Editing Direction finding (section)

===Operation===
[[File:US Navy model DAQ high frequency radio direction finder 2.jpg|right|thumbnail|World War II US Navy high frequency radio direction finder]]

One form of radio direction finding  works by comparing the signal strength of a directional [[Antenna (electronics)|antenna]] pointing in different directions.  At first, this system was used by land and marine-based radio operators, using a simple rotatable loop antenna linked to a degree indicator.  This system was later adopted for both ships and aircraft, and was widely used in the 1930s and 1940s.  On pre-[[World War II]] aircraft, RDF antennas are easy to identify as the circular loops mounted above or below the fuselage. Later loop antenna designs were enclosed in an aerodynamic, teardrop-shaped fairing. In ships and small boats, RDF receivers first employed large metal loop antennas, similar to aircraft, but usually mounted atop a portable battery-powered receiver.

In use, the RDF operator would first tune the receiver to the correct frequency, then manually turn the loop, either listening or watching an [[S meter]] to determine the direction of the ''null'' (the direction at which a given signal is weakest) of a [[long wave]] (LW) or [[medium wave]] (AM) broadcast beacon or station (listening for the null is easier than listening for a peak signal, and normally produces a more accurate result).  This null was symmetrical, and thus identified both the correct degree heading marked on the radio's compass rose as well as its 180-degree opposite.  While this information provided a baseline from the station to the ship or aircraft, the navigator still needed to know beforehand if he was to the east or west of the station in order to avoid plotting a course 180-degrees in the wrong direction.  By taking bearings to two or more broadcast stations and plotting the intersecting bearings, the navigator could locate the relative position of his ship or aircraft.

Later, RDF sets were equipped with rotatable [[Loop antenna#AM broadcast receiving antennas|ferrite loopstick]] antennas, which made the sets more portable and less bulky.  Some were later partially automated by means of a motorized antenna (ADF).  A key breakthrough was the introduction of a secondary vertical whip or [[Loop antenna|'sense' antenna]] that substantiated the correct bearing and allowed the navigator to avoid plotting a bearing 180 degrees opposite the actual heading. The U.S. Navy RDF model SE 995 which used a sense antenna was in use during World War I.<ref>Gebhard, Louis A "Evolution of Naval Radio-Electronics and Contributions of the Naval Research Laboratory" (1979)</ref> After World War II, there were many small and large firms making direction finding equipment for mariners, including [[Raytheon|Apelco]], Aqua Guide, [[Bendix Corporation|Bendix]], Gladding (and its marine division, Pearce-Simpson), Ray Jefferson, [[Raytheon]], and [[Sperry Marine|Sperry]].  By the 1960s, many of these radios were actually made by Japanese electronics manufacturers, such as [[Panasonic]], [[Fuji Onkyo]], and [[Yoji Ito|Koden Electronics Co., Ltd.]]  In aircraft equipment, Bendix and [[Sperry Corporation|Sperry-Rand]] were two of the larger manufacturers of RDF radios and navigation instruments.