t be thinking you can run this from a PP3/1604/6LR61 battery – it will go flat in no time. Rather, these are circuits you might consider running from a substantial rechargeable battery pack or a 9V wall wart or stomp box supply. If you are not familiar with phantom powering, you should read my previous article on the subject here. The first circuit is a capacitive charge-pump design based on a TC1044S charge pump. I chose the Microchip product (over the industry standard 7660 device) because it is cheap, it works with a supply voltage up to 12V, and you can double the switching frequency of the original. Here’s the schematic:-

The TC1044S, along with D3, D4, C7 and C8 form a conventional charge-pump voltage doubler as figure 8 on page 6 of the Microchip data sheet. In this case, however, we have added a further six stages of charge pumpage to give a no-load voltage of about 63V at the cathode of D16. As soon as you load up the output of the charge pump, the voltage starts to droop – so after all the charge-pump stages, we have a linear regulator to drop the voltage down to 48V. The regulator is an LM317AHV (Fairchild) or LM317HV (National Semiconductor), which is the highvoltage version of the ’317. Also, the LM317 is a floating design, so (given that the LM317 drop-out voltage is about 1.5V), as the input voltage drops below 49.5V, so the output tracks at approximately 1.5V less than the input voltage; which is quite cool, because if you do run the circuit on batteries, then as the input voltage drops below 9V, so the output voltage falls off gracefully (instead of falling off a cliff as the regulator goes into shutdown). D3~D16 can be any small signal Schottky diode (the lower the Vf, the better). D1 and D2 can be almost any general rectifier at all, e.g. 1n914, 1n4148, 1n4001~7. The BAV21 is a beefy 1N4148. There are some drawbacks to this design. Firstly, it has significant switching losses. The quiescent current for a 1-stage charge pump is about zero. The quiescent current for the above design (bearing in mind that the LM317 has a minimum load current of 3.5mA to maintain output regulation) is 27mA @ 9.2V. And, when you put a significant load on the output, the conversion efficiency for the load current is about 65%, which is pants. Secondly, in order to provide 10mA at 48V as per IEC61938 spec., you need an input voltage of at least 9.5V. Thirdly, it will only do one ‘lot’ of phantom power, and fourthly, it switches at 20kHz, which may be a problem for you if you’re doing high-quality digital audio. So, cue design number two. This is straight off the Texas Instruments WEBENCH online design package. I specified a switcher with 8.5V to 14.5V input, 48V output at 20mA. Here’s the schematic: -

Before we go any further, it’s worth noting that Ti have a number of switching devices which are less overkill than the above circuit, but – without exception – all of the devices are only available in tiny surface-mount packages and as such, not Stompville style. So this design has the good old LM2585 at its heart. The switching frequency is 100kHz, which is good for digital audio sample rates up to 48kHz. If you’re sampling at 96kHz, good luck to you. There’s not much to say about the circuit – except that you have a wide choice of suitable inductors. Basically you will likely get away with any inductor that has inductance of 0.56 ~ 1.5 mH (or, if you like 560 ~ 1500 uH), providing the inductor has a current rating in excess of 0.5A and a d.c. resistance below about 2 Ohms. Note that an iron-powder toroidal-cored inductor will have better EMC performance than a ferrite-cored bobbin inductor and is therefore preferable. I used a Bourns 2124-H-RC from Farnell (1mH, toroidal, 0.4 Ohm, 1.3A), because it was the best-value option they had in stock on the day. I have a Panasonic ELC12D561E (560 uH ferrite bobbin inductor) to try as a absolute-minimum suitable spec. part, but I haven’t got round to trying it yet. Also, D1 can be any Schottky diode with a Vrrm of 100V (minimum) and a current rating of 1 A (minimum). As you would expect, this design works flawlessly, maintaining a steady output of 48V/10mA from a supply between about 3.6 ~ 24V d.c. (You can go higher, but 24V was the highest voltage I had available when I wrote this article). Here’s photo’s of the two designs. The PCB’s are designed to fit a Hammond 27134PSLA or 1590B stomp box in a similar manner to my re-Stomp re-amping project: -

Note that the LM2585 design has a surface-mount LM2585S on the rear. This is just about the smallest surface-mount item I am prepared to solder by hand! Finally, here’s a table comparing the specification and relative performance of the two designs:-

Notes: 1.

TC1044S design has a startup time of several seconds if all the capacitors are discharged.

2. For IEC61938 compliance. The TC1044S circuit will run at voltages down to 1.5V. For a 9V d.c. input you can get about 7mA at 48V output. The LM2585 design will probably accept input voltage in excess of 24V. 3. The TC1044S circuit will produce more than 10mA, but the input voltage will need to be increased accordingly to maintain output voltage. The TC1044S has a maximum input voltage of 12V, so running the circuit off an unregulated 12V supply (even a 12V charger supply that runs to 13.8V) is not an option. 4. The LM2585 design is based around a load current of 20mA. If you want to pull significant current from the LM2585 design, you may need to change the inductor, the smoothing capacitors and the compensation network. Best to go back to the Ti WEBENCH software and see what you get. 5. This is calculated by taking the quiescent (no load current) as a baseline and then calculating the efficiency of the conversion of an additional 10mA of output current at 48V. Two final caveats: Firstly, the LM317HV data sheet specifies a standard value for R1 of 240 Ohms. However, this would cause the network of R1, PR1, R2 to consume several mA. So R1 is increased to 1k2 Ohm and PR1 and R2 increased accordingly. This gives a quiescent current through the R1, PR2, R2 network of approximately 1mA at 48V. With an LED having Vf = 1.7V, the current through the LED is 2.1 mA. The data sheet specifies a minimum load current on the LM317 of 3.5 mA for proper regulation. On the sample I had, 3.1 mA (i.e. 1 mA + 2.1 mA) was enough to get the regulator working properly. So, if you build the TC1044S design, don’t omit the LED and, if you find that the LM317 is not regulating, reduce the value of R3 to increase the LED current.

Secondly, pay careful attention to the voltage ratings of the electrolytic capacitors as shown on the schematics. If you over-voltage an electrolytic, it will overheat and its useful life will be severely shortened. It may even explode!