Automatic Direction Finder (ADF) Components

The airborne equipment includes two antennas, a receiver, and the indicator instrument. The "sense" antenna (non-directional) receives signals with nearly equal efficiency from all directions. The "loop" antenna receives signals better from two directions (bi-directional). When the loop and sense antenna inputs are processed together in the automatic direction finder (ADF) radio, the result is the ability to receive a radio signal well in all directions but one, thus resolving all directional ambiguity.

The indicator instrument can be one of three kinds: the fixed card automatic direction finder (ADF), movable-card automatic direction finder (ADF), or the radio magnetic indicator (RMI). The fixed-card automatic direction finder -ADF- (also known as the relative bearing indicator (RBI)), always indicates zero at the top of the instrument, and the needle indicates the RB to the station. Relative bearing on a fixed-card indicator figure indicates a RB of 135°, and if the MH is 45°, the MB to the station is 180°. (MH + RB = MB to the station.)

The movable-card automatic direction finder (ADF) allows the pilot to rotate the aircraft's present heading to the top of the instrument so that the head of the needle indicates MB to the station and the tail indicates MB from the station. Relative bearing on a movable-card indicator figure indicates a heading of 45°, the MB to the station is 180°, and the MB from the station is 360°.  

Relative bearing: The number of degrees measured clockwise between the heading of the aircraft and the direction from which the bearing is taken.

Magnetic heading (MH): The direction an aircraft is pointed with respect to magnetic north.
Magnetic bearing (MB): The direction to or from a radio transmitting station measured relative to magnetic north.

The RMI differs from the movable-card automatic direction finder (ADF) in that it automatically rotates the azimuth card (remotely controlled by a gyrocompass) to represent aircraft heading. The RMI has two needles, which can be used to indicate navigation information from either the automatic direction finder (ADF) or the VOR receivers. When a needle is being driven by the automatic direction finder (ADF), the head of the needle indicates the MB TO the station tuned on the ADF (automatic direction finder) receiver. The tail of the needle is the bearing FROM the station. When a VOR receiver drives a needle of the RMI, the needle indicates where the aircraft is radial with respect to the VOR station. The needle points to the bearing TO the station, as read on the azimuth card. The tail of the needle points to the radial of the VOR the aircraft is currently on or crossing. Radio Magnetic indicator figure indicates a heading of 005°, the MB to the station is 015°, and the MB from the station is 195°.