Editing Direction finding (section)

===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.]]