Direction finding (DF) refers to the establishment of the direction from which a received signal was transmitted. This can refer to radio or other forms of wireless communication. By combining the direction information from two or more suitably spaced receivers (or a single mobile receiver), the source of a transmission may be located in space via triangulation. Radio direction finding (RDF) equipment is used for land-mobile, fixed-site, vehicle tracking and location, shipboard, and airborne radio direction finding missions. Radio transmitters for air and sea navigation are known as beacons and are the radio equivalent to a lighthouse. Radio Direction Finder was once the primary form of aircraft and marine navigation.

Direction finding often requires an antenna that is directional – that is, more sensitive in certain directions than in others. Many antenna designs exhibit this property. For example, a Yagi antenna has quite pronounced directionality, so the source of a transmission can be determined simply by pointing it in the direction where the maximum signal level is obtained. However, to establish direction to great accuracy requires much more sophisticated techniques. A simple form of directional antenna is the loop aerial.

Automatic direction finder (ADF) is a marine or aircraft radio-navigation instrument that automatically and continuously displays the relative bearing from the ship or aircraft to a suitable radio station. Unlike the RDF, the ADF operates without direct intervention, and continuously displays the direction of the tuned beacon. Radio Direction Finding (RDF) technology is typically done at considerable distance (from yards to miles and beyond). To do that, transmitters require long-term power sources (such as batteries) and antennas. In addition, the transmitted signal can be detected by anyone with a receiver tuned to the proper frequency, so these systems are not covert.