Editing Radar (section)

===Antenna design===
{{main|Antenna (radio)}}
[[File:Electronics Technician - Volume 7 - Figure 2-48.jpg|thumb|AS-3263/SPS-49(V) antenna (US Navy)]]
Radio signals broadcast from a single antenna will spread out in all directions, and likewise a single antenna will receive signals equally from all directions. This leaves the radar with the problem of deciding where the target object is located.

Early systems tended to use [[omnidirectional antenna|omnidirectional broadcast antennas]], with directional receiver antennas which were pointed in various directions. For instance, the first system to be deployed, Chain Home, used two straight antennas at [[right angle]]s for reception, each on a different display. The maximum return would be detected with an antenna at right angles to the target, and a minimum with the antenna pointed directly at it (end on). The operator could determine the direction to a target by [[rotation|rotating]] the antenna so one display showed a maximum while the other showed a minimum.
One serious limitation with this type of solution is that the broadcast is sent out in all directions, so the amount of energy in the direction being examined is [[inverse-square law|a small part]] of that transmitted. To get a reasonable amount of power on the "target", the transmitting aerial should also be directional.

====Parabolic reflector====
[[File:SPS-10 radar antenna on a Knox class frigate.jpg|thumb|right|Surveillance radar antenna]]
{{main|Parabolic antenna}}
More modern systems use a steerable [[parabola|parabolic]] "dish" to create a tight broadcast beam, typically using the same dish as the receiver. Such systems often combine two radar frequencies in the same antenna in order to allow automatic steering, or ''radar lock''.

Parabolic reflectors can be either symmetric parabolas or spoiled parabolas:
Symmetric parabolic antennas produce a narrow "pencil" beam in both the X and Y dimensions and consequently have a higher gain. The [[NEXRAD]] [[Pulse-Doppler]] weather radar uses a symmetric antenna to perform detailed volumetric scans of the atmosphere. Spoiled parabolic antennas produce a narrow beam in one dimension and a relatively wide beam in the other. This feature is useful if target detection over a wide range of angles is more important than target location in three dimensions. Most 2D surveillance radars use a spoiled parabolic antenna with a narrow azimuthal beamwidth and wide vertical beamwidth. This beam configuration allows the radar operator to detect an aircraft at a specific azimuth but at an indeterminate height. Conversely, so-called "nodder" height finding radars use a dish with a narrow vertical beamwidth and wide azimuthal beamwidth to detect an aircraft at a specific height but with low azimuthal precision.

====Types of scan====
* Primary Scan: A scanning technique where the main antenna aerial is moved to produce a scanning beam, examples include circular scan, sector scan, etc.
* Secondary Scan: A scanning technique where the antenna feed is moved to produce a scanning beam, examples include conical scan, unidirectional sector scan, lobe switching, etc.
* Palmer Scan: A scanning technique that produces a scanning beam by moving the main antenna and its feed. A Palmer Scan is a combination of a Primary Scan and a Secondary Scan.
* [[Conical scanning]]: The radar beam is rotated in a small circle around the "boresight" axis, which is pointed at the target.

====Slotted waveguide====
[[File:Radar antennas on USS Theodore Roosevelt SPS-64.jpg|right|thumb|Slotted waveguide antenna]]
{{Main|Slotted waveguide}}

Applied similarly to the parabolic reflector, the slotted waveguide is moved mechanically to scan and is particularly suitable for non-tracking surface scan systems, where the vertical pattern may remain constant. Owing to its lower cost and less wind exposure, shipboard, airport surface, and harbour surveillance radars now use this approach in preference to a parabolic antenna.

====Phased array====
[[File:PAVE PAWS Radar Clear AFS Alaska.jpg|thumb|right|[[Phased array]]: Not all radar antennas must rotate to scan the sky.]]
{{Main|Phased array}}
Another method of steering is used in a [[phased array]] radar.

Phased array antennas are composed of evenly spaced similar antenna elements, such as aerials or rows of slotted waveguide. Each antenna element or group of antenna elements incorporates a discrete phase shift that produces a phase gradient across the array. For example, array elements producing a 5 degree phase shift for each wavelength across the array face will produce a beam pointed 5 degrees away from the centerline perpendicular to the array face. Signals travelling along that beam will be reinforced. Signals offset from that beam will be cancelled. The amount of reinforcement is [[antenna gain]]. The amount of cancellation is side-lobe suppression.<ref>{{cite web|url=http://mit.edu/6.933/www/Fall2000/mode-s/sidelobe.html|title=Side-Lobe Suppression|publisher=MIT|access-date=11 September 2012|archive-date=31 March 2012|archive-url=https://web.archive.org/web/20120331085410/http://mit.edu/6.933/www/Fall2000/mode-s/sidelobe.html|url-status=dead}}</ref>

Phased array radars have been in use since the earliest years of radar in World War II ([[Mammut radar]]), but electronic device limitations led to poor performance. Phased array radars were originally used for missile defence (see for example [[Safeguard Program]]). They are the heart of the ship-borne [[Aegis Combat System]] and the [[MIM-104 Patriot|Patriot Missile System]]. The massive redundancy associated with having a large number of array elements increases reliability at the expense of gradual performance degradation that occurs as individual phase elements fail. To a lesser extent, phased array radars have been used in [[weather]] [[surveillance]]. As of 2017, NOAA plans to implement a national network of multi-function phased array radars throughout the United States within 10 years, for meteorological studies and flight monitoring.<ref>{{cite web|url=http://www.nssl.noaa.gov/projects/mpar/|title=Multi-function Phased Array Radar (MPAR) Project|author=National Severe Storms Laboratory|publisher=NOAA|access-date=8 February 2017|author-link=National Severe Storms Laboratory|archive-date=2 February 2017|archive-url=https://web.archive.org/web/20170202073609/http://www.nssl.noaa.gov/projects/mpar/|url-status=live}}</ref>

Phased array antennas can be built to conform to specific shapes, like missiles, infantry support vehicles, ships, and aircraft.

As the price of electronics has fallen, phased array radars have become more common. Almost all modern military radar systems are based on phased arrays, where the small additional cost is offset by the improved reliability of a system with no moving parts. Traditional moving-antenna designs are still widely used in roles where cost is a significant factor such as air traffic surveillance and similar systems.

Phased array radars are valued for use in aircraft since they can track multiple targets. The first aircraft to use a phased array radar was the [[B-1B Lancer]]. The first fighter aircraft to use phased array radar was the [[Mikoyan MiG-31]]. The MiG-31M's SBI-16 [[Zaslon]] [[passive electronically scanned array]] radar was considered to be the world's most powerful fighter radar,{{citation needed|date=January 2023}} until the [[AN/APG-77]] [[active electronically scanned array]] was introduced on the [[Lockheed Martin F-22 Raptor]].

Phased-array [[interferometry]] or [[aperture synthesis]] techniques, using an array of separate dishes that are phased into a single effective aperture, are not typical for radar applications, although they are widely used in [[radio astronomy]]. Because of the [[thinned array curse]], such multiple aperture arrays, when used in transmitters, result in narrow beams at the expense of reducing the total power transmitted to the target. In principle, such techniques could increase spatial resolution, but the lower power means that this is generally not effective.

[[Synthetic aperture radar|Aperture synthesis]] by post-processing motion data from a single moving source, on the other hand, is widely used in space and [[airborne radar system]]s.