In the context of aviation and aerospace, an Automatic Direction Finder (ADF) is a radio navigation system that uses a directional antenna to determine the direction of a nearby radio transmitter, typically a Non-Directional Beacon (NDB). The system is used to help pilots and navigators determine their bearing, or direction, relative to the transmitter.

An ADF typically consists of a directional antenna, a receiver, and a display unit. The antenna is usually a loop antenna or a sense antenna, which is designed to receive radio signals and produce a directional pattern. The receiver tunes into the frequency of the NDB, and the display unit shows the direction of the signal, usually as a needle or pointer on a dial.

When a pilot tunes into an NDB frequency, the ADF system detects the signal and displays the direction of the transmitter relative to the aircraft's current heading. This direction is usually shown as a magnetic bearing, which can be used to determine the aircraft's position and course.

ADF systems were widely used in aviation navigation before the advent of more modern navigation systems, such as GPS and VOR (VHF Omnidirectional Range). While ADF is still used in some aircraft, it has largely been replaced by more accurate and reliable navigation systems.

In summary, an Automatic Direction Finder (ADF) is a radio navigation system that uses a directional antenna to determine the direction of a nearby radio transmitter, helping pilots and navigators determine their bearing and position in aviation and aerospace applications.