Rdf Continuation 1i3z5e

AURAL NULL DIRECTION FINDER Components and Types of Operation An aural null direction finder comprises essentially a directive array confined with a sensitive and highly stable receiver. For mobile installations, as on ship board or aircraft, a loop antenna is almost invariably used. The elements consist of  The loop antennas  A non directional vertical or sense antenna  A sense coupling circuit  A loop tuning and coupling circuit  A balancer circuit (usually used in ship board only)  A receiver  Three types of operation 1. Communication reception of modulated continuous waves or continuous wave signals. This is often carried out by means of the non-directional vertical antenna alone, or the loop may be used at or near its operations. 2. Bilateral Operations. Here the signal is picked up on the loop, which has of course two directions of null pick up. The result is an accurate bearing but with 180˚ ambiguity. If a balancer circuit is used, the signal is also coupled into the loop input circuit in phase quadrature from the nondirectional antenna and is adjusted in amplitude to give the sharpest loop minima. Either modulated CW and CW reception is possible. 3. Unilateral Receptions. This is also referred to as sense determination or operation with a cardiod pattern. Signals from the loop and the vertical antennas are combined in proper phase relationship to give a single null pattern. This mode of operation permits identification of the appropriate null as determined in bilateral operation, but the accuracy of the unilateral null is insufficient to allow precise bearing determination by this means alone. Loop circuits are almost invariably tuned either directly by means of a capacitor or through one or more coupling transformers. Satisfactorily operation is easily obtained at distances up to 35 ft separation between the loop and receiver proper. In modern direction finders the loop antenna is always shielded and is usually balanced to the ground. These precautions serve to minimise antenna effect and in the case of aircraft direction finders to reduce precipitation static to a minimum when the vertical antenna is coupled into the receiver at the same time. An example of Shipboard Direction Finder Circuits An aural-null system when it is used in marine service, it incorporates means of obtaining bilateral bearings including a balancer circuit and means for obtaining “sense”. The basic circuit for one band is shown in the figure on shipboard, the presence of metallic objects such as guys, stacks etc. tends to produce, by re-radiating, undesirable voltages in the loop, provision is made in this circuit to introduce a quadrature voltage which can be adjusted to cancel the undesirable picked-up voltage.

Bilateral Operation. Under these conditions this circuit contains the loop inductance L1 together with two auxiliary coils L3 and L4. All these are resonated by means of capacitor C1, which is balanced with respect to the ground. The output of the loop circuit is balanced to the ground but is fed to the single-sided amplifier V1 in such a fashion that only half of the voltage developed across C1 is utilized. ADCOCK DIRECTION FINDERS The horizontal of a loop are responsible for bearing error due to night effect. If these horizontal conductors could be eliminated or their effect nullified, accuracy would be improved. In the case of a single turn loop this can be done by removing the upper conductor, and shielding or balancing out the effect of the lower horizontal wire. Such a directive array is known as an Adcock Antenna. Two types of Adcock Systems

Comparison between Adcock and Loop direction finders 1. The loop antenna direction finder suffers from vertical and polarisation errors but in Adcock direction finder system these errors are minimised. 2. The loop direction finders are suitable at lower frequency, whereas Adcock direction finder is suitable for higher frequency. 3. The induced voltage in Adcock system is less in comparison to loop. 4. A moderate inductive internal impedance is present in the loop direction finder system whereas the Adcock direction finder presents high internal capacitive impedance.

GONIOMETER When it is desirable to avoid the necessity of physically rotating a loop or Adcock antenna, as when their dimensions are very large, it is possible to obtain the effect of rotation by employing two such antenna, at right angles to each other and combining their output in a goniometer as shown in the figure. The goniometer consists of two primary coils, one for each antenna, arranged at right angles to each other and coupled to a secondary as shown.

AUTOMATIC DIRECTION FINDERS (ADF) In distinction to the direction finders, there are numerous types in which the proper bearing is determined by means of a visual device. In this device, the operator is not required to listen for a minimum signal as in an aural-null type, reception with phones being needed only for station identification or for monitoring.

Visual direction finders may be grouped in two main classes. 1. Automatic direction finders (radio com) airborne equipment 2. Right-left type (Homing) NDB (Nondirectional Beacons) ground equipment Basically the airborne equipment such as ADF consists of the following: 1. Antenna system – consists of omnidirectional sense antenna and rotatable loop, but now a days fixed loop and electronic goniometer are mostly used. 2. Receiver – consists of RF and detector sections. The frequency range is between 195 to 1500 kHz. The output of the receiver is directional signal and indent. Speaker and headphone are used for voice and indent. The directional signals are fed to the display part of the receiver. 3. Display – can be Relative Bearing Indicator (RBI) or Radio Magnetic Indicator (RMI). Elements of Automatic Direction Finders

In the following condition the ADF will not be able to give accurate bearing: In case of heavy lighting it fails to indicate correct direction due to interference by the lighting. In case of heavy wind storm which makes aircraft to drift perpendicular to the direction of motion causes misleading of homing direction because the radio com will always be pointed towards the runway field. RADIO BEACON Radio beams works on the principle of the wireless light noise. it is being used in various forms such as 1. Flashing radio beacon 2. Revolving radio beacons 3. Radio-range beacons Flashing Radio Beacons Flashing radio beacons are the same as omnidirectional flashing lighthouse. The morse signal radiated at predefined intervals takes the place of the flashing of light houses and makes recognition easier. These beacons operate at frequency of 300 kHz and it may operate continuously in fog. Revolving Beacons Revolving beacons are used to determine the direction or bearing from a fixed point without the use of direction finding apparatus in the craft. A very simple receiving equipment is required to observe the instant at which a sharply focussed beam sweeps the sector or to detect a rise and fall in signal strength according to the beam rotation. All forms of revolving loop transmitter are liable to night effect. Radio Range Beacons Radio range beacons are operating at medium frequencies, are fixed in direction, and are used to assist the aircraft to land safely.