Editing Direction finding (section)

==Equipment==
[[File:Earhart-electra 10.jpg|thumb|right|[[Amelia Earhart]]'s [[Lockheed Model 10 Electra]] with the circular ''RDF'' aerial visible above the cockpit]]

A '''radio direction finder''' ('''RDF''') is a device for finding the direction, or [[Bearing (navigation)|''bearing'']], to a [[radio]] source. The act of measuring the direction is known as '''radio direction finding''' or sometimes simply '''direction finding''' ('''DF'''). Using two or more measurements from different locations, the location of an unknown transmitter can be determined; alternately, using two or more measurements of known transmitters, the location of a vehicle can be determined. RDF is widely used as a [[radio navigation]] system, especially with boats and aircraft.

RDF systems can be used with any radio source, although the size of the receiver antennas are a function of the [[wavelength]] of the signal; very long wavelengths (low frequencies) require very large antennas, and are generally used only on ground-based systems. These wavelengths are nevertheless very useful for marine [[navigation]] as they can travel very long distances and "over the horizon", which is valuable for ships when the line-of-sight may be only a few tens of kilometres. For aircraft, where the horizon at altitude may extend to hundreds of kilometres, higher frequencies can be used, allowing much smaller antennas. An automatic direction finder, often capable of being tuned to commercial [[AM radio]] transmitters, is a feature of almost all modern aircraft.

For the military, RDF systems are a key component of [[signals intelligence]] systems and methodologies. The ability to locate the position of an enemy transmitter has been invaluable since [[World War I]], and it played a key role in [[World War II]]'s [[Battle of the Atlantic]]. It is estimated that the UK's advanced "[[high-frequency direction finding|huff-duff]]" systems were directly or indirectly responsible for 24% of all [[U-boat]]s sunk during the war.<ref>{{cite web |first=Arthur O. |last=Bauer |url=http://www.xs4all.nl/~aobauer/HFDF1998.pdf |title=HF/DF An Allied Weapon against German U-boats 1939–1945 |date=27 December 2004 |access-date=2008-01-26}} A paper on the technology and practice of the HF/DF systems used by the Royal Navy against U-boats in World War II</ref> Modern systems often use [[phased array]] antennas to allow rapid [[beam forming]] for highly accurate results. These are generally integrated into a wider [[electronic warfare]] suite.

Several distinct generations of RDF systems have been used over time, following new developments in electronics. Early systems used mechanically rotated antennas that compared signal strengths from different directions, and several electronic versions of the same concept followed. Modern systems use the comparison of [[phase (waves)|phase]] or [[doppler effect|doppler techniques]] which are generally simpler to automate. Modern [[pseudo-Doppler direction finder]] systems consist of a number of small antennas fixed to a circular card, with all of the processing performed by software.

Early British [[radar]] sets were also referred to as RDF, which was a deception tactic. However, the terminology was not inaccurate; the [[Chain Home]] systems used separate omnidirectional broadcasters and large RDF receivers to determine the location of the targets.<ref name="battleofbritain1940.net"/>

===Antennas===
In one type of direction finding, a directional [[antenna (electronics)|antenna]] is used which 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 by pointing it in the direction where the maximum signal level is obtained. Since the directional characteristics can be very broad, large antennas may be used to improve precision, or null techniques used to improve angular resolution.

[[File:20070917-Piraeus-TB AgiaVarvara.jpg|thumb|The crossed-loops antenna atop the mast of a tug boat is a direction-finding design.]]

====Null finding with loop antennas====
{{Main|Null (radio)}}
A simple form of directional antenna is the [[loop antenna|loop aerial]]. This consists of an open loop of [[wire]] on an insulating frame, or a metal ring that forms the antenna's loop element itself; often the diameter of the loop is a tenth of a wavelength or smaller at the target frequency. Such an antenna will be ''least'' sensitive to signals that are perpendicular to its face and ''most'' responsive to those arriving edge-on. This is caused by the phase of the received signal: The difference in electrical phase along the rim of the loop at any instant causes a difference in the voltages induced on either side of the loop.

Turning the plane of the loop to "face" the signal so that the arriving phases are identical around the entire rim will not induce any current flow in the loop. So simply turning the antenna to produce a ''minimum'' in the desired signal will establish two possible directions (front and back) from which the radio waves could be arriving. This is called a ''null'' in the signal, and it is used instead of the strongest signal direction, because small angular deflections of the loop aerial away from its null positions produce much more abrupt changes in received current than similar directional changes around the loop's strongest signal orientation. Since the null direction gives a clearer indication of the signal direction – the null is "sharper" than the max – with loop aerial the null direction is used to locate a signal source.

A "sense antenna" is used to resolve the two direction possibilities; the sense aerial is a non-directional antenna configured to have the same sensitivity as the loop aerial. By adding the steady signal from the sense aerial to the alternating signal from the loop signal as it rotates, there is now only one position as the loop rotates 360° at which there is zero current. This acts as a phase reference point, allowing the correct null point to be identified, removing the 180° ambiguity. A [[dipole antenna]] exhibits similar properties, as a small loop, although its null direction is not as "sharp".

====Yagi antenna for higher frequencies====
The [[Yagi antenna|Yagi-Uda antenna]] is familiar as the common [[VHF]] or [[Ultra high frequency|UHF]] [[television]] aerial. A Yagi antenna uses multiple dipole elements, which include "reflector" and "director" dipole elements. The "reflector" is the longest dipole element and blocks nearly all the signal coming from behind it, hence a Yagi has no front vs. back directional ambiguity: The maximum signal only occurs when the narrowest end of the Yagi is aimed in the direction from which the radio waves are arriving. With a sufficient number of shorter "director" elements, a Yagi's maximum direction can be made to approach the sharpness of a small loop's null. {{citation needed|date=October 2023}}

====Parabolic antennas for extremely high frequencies====
For much higher frequencies still, such as [[millimeter waves]] and [[microwaves]], [[parabolic antenna]]s or [[Dish antenna|"dish" antennas]] can be used. Dish antennas are highly directional, with the [[parabola|parabolic shape]] directing received signals from a very narrow angle into a small receiving element mounted at the focus of the parabola.

====Electronic analysis of two antennas' signals====
More sophisticated techniques such as [[phased array]]s are generally used for highly accurate direction finding systems. The modern systems are called [[goniometer#radio goniometer anchor|goniometers]] by analogy to [[World War II|WW&nbsp;II]] directional circuits used to measure direction by comparing the differences in two or more matched reference antennas' received signals, used in old [[signals intelligence]] (SIGINT). A modern [[helicopter]]-mounted direction finding system was designed by [[ESL Incorporated]] for the U.S. Government as early as 1972.

[[TDOA|Time difference of arrival]] techniques compare the arrival time of a radio wave at two or more different antennas and deduce the direction of arrival from this timing information. This method can use mechanically simple non-moving omnidirectional antenna elements fed into a multiple channel receiver system.

[[File:B-17F "Tom Paine" of the 388th Bomb Group, WW2.jpg|thumb|upright|right|The RDF antenna on this B-17F is located in the prominent teardrop housing under the nose.]]

===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.

===Single-channel DF===
Single-channel DF uses a multi-antenna array with a single channel radio receiver. This approach to DF offers some advantages and drawbacks. Since it only uses one receiver, mobility and lower power consumption are benefits. Without the ability to look at each antenna simultaneously (which would be the case if one were to use multiple receivers, also known as N-channel DF) more complex operations need to occur at the antenna in order to present the signal to the receiver.

The two main categories that a single channel DF algorithm falls into are ''amplitude comparison'' and ''phase comparison''. Some algorithms can be hybrids of the two.

====Pseudo-doppler DF technique====
The [[Doppler radio direction finding|pseudo-doppler technique]] is a phase based DF method that produces a bearing estimate on the received signal by measuring the [[doppler shift]] induced on the signal by sampling around the elements of a circular array.  The original method used a single antenna that physically moved in a circle but the modern approach uses a multi-antenna circular array with each antenna sampled in succession.

====Watson–Watt, or Adcock-antenna array====
{{main article|Adcock antenna}}
The [[Robert Watson-Watt|Watson-Watt]] technique uses two antenna pairs to perform an amplitude comparison on the incoming signal. The popular Watson-Watt method uses an array of two orthogonal coils (magnetic dipoles) in the horizontal plane, often completed with an omnidirectional vertically polarized electric dipole to resolve 180° ambiguities.

The [[Adcock antenna|Adcock antenna array]] uses a pair of monopole or dipole antennas that takes the vector difference of the received signal at each antenna so that there is only one output from each pair of antennas. Two of these pairs are co-located but perpendicularly oriented to produce what can be referred to as the N–S (North-South) and E–W (East-West) signals that will then be passed to the receiver.  In the receiver, the bearing angle can then be computed by taking the [[arctangent]] of the ratio of the N–S to E–W signal.

====Correlative interferometer====
The basic principle of the correlative interferometer consists in comparing the measured phase differences with the phase differences obtained for a DF antenna system of known configuration at a known wave angle (reference data set). For this, at least three antenna elements (with omnidirectional reception characteristics) must form a non-collinear basis. The comparison is made for different azimuth and elevation values of the reference data set. The bearing result is obtained from a correlative and stochastic evaluation for which the [[Correlation and dependence|correlation coefficient]] is at a maximum. If the direction finding antenna elements have a directional antenna pattern, then the amplitude may be included in the comparison. 
	
Typically, the correlative interferometer DF system consists of more than five antenna elements. These are scanned one after the other via a specific switching matrix. In a multi-channel DF system n antenna elements are combined with m receiver channels to improve the DF-system performance.