An automatic direction finder is tuned to a nondirectional beacon, which transmits a radio wave that consists of an electric field and a magnetic field component propagating perpendicular to one another. An antenna on the automatic direction finder receives this wave and interprets it in order to continuously determine the direction of the beacon.
An automatic direction finder contains a loop antenna, which measures the phase difference induced by the magnetic field of the radio wave in the ferrite-based windings contained in the loop antenna. Using this phase difference, the loop antenna calculates two possible directions from which the nondirectional signal beacon may originate. A second antenna, called a sense antenna, determines which one of those directions is correct. The design of modern planes combines the loop and sense antennas into one unit, while older planes have two antennas.
In marine automatic direction finders, the antenna typically rotates and contains a compass that shows the direction of the nondirectional beacon signal. In planes, the automatic direction finder typically contains a radio-magnetic indicator, which both shows the location of the beacon and the distance from the aircraft to the beacon.
An automatic direction finder typically has two modes: an ANT mode and an ADF mode. While in ADF mode, the finder continuously points in the direction of the beacon, while ANT mode disables the loop antenna.