The practice and principle of locating the origin of a radio signal. A discriminating antenna and some form of receiver is required. Automatic direction finding carries out the process automatically using either a rotating directional antenna or two such antennas at right angles. The rotating antenna is often used in conjunction with a cathode-ray tube (CRT) as an indicator to display strength of signal against direction. The direction of maximum strength is the bearing of the radio source. When a CRT is used in this way the term cathode-ray direction finding is sometimes applied. Frame direction finding employs a loop antenna with a polar figure-of-eight directional response. At the point of zero signal the frame of the antenna points along the direction of propagation.

Direction finding in mountainous areas or in urban areas containing many very tall buildings can be subject to error because of reflections from the mountains or buildings. This is termed the mountain effect.