automatic direction finder

an aircraft radio direction finder for the automatic location of a bearing on transmitting ground stations. It usually consists of a receiver-indicator device coupled with a tracking system and two antennas—a directional loop and a nondirectional single wire antenna.

The signals from the radio station on which a bearing is being taken are received by both antennas and combined: the signal from the loop is switched with the frequency of a reference generator, producing an amplitude-modulated voltage at the receiver input. By comparing the voltages at the outputs of the receiver and the reference generator, an error signal is produced and then fed to the tracking system. The tracking system automatically rotates the loop to the position of minimum signal, which coincides with the bearing of the radio station. Sensors detect the angle of rotation of the loop and transmit it to an indicator with a pointer showing the relative angle of the radio station, that is, the angle between the longitudinal axis of the aircraft and the direction to the radio station.

Automatic direction finders generally operate with non-directional radio beacons, less often with other types, such as radio broadcast stations. Such devices make it possible to guide aircraft to the vicinity of an airfield, make a landing approach, and solve other navigational problems.