Editing Phased array (section)

== Types ==
Phased arrays take multiple forms. However, the four most common are the passive electronically scanned array (PESA), active electronically scanned array (AESA), hybrid beam forming phased array, and digital beam forming (DBF) array.<ref>{{cite book |last1=Sturdivant|first1=Rick|last2=Quan|first2=Clifton|last3=Chang|first3=Enson|title=Systems Engineering of Phased Arrays |date=2018 |publisher=Artech House |isbn=978-1630814885}}</ref>

A ''passive phased array'' or ''[[passive electronically scanned array]]'' (PESA) is a phased array in which the antenna elements are connected to a single [[transmitter]] and/or [[radio receiver|receiver]], as shown in the first animation at top. PESAs are the most common type of phased array. Generally speaking, a PESA uses one receiver/exciter for the entire array.

An ''active phased array'' or ''[[active electronically scanned array]]'' (AESA) is a phased array in which each antenna element has an analog transmitter/receiver (T/R) module<ref>{{cite book |last1=Sturdivant|first1=Rick|last2=Harris|first2=Mike|title=Transmit Receive Modules for Radar and Communication Systems |date=2015 |publisher=Artech House |location=Norwood, MA |isbn=978-1608079797}}</ref> which creates the phase shifting required to electronically steer the antenna beam.  Active arrays are a more advanced, second-generation phased-array technology that are used in military applications; unlike PESAs they can radiate several beams of radio waves at multiple frequencies in different directions simultaneously. However, the number of simultaneous beams is limited by practical reasons of electronic packaging of the beam formers to approximately three simultaneous beams for an AESA{{Citation needed|date=May 2025}}. Each beam former has a receiver/exciter connected to it.

A ''digital beam forming (DBF) phased array'' has a digital receiver/exciter at each element in the array. The signal at each element is digitized by the receiver/exciter. This means that antenna beams can be formed digitally in a field programmable gate array (FPGA) or the array computer. This approach allows for multiple simultaneous antenna beams to be formed.

A ''hybrid beam forming phased array'' can be thought of as a combination of an AESA and a digital beam forming phased array. It uses subarrays that are active phased arrays (for instance, a subarray may be 64, 128 or 256 elements and the number of elements depends upon system requirements). The subarrays are combined to form the full array. Each subarray has its own digital receiver/exciter. This approach allows clusters of simultaneous beams to be created.

A ''[[conformal antenna]]''<ref name=" Pandey">{{cite book
 | last1  =  Pandey
 | first1 = Anil 
 | title  = Practical Microstrip and Printed Antenna Design
 | publisher = Artech House
 | date   = 2019
 | location = Bostan
 | pages  = 443
 | language = en
 | url    = https://us.artechhouse.com/Practical-Microstrip-and-Printed-Antenna-Design-P2002.aspx
 | isbn   = 9781630816681
 }}</ref> is a phased array in which the individual antennas, instead of being arranged in a flat plane, are mounted on a curved surface.  The phase shifters compensate for the different path lengths of the waves due to the antenna elements' varying position on the surface, allowing the array to radiate a plane wave.  Conformal antennas are used in aircraft and missiles, to integrate the antenna into the curving surface of the aircraft to reduce aerodynamic drag.
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=== Time and frequency domains ===
{{unreferenced section|date=December 2016}}
{{Main|Beamforming}}
There are two main types of beamformers. These are [[time domain]] beamformers and [[frequency domain]] beamformers. From a theoretical point of view, both are in principle the same operation, with just a [[Fourier transform]] allowing conversion from one to the other type.

A graduated attenuation window is sometimes applied across the face of the array to improve side-lobe suppression performance, in addition to the phase shift.

Time domain beamformer works by introducing time delays. The basic operation is called "delay and sum". It delays the incoming signal from each array element by a certain amount of time, and then adds them together. A [[Butler matrix]] allows several beams to be formed simultaneously, or one beam to be scanned through an arc. The most common kind of time domain beam former is serpentine waveguide. Active phased array designs use individual delay lines that are switched on and off. [[Yttrium iron garnet]] phase shifters vary the phase delay using the strength of a magnetic field.

There are two different types of frequency domain beamformers.

The first type separates the different frequency components that are present in the received signal into multiple frequency bins (using either a [[Discrete Fourier transform]] (DFT) or a [[filterbank]]). When different delay and sum beamformers are applied to each frequency bin, the result is that the main lobe simultaneously points in multiple different directions at each of the different frequencies. This can be an advantage for communication links, and is used with the [[SPS-48]] radar.

The other type of frequency domain beamformer makes use of Spatial Frequency. Discrete samples are taken from each of the individual array elements. The samples are processed using a DFT. The DFT introduces multiple different discrete phase shifts during processing. The outputs of the DFT are individual channels that correspond with evenly spaced beams formed simultaneously. A 1-dimensional DFT produces a fan of different beams. A 2-dimensional DFT produces beams with a [[pineapple]] configuration.

These techniques are used to create two kinds of phased array.
:* Dynamic{{Snd}} an array of variable phase shifters are used to move the beam
:* Fixed{{Snd}} the beam position is stationary with respect to the array face and the whole antenna is moved

There are two further sub-categories that modify the kind of dynamic array or fixed array.
:* Active{{Snd}} amplifiers or processors are in each phase shifter element
:* Passive{{Snd}} large central amplifier with attenuating phase shifters

=== Dynamic phased array ===
Each array element incorporates an adjustable phase shifter. These are collectively used to move the beam with respect to the array face.

Dynamic phased arrays require no physical movement to aim the beam. The beam is moved electronically. This can produce antenna motion fast enough to use a small pencil beam to simultaneously track multiple targets while searching for new targets using just one radar set, a capability known as ''track while search''.

As an example, an antenna with a 2-degree beam with a pulse rate of 1&nbsp;kHz will require approximately 8 seconds to cover an entire hemisphere consisting of 8,000 pointing positions. This configuration provides 12 opportunities to detect a {{convert|1000|m/s|mph km/h|abbr=on}} vehicle over a range of {{convert|100|km|mi|abbr=on}}, which is suitable for military applications.{{citation needed|date=July 2014}}

The position of mechanically steered antennas can be predicted, which can be used to create [[electronic countermeasures]] that interfere with radar operation. The flexibility resulting from phased array operation allows beams to be aimed at random locations, which eliminates this vulnerability. This is also desirable for military applications.

=== Fixed phased array ===
[[File:Antenna-tower-collinear-et-al.jpg|thumb|130px|An antenna tower consisting of a fixed phase collinear antenna array with four elements]]
Fixed phased array antennas are typically used to create an antenna with a more desirable form factor than the conventional [[parabolic reflector]] or [[cassegrain reflector]]. Fixed phased arrays incorporate fixed phase shifters. For example, most commercial FM Radio and TV antenna towers use a [[collinear antenna array]], which is a fixed phased array of dipole elements.

In radar applications, this kind of phased array is physically moved during the track and scan process. There are two configurations.
:* Multiple frequencies with a delay-line
:* Multiple adjacent beams

The [[SPS-48]] radar uses multiple transmit frequencies with a serpentine delay line along the left side of the array to produce vertical fan of stacked beams. Each frequency experiences a different phase shift as it propagates down the serpentine delay line, which forms different beams. A filter bank is used to split apart the individual receive beams. The antenna is mechanically rotated.

[[Semi-active radar homing]] uses [[monopulse radar]] that relies on a fixed phased array to produce multiple adjacent beams that measure angle errors. This form factor is suitable for [[gimbal]] mounting in missile seekers.

=== Active phased array ===
[[Active electronically scanned array|Active electronically-scanned array]]s (AESA) elements incorporate transmit amplification with [[phase shift]] in each [[Transceiver|antenna element]] (or group of elements). Each element also includes receive pre-amplification. The phase shifter setting is the same for transmit and receive.<ref>Active Electronically Steered Arrays{{Snd}} A Maturing Technology (ausairpower.net)</ref>

Active phased arrays do not require phase reset after the end of the transmit pulse, which is compatible with [[Doppler radar]] and [[pulse-Doppler radar]].

=== Passive phased array ===
[[Passive phased array]]s typically use large amplifiers that produce all of the microwave transmit signal for the antenna. Phase shifters typically consist of waveguide elements controlled by magnetic field, voltage gradient, or equivalent technology.<ref>{{cite web|url=http://scholarworks.sjsu.edu/cgi/viewcontent.cgi?article=5082&context=etd_theses&sei-redir=1&referer=http%3A%2F%2Fwww.google.com|title=YIG-sphere-based phase shifter for X-band phased array applications|publisher=Scholarworks|url-status=live|archive-url=https://web.archive.org/web/20140527212406/http://scholarworks.sjsu.edu/cgi/viewcontent.cgi?article=5082&context=etd_theses&sei-redir=1&referer=http%3A%2F%2Fwww.google.com|archive-date=2014-05-27}}</ref><ref>{{cite web|url=http://www.microwaves101.com/encyclopedia/phaseshifters_ferro.cfm|title=Ferroelectric Phase Shifters|publisher=Microwaves 101|url-status=live|archive-url=https://web.archive.org/web/20120913014513/http://www.microwaves101.com/encyclopedia/phaseshifters_ferro.cfm|archive-date=2012-09-13}}</ref>

The phase shift process used with passive phased arrays typically puts the receive beam and transmit beam into diagonally opposite quadrants. The sign of the phase shift must be inverted after the transmit pulse is finished and before the receive period begins to place the receive beam into the same location as the transmit beam. That requires a phase impulse that degrades sub-clutter visibility performance on Doppler radar and Pulse-Doppler radar. As an example, [[Yttrium iron garnet]] phase shifters must be changed after transmit pulse quench and before receiver processing starts to align transmit and receive beams. That impulse introduces FM noise that degrades clutter performance.

Passive phased array design is used in the AEGIS Combat System<ref>{{cite web|url=http://apps.dtic.mil/dtic/tr/fulltext/u2/a460426.pdf|title=Total Ownership Cost Reduction Case Study: AEGIS Radar Phase Shifters|publisher=Naval Postgraduate School|url-status=live|archive-url=https://web.archive.org/web/20160303234942/http://www.dtic.mil/dtic/tr/fulltext/u2/a460426.pdf|archive-date=2016-03-03}}</ref> for [[Direction of arrival|direction-of-arrival]] estimation.