The Automatic Direction Finder is a primary radio navigational aid, providing a pilot or operator with a direct indication of the relative bearing of any radio transmitter to which the equipment is tuned, normally in the band 150 to 1,800 Kc/s

Using a single transmitter the co-ordinates of which are known, a simple homing can be obtained and with the use of two or more transmitters it is possible to obtain a position fix

To this end small beacon transmitters are often installed at airfields Most homing procedures terminate with the passage of the aircraft over the top of the beacon and this is indicated by a reversal of the bearing indicator needle