Direction finding (DF), or radio direction finding (RDF), is the use of radio waves to determine the direction to a radio source. The source may be a cooperating radio transmitter or may be an inadvertant source, a naturally-occurring radio source, or an illicit or enemy system. Radio direction finding differs from radar in that only the direction is determined by any one receiver; a radar system usually also gives a distance to the object of interest, as well as direction. By triangulation, the location of a radio source can be determined by measuring its direction from two or more locations. Radio direction finding is used in radio navigation for ships and aircraft, to locate emergency transmitters for search and rescue, for tracking wildlife, and to locate illegal or interfering transmitters. During the Second World War, radio direction finding was used by both sides to locate and direct aircraft, surface ships, and submarines.

A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that for transmitting is usually fed by a balanced power source or for receiving feeds a balanced load. Within this physical description there are two distinct types:Large loop antennasLarge loops are also called self-resonant loop antennas or full-wave loops; they have a perimeter close to one or more whole wavelengths at the operating frequency, which makes them self-resonant at that frequency. They are the most efficient of all antenna types for both transmission and reception. Large loop antennas have a two-lobe radiation pattern at their first, full-wave resonance, peaking in both directions perpendicular to the plane of the loop Large loops are the most efficient, by an order of magnitude, of all antenna designs of similar size.Halo antennasHalos are often explained as shortened dipoles that have been bent into a circular loop, with the ends not quite touching. Some writers prefer to exclude them from loop antennas, since they can be well-understood as bent dipoles, others make halos an intermediate category between large and small loops, or the extreme upper size limit for small transmitting loops: In shape and performance halo antennas are very similar to small loops, only distinguished by being self resonant and having much higher radiation resistance. Small loop antennasSmall loops are also called magnetic loops or tuned loops; they have a perimeter smaller than half the operating wavelength. They are used mainly as receiving antennas, but are sometimes used for transmission despite their reduced efficiency; loops with a circumference smaller than about 1/ 10  wavelength become so inefficient they are rarely used for transmission. A common example of small loop is the ferrite (loopstick) antenna used in most AM broadcast radios. The radiation pattern of small loop antennas is maximum at directions within the plane of the loop, so perpendicular to the maxima of large loops.Small loops divide into two sub-types, depending on the purpose they are optimized for:ReceivingSmall receiving loops are compact antennas optimized to capture radio waves much longer than their size, where full-sized antennas would be either infeasible or impossible. If their perimeters are kept shorter than 1/ 10  wave, they have exceptionally precise "null" directions which gives a tiny antenna for exceedingly accurate direction-finding, better than most moderately large antennas, and as good as many huge antennas.TransmittingSmall transmitting loops are optimized for compact antennas that are the "least-worst" signal radiators. Small antennas of any kind are inefficient, but when a full-sized antenna is not practical, making a small loop with a perimeter as close to 1/ 2  wave as possible makes the small loop better for transmitting, although it sacrifices or outright loses the precise "null" direction of smaller small loops.