Automatic direction finder

[3][4] ADF receivers are normally tuned to aviation or marine NDBs (Non-Directional Beacon) operating in the LW band between 190 – 535 kHz.

The operator tunes the ADF receiver to the correct frequency and verifies the identity of the beacon by listening to the Morse code signal transmitted by the NDB.

[5] On marine ADF receivers, the motorized ferrite-bar antenna atop the unit (or remotely mounted on the masthead) would rotate and lock when reaching the null of the desired station.

On aviation ADFs, the unit automatically moves a compass-like pointer (RMI) to show the direction of the beacon.

The electronic sensors listen for the trough that occurs when the antenna is at right angles to the signal, and provide the heading to the station using a direction indicator.

ADF receivers can be used to determine current position, track inbound and outbound flight path, and intercept a desired bearing.

[7] As an aircraft nears an NDB station, the ADF becomes increasingly sensitive, small lateral deviations result in large deflections of the needle which sometimes shows erratic left/right oscillations.

Homing is flying the aircraft on the heading required to keep the needle pointing directly to the 0° (straight ahead) position.

Homing is regarded as poor piloting technique because the aircraft may be blown significantly or dangerously off-course by a cross-wind, and will have to fly further and for longer than the direct track.