Editing Direction finding (section)

===Location of illegal, secret or hostile transmitters – SIGINT===
{{See also|High frequency direction finding|SIGINT}}
[[Image:British Post Office interference finding truck 1927.jpg|thumb|British Post Office RDF lorry from 1927 for finding unlicensed [[amateur radio]] transmitters.  It was also used to find [[regenerative receiver]]s  which radiated interfering signals due to feedback, a big problem at the time.]]

In World War II considerable effort was expended on identifying secret transmitters in the United Kingdom (UK) by direction finding. The work was undertaken by the [[Radio Security Service]] (RSS also MI8). Initially three U Adcock HF DF stations were set up in 1939 by the General Post Office. With the declaration of war, [[MI5]] and [[MI8|RSS]] developed this into a larger network. One of the problems with providing coverage of an area the size of the UK was installing sufficient DF stations to cover the entire area to receive [[skywave]] signals reflected back from the ionised layers in the upper atmosphere. Even with the expanded network, some areas were not adequately covered and for this reason up to 1700 voluntary interceptors (radio amateurs) were recruited to detect illicit transmissions by [[ground wave]]. In addition to the fixed stations, RSS ran a fleet of mobile DF vehicles around the UK. If a transmitter was identified by the fixed DF stations or voluntary interceptors, the mobile units were sent to the area to home in on the source. The mobile units were HF Adcock systems.

By 1941 only a couple of illicit transmitters had been identified in the UK; these were German agents that had been "turned" and were transmitting under MI5 control. Many illicit transmissions had been logged emanating from German agents in occupied and neutral countries in Europe. The traffic became a valuable source of intelligence, so the control of RSS was subsequently passed to MI6 who were responsible for secret intelligence originating from outside the UK. The direction finding and interception operation increased in volume and importance until 1945.

The HF Adcock stations consisted of four 10{{nbsp}}m vertical antennas surrounding a small wooden operators hut containing a receiver and a radio-[[goniometer]] which was adjusted to obtain the bearing. MF stations were also used which used four guyed 30{{nbsp}}m lattice tower antennas. In 1941, RSS began experimenting with spaced loop direction finders, developed by the Marconi company and the UK [[United Kingdom National Physical Laboratory|National Physical Laboratories]]. These consisted of two parallel loops 1 to 2{{nbsp}}m square on the ends of a rotatable 3 to 8{{nbsp}}m beam. The angle of the beam was combined with results from a radiogoniometer to provide a bearing. The bearing obtained was considerably sharper than that obtained with the U Adcock system, but there were ambiguities which prevented the installation of 7 proposed S.L DF systems. The operator of an SL system was in a metal underground tank below the antennas. Seven underground tanks were installed, but only two SL systems were installed at Wymondham, Norfolk and Weaverthorp in Yorkshire. Problems were encountered resulting in the remaining five underground tanks being fitted with Adcock systems. The rotating SL antenna was turned by hand which meant successive measurements were a lot slower than turning the dial of a goniometer.

Another experimental spaced loop station was built near Aberdeen in 1942 for the Air Ministry with a semi-underground concrete bunker. This, too, was abandoned because of operating difficulties. By 1944, a mobile version of the spaced loop had been developed and was used by RSS in France following the D-Day invasion of Normandy.

The US military used a shore based version of the spaced loop DF in World War II called "DAB". The loops were placed at the ends of a beam, all of which was located inside a wooden hut with the electronics in a large cabinet with [[cathode-ray-tube]] display at the centre of the beam and everything being supported on a central axis. The beam was rotated manually by the operator.

The [[Royal Navy]] introduced a variation on the shore based HF DF stations in 1944 to track U-boats in the North Atlantic. They built groups of  five DF stations, so that bearings from individual stations in the group could be combined and a mean taken. Four such groups were built in Britain at [[Ford End]], Essex, Goonhavern, Cornwall, Anstruther and Bowermadden in the Scottish Highlands. Groups were also built in Iceland, Nova Scotia and Jamaica. The anticipated improvements were not realised but later statistical work improved the system and the Goonhavern and Ford End groups continued to be used during the Cold War.  The Royal Navy also deployed direction finding equipment on ships tasked to [[anti-submarine warfare]] in order to try to locate German submarines, e.g. [[Captain class frigate]]s were fitted with a  medium frequency direction finding antenna (MF/DF) (the antenna was fitted in front of the bridge) and high frequency direction finding (HF/DF, "Huffduff") Type FH 4 antenna (the antenna was fitted on top of the mainmast).<ref name="WSInt III">Elliott (1972), p. 264</ref>

A comprehensive reference on World War II wireless direction finding was written by Roland Keen, who was head of the engineering department of RSS at Hanslope Park. The DF systems mentioned here are described in detail in his 1947 book ''Wireless Direction Finding''.<ref>
{{cite book
 | last=Keen | first=R 
 | title=Wireless Direction Finding 
 | edition=4th
 | year=1947
 | publisher=Iliffe
 | location=London, UK
}}</ref>

At the end of World War II a number of RSS DF stations continued to operate into the Cold War under the control of GCHQ the British SIGINT organisation.

Most direction finding effort within the UK now (2009) is directed towards locating unauthorised "[[pirate radio|pirate]]" FM broadcast radio transmissions. A network of remotely operated VHF direction finders are used mainly located around the major cities. The transmissions from mobile telephone handsets are also located by a form of direction finding using the comparative signal strength at the surrounding local "cell" receivers. This technique is often offered as evidence in UK criminal prosecutions and, almost certainly, for SIGINT purposes.<ref>
{{cite book
 | title=Electronic Countermeasures
 | editor1=J.A. Biyd |editor2=D.B. Harris |editor3=D.D. King |editor4=H.W. Welch Jr.
 | author=deRosa, L.A.
 | publisher=Peninsula Publishing
 | isbn=0-932146-00-7
 | chapter=Direction Finding
 | location=Los Altos, CA
 | year=1979
}}</ref>