Editing Direction finding (section)

==History==

===Early mechanical systems===
[[File:Wade performing RDF.jpg|thumb|W.G. Wade of the National Bureau of Standards uses a large multi-loop antenna to perform RDF in this 1919 photo. This is a fairly small unit for the era.]]

The earliest experiments in RDF were carried out in 1888 when [[Heinrich Hertz]] discovered the directionality of an [[loop antenna|open loop of wire]] used as an antenna. When the antenna was aligned so it pointed at the signal it produced maximum gain, and produced zero signal when face on. This meant there was always an ambiguity in the location of the signal: it would produce the same output if the signal was in front or back of the antenna. Later experimenters also used [[dipole antenna]]s, which worked in the opposite sense, reaching maximum gain at right angles and zero when aligned. RDF systems using mechanically swung loop or dipole antennas were common by the turn of the 20th century. Prominent examples were patented by [[John Stone Stone]] in 1902 (U.S. Patent 716,134) and [[Lee de Forest]] in 1904 (U.S. Patent 771,819), among many other examples.

By the early 1900s, many experimenters were looking for ways to use this concept for locating the position of a transmitter. Early radio systems generally used [[medium wave]] and [[longwave]] signals. Longwave in particular had good long-distance transmission characteristics due to their limited interaction with the ground, and thereby provided excellent [[great circle route]] [[ground wave propagation]] that pointed directly to the transmitter. Methods of performing RDF on longwave signals was a major area of research during the 1900s and 1910s.{{sfn|Yeang|2013|p=187}}

Antennas are generally sensitive to signals only when they have a length that is a significant portion of the wavelength, or larger. Most antennas are at least {{frac|1|4}} of the wavelength, more commonly {{frac|1|2}} – the [[half-wave dipole]] is a very common design. For longwave use, this resulted in loop antennas tens of feet on a side, often with more than one loop connected together to improve the signal. Another solution to this problem was developed by the [[Marconi]] company in 1905. This consisted of a number of horizontal wires or rods arranged to point outward from a common center point. A movable switch could connect opposite pairs of these wires to form a dipole, and by rotating the switch the operator could hunt for the strongest signal.{{sfn|Baker|2013|p=150}} The [[US Navy]] overcame this problem, to a point, by mounting antennas on ships and sailing in circles.{{sfn|Howeth|1963|p=261}} Such systems were unwieldily and impractical for many uses.{{sfn|Yeang|2013|p=188}}

===Bellini–Tosi===
[[File:Radiogoniometer_RN_S25_internal_workings.jpg|thumb|right|This Royal Navy model is typical of B–T goniometers. The two sets of "field coils" and the rotating "sense coil" are visible.]]
A key improvement in the RDF concept was introduced by Ettore Bellini and Alessandro Tosi in 1909 (U.S. Patent 943,960). Their system used two such antennas, typically triangular loops, arranged at right angles. The signals from the antennas were sent into coils wrapped around a wooden frame about the size of a [[Beverage can|pop can]], where the signals were re-created in the area between the coils. A separate loop antenna located in this area could then be used to hunt for the direction, without moving the main antennas. This made RDF so much more practical that it was soon being used for navigation on a wide scale, often as the first form of aerial navigation available, with ground stations homing in on the aircraft's radio set. [[Bellini–Tosi direction finder]]s were widespread from the 1920s into the 1950s.

Early RDF systems were useful largely for long wave signals. These signals are able to travel very long distances, which made them useful for long-range navigation. However, when the same technique was being applied to higher frequencies, unexpected difficulties arose due to the reflection of high frequency signals from the [[ionosphere]]. The RDF station might now receive the same signal from two or more locations, especially during the day, which caused serious problems trying to determine the location. This led to the 1919 introduction of the [[Adcock antenna]] (UK Patent 130,490), which consisted of four separate monopole antennas instead of two loops, eliminating the horizontal components and thus filtering out the [[sky wave]]s being reflected down from the ionosphere. Adcock antennas were widely used with Bellini–Tosi detectors from the 1920s on.

The US Army Air Corps in 1931 tested a primitive radio compass that used commercial stations as the beacon.<ref>[https://books.google.com/books?id=8ycDAAAAMBAJ&dq=Popular+Science+1931+plane&pg=PA54 "Broadcast Station Can Guide Flyer", April 1931, Popular Science]</ref>

===Huff-duff===
[[File:HMS Belfast - Huff Duff.jpg|thumb|upright|right|FH4 "Huff-duff" equipment on the museum ship {{HMS|Belfast|C35|6}}]]

A major improvement in the RDF technique was introduced by [[Robert Watson-Watt]] as part of his experiments to locate [[lightning]] strikes as a method to indicate the direction of thunderstorms for sailors and airmen. He had long worked with conventional RDF systems, but these were difficult to use with the fleeting signals from the lightning. He had early on suggested the use of an [[oscilloscope]] to display these near instantly, but was unable to find one while working at the [[Met Office]]. When the office was moved, his new location at a radio research station provided him with both an [[Adcock antenna]] and a suitable oscilloscope, and he presented his new system in 1926.

In spite of the system being presented publicly, and its measurements widely reported in the UK, its impact on the art of RDF seems to be strangely subdued. Development was limited until the mid-1930s, when the various British forces began widespread development and deployment of these "[[high-frequency direction finding]]", or "huff-duff" systems. To avoid RDF, the Germans had developed a method of broadcasting short messages under 30 seconds, less than the 60 seconds that a trained Bellini-Tosi operator would need to determine the direction. However, this was useless against huff-duff systems, which located the signal with reasonable accuracy in seconds. The Germans did not become aware of this problem until the middle of the war, and did not take any serious steps to address it until 1944. By that time huff-duff had helped in about one-quarter of all successful attacks on the U-boat fleet.

===Post-war systems===
Several developments in electronics during and after the [[Second World War]] led to greatly improved methods of comparing the phase of signals. In addition, the [[phase-locked loop]] (PLL) allowed for easy tuning in of signals, which would not drift. Improved [[vacuum tube]]s and the introduction of the [[transistor]] allowed much higher frequencies to be used economically, which led to widespread use of VHF and UHF signals. All of these changes led to new methods of RDF, and its much more widespread use.

In particular, the ability to compare the phase of signals led to phase-comparison RDF, which is perhaps the most widely used technique today. In this system the loop antenna is replaced with a single square-shaped [[ferrite core]], with loops wound around two perpendicular sides. Signals from the loops are sent into a phase comparison circuit, whose output phase directly indicates the direction of the signal. By sending this to any manner of display, and locking the signal using PLL, the direction to the broadcaster can be continuously displayed. Operation consists solely of tuning in the station, and is so automatic that these systems are normally referred to as [[automatic direction finder]].

Other systems have been developed where more accuracy is required. [[#Pseudo-doppler DF technique|Pseudo-doppler radio direction finder]] systems use a series of small dipole antennas arranged in a ring and use electronic switching to rapidly select dipoles to feed into the receiver. The resulting signal is processed and produces an audio tone. The phase of that audio tone, compared to the antenna rotation, depends on the direction of the signal. Doppler RDF systems have widely replaced the huff-duff system for location of fleeting signals.

=== 21st century ===
The various procedures for radio direction finding to determine position at [[ocean|sea]] are no longer part of the maritime safety system [[GMDSS]], which has been in force since 1999. The striking cross frame antenna with attached auxiliary antenna can only be found on the signal masts of some older ships because they do not interfere there and dismantling would be too expensive.

Modern positioning methods such as GPS, DGPS, radar and the now-outdated Loran C have radio direction finding methods that are imprecise for today's needs.

Radio direction finding networks also no longer exist.<ref>{{Cite web |title=Die Geschichte des Funkpeilens |url=http://www.seefunknetz.de/peilen.htm |access-date=2023-08-11 |website=www.seefunknetz.de}}</ref>  However rescue vessels, such as [[RNLI]] lifeboats in the UK, and Search and Rescue helicopters have direction finding receivers for marine VHF  signals and the 121.5&nbsp;MHz homing signals incorporated in [[EPIRB]] and PLB beacons, although modern GPS-EPIRBS and AIS beacons are slowly making these redundant.