View source for Phased array

{{Short description|Array of antennas creating a steerable beam}}
{{Use American English|date=May 2023}}

[[File:Phasearray.gif|thumb|upright=1.3|Animation showing the [[radiation pattern]] of a phased array of 15 antenna elements spaced a quarter wavelength apart as the [[phase shift|phase difference]] between adjacent antennas is swept between −120 and 120 degrees. The dark area is the beam or [[main lobe]], while the light lines fanning out around it are [[sidelobe]]s.]]

In [[antenna (radio)|antenna]] theory, a phased array consists of many small antennas, positioned in an array, next to each other. 

Instead of sending a [[radio wave]] with one antenna, a computer system can shift the phase of the original wave by fractions of a [[wavelength]] for each individual antenna in the array. The [[frequency]] of the signal is not changed, only the [[Phase (waves)|phase]] by precisely delaying the signal for each antenna in the array, resulting in a shift of the [[Crest and trough|wave crest and trough]] for each antenna.

This allows the radio wave being sent out to be concentrated in a specific direction by creating a constructive [[Wave interference|interference pattern]] from the shifted phases of each individual wave. The resulting signal just looks like any other radio wave, being the strongest in the desired direction, where the phase of all individual waves align and constructively interfere.

Phased arrays are mainly used at the high frequency end of the [[radio spectrum]]. Smaller wavelengths can be sent with smaller antennas, allowing the total size of the array to be practical.

==Description==
[[File:Phased array animation with arrow 10frames 371x400px 100ms.gif|thumb|upright=1.25|Animation showing how a phased array works. It consists of an array of antenna elements ''(A)'' powered by a [[transmitter]] ''(TX)''.  The feed current for each element passes through a [[phase shifter]] ''(φ)'' controlled by a computer ''(C).'' The moving red lines show the wavefronts of the radio waves emitted by each element.  The individual wavefronts are spherical, but they combine ([[Superposition principle|superpose]]) in front of the antenna to create a [[plane wave]]. The phase shifters delay the radio waves progressively going up the line so each antenna emits its wavefront later than the one below it.  This causes the resulting plane wave to be directed at an angle ''θ'' to the antenna's axis.  By changing the phase shifts, the computer can instantly change the angle ''θ'' of the beam.  Most phased arrays have two-dimensional arrays of antennas instead of the linear array shown here, and the beam can be steered in two dimensions. The velocity of the radio waves shown have been slowed down.]] 

A phased array is an '''electronically scanned array''', a computer-controlled [[Antenna array|array of antennas]] which creates a [[radio beam|beam of radio waves]] that can be electronically steered to point in different directions without moving the antennas.<ref name="Brown2">{{cite book
 | last  = Brown
 | first = Arik D. 
 | title = Active Electronically Scanned Arrays: Fundamentals and Applications
 | publisher = Wiley-IEEE Press
 | date   = 2021
 | url    = https://www.wiley.com/en-us/Active+Electronically+Scanned+Arrays%3A+Fundamentals+and+Applications-p-9781119749059
 | isbn   = 978-1-119-74905-9
 }}</ref><ref name="Brown1">{{cite book
 | last = Brown
 | first = Arik D. 
 | editor-first = Arik D.
 | editor-last = Brown 
 | title = Electronically Scanned Arrays: MATLAB Modeling and Simulation, 1st Ed.
 | publisher = CRC Press
 | date   = 2012
 | doi = 10.1201/b12044 
 | url    = https://www.taylorfrancis.com/books/edit/10.1201/b12044/electronically-scanned-arrays-arik-brown
 | isbn = 9781315217130
 }}</ref><ref name="Milligan">{{cite book
 | last = Milligan
 | first = Thomas A. 
 | title  = Modern Antenna Design, 2nd Ed.
 | publisher = John Wiley & Sons
 | date   = 2005
 | url    = https://books.google.com/books?id=PPyDQXAd09kC&q=%22phased+array%22&pg=PA102
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 }}</ref><ref name="Balanis">{{cite book
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 | first = Constantine A. 
 | title  = Antenna Theory: Analysis and Design, 4th Ed.
 | publisher = John Wiley & Sons
 | date   = 2015
 | pages  = 302–303
 | url    = https://books.google.com/books?id=PTFcCwAAQBAJ&q=%22phased%20array%22&pg=PA303
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 | title  = Antenna Theory and Design
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 | url    = https://books.google.com/books?id=xhZRA1K57wIC&q=%22phased+array%22&pg=PA315
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 | title  = Comprehensive Dictionary of Electrical Engineering
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 | title  = Array and Phased Array Antenna Basics
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 }}</ref>{{Excessive citations inline|date=March 2024}}
The general theory of an electromagnetic phased array also finds applications in ultrasonic and medical imaging application (''[[phased array ultrasonics]]'') and in optics ''[[optical phased array]]''.

In a simple [[array antenna]], the [[radio frequency]] current from the [[transmitter]] is fed to multiple individual antenna elements with the proper [[phase (waves)|phase]] relationship so that the radio waves from the separate elements combine ([[superposition principle|superpose]]) to form beams, to increase power radiated in desired directions and suppress radiation in undesired directions. 

In a phased array, the power from the transmitter is fed to the radiating elements through devices called ''[[phase shifter]]s'', controlled by a computer system, which can alter the phase or signal delay electronically, thus steering the beam of radio waves to a different direction.  Since the size of an antenna array must extend many wavelengths to achieve the high gain needed for narrow beam-width, phased arrays are mainly practical at the high [[frequency]] end of the radio spectrum, in the [[ultrahigh frequency|UHF]] and [[microwave]] bands, in which the operating wavelengths are conveniently small.

Phased arrays were originally conceived for use in military [[radar]] systems, to steer a beam of radio waves quickly across the sky to detect planes and missiles. These systems are now widely used and have spread to civilian applications such as [[5G]] [[MIMO]] for cell phones. The phased array principle is also used in [[acoustics]], and phased arrays of [[transducer|acoustic transducers]] are used in medical [[ultrasound imaging]] scanners ([[phased array ultrasonics]]), [[hydrocarbon exploration|oil and gas prospecting]] ([[reflection seismology]]), and military [[sonar]] systems.

The term "phased array" is also used to a lesser extent for non steerable [[array antenna]]s in which the phase of the feed power and thus the radiation pattern of the antenna array is fixed.<ref name="Visser"/><ref>{{FS1037C MS188}} [http://glossary.its.bldrdoc.gov/fs-1037/dir-027/_3979.htm Definition of Phased Array] {{webarchive|url=https://web.archive.org/web/20041021132428/http://glossary.its.bldrdoc.gov/fs-1037/dir-027/_3979.htm |date=2004-10-21 }}. Accessed 27 April 2006.</ref> For example, AM broadcast radio antennas consisting of multiple [[mast radiator]]s fed so as to create a specific radiation pattern are also called "phased arrays".

== 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. 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.
{{breakafterimages}}

== History ==
[[Image:Braun phased array antenna 1905.png|thumb|[[Ferdinand Braun]]'s 1905 directional antenna, which used the phased array principle, consisting of three monopole antennas in an equilateral triangle. A quarter-wave delay in the feedline of one antenna caused the array to radiate in a beam. The delay could be switched manually into any of the three feeds, rotating the antenna beam by 120°.]]
{{multiple image
| align = right
| direction = horizontal
| header   =
| image1   = PAVE PAWS Radar Clear AFS Alaska.jpg
| caption1 = US [[PAVE PAWS]] [[active electronically scanned array|active phased array]] ballistic missile detection radar in Alaska. Completed in 1979, it was one of the first active phased arrays.
| width1   = 180
| image2   = Cape Cod Air Station - HAER MA-151-A - 384568pu.jpg
| caption2 = Closeup of some of the 2677 crossed dipole antenna elements that make up the plane array. This antenna produced a narrow "pencil" beam only 2.2° wide.
| width2   = 195
| footer   =
}}
[[File:PAVE PAWS&BMEWS.svg|thumb|[[BMEWS]] & [[PAVE PAWS]] radars]]
[[Image:Mammut Hoarding radar illustration.png|thumb|Mammut phased-array radar, [[World War II]]]]

Phased array transmission was originally shown in 1905 by [[Nobel Prize|Nobel]] laureate [[Karl Ferdinand Braun]] who demonstrated enhanced transmission of [[radio]] waves in one direction.<ref>{{cite book |url=https://www.nobelprize.org/prizes/physics/1909/braun/lecture/|chapter=Electrical Oscillations and Wireless Telegraphy|title=Nobel Lectures, Physics 1901-1921|publisher=Elsevier|location=Amsterdam|year=1967|orig-date=Delivered 11 December 1909|last=Braun|first=Karl Ferdinand|access-date=29 July 2023|via=nobelprize.org}} Braun's Nobel Prize lecture. The phased array section is on pages 239–240.</ref><ref>"Die Strassburger Versuche über gerichtete drahtlose Telegraphie" (The Strassburg experiments on directed wireless telegraphy), ''Elektrotechnische und Polytechnische Rundschau'' (Electrical technology and polytechnic review [a weekly]), (1 November 1905). This article is summarized (in German) in:
Adolf Prasch, ed., ''Die Fortschritte auf dem Gebiete der Drahtlosen Telegraphie'' [Progress in the field of wireless telegraphy] (Stuttgart, Germany: Ferdinand Enke, 1906), vol. 4, [https://books.google.com/books?id=ZAAMAAAAYAAJ&pg=RA1-PA184 pages 184–185].</ref> During [[World War II]], Nobel laureate [[Luis Walter Alvarez|Luis Alvarez]] used phased array transmission in a rapidly [[Beam steering|steerable]] [[radar]] system for "[[ground-controlled approach]]", a system to aid in the landing of aircraft. At the same time, the GEMA in Germany built the [[Mammut radar|Mammut]] 1.<ref>http://www.100jahreradar.de/index.html?/gdr_5_deutschefunkmesstechnikim2wk.html {{webarchive|url=https://web.archive.org/web/20070929154159/http://www.100jahreradar.de/index.html?%2Fgdr_5_deutschefunkmesstechnikim2wk.html |date=2007-09-29 }} Mamut1 first early warning PESA Radar</ref> It was later adapted for [[radio astronomy]] leading to [[Nobel Prize for Physics|Nobel Prizes for Physics]] for [[Antony Hewish]] and [[Martin Ryle]] after several large phased arrays were developed at the [[University of Cambridge]] [[Interplanetary Scintillation Array]]. This design is also used for [[radar]], and is generalized in [[interferometry|interferometric]] radio antennas.

In 1966, most phased-array radars use ferrite phase shifters or traveling-wave tubes to dynamically adjust the phase.
The AN/SPS-33 -- installed on the nuclear-powered ships Long Beach and Enterprise around 1961 -- was claimed to be the only operational 3-D phased array in the world in 1966.
The AN/SPG-59 was designed to generate multiple tracking beams from the transmitting array and simultaneously program independent receiving arrays.
The first civilian 3D phased array was built in 1960 at the National Aviation Facilities Experimental Center; but was abandoned in 1961.<ref>
W. J. Evanzia.
[https://www.worldradiohistory.com/Archive-Electronics/60s/66/Electronics-1966-06-27.pdf "Faster, lighter 3-D radars in sight for tactical warfare"].
Electronics.
1966.
p. 81, 83, 87.
</ref>

In 2004, [[California Institute of Technology|Caltech]] researchers demonstrated the first integrated silicon-based phased array receiver at 24&nbsp;GHz with 8 elements.<ref>{{Cite web|url=http://chic.caltech.edu/wp-content/uploads/2013/06/fullyintegrated24ghz.pdf|title=A Fully Integrated 24GHz 8-Path Phased-Array Receiver in Silicon|url-status=live|archive-url=https://web.archive.org/web/20180511100234/http://chic.caltech.edu/wp-content/uploads/2013/06/fullyintegrated24ghz.pdf|archive-date=2018-05-11}}</ref> This was followed by their demonstration of a CMOS 24&nbsp;GHz phased array transmitter in 2005<ref>{{Cite web|url=http://chic.caltech.edu/wp-content/uploads/2013/05/11.7_final.pdf|title=A 24GHz Phased-Array Transmitter in 0.18μm CMOS|url-status=live|archive-url=https://web.archive.org/web/20180511100233/http://chic.caltech.edu/wp-content/uploads/2013/05/11.7_final.pdf|archive-date=2018-05-11}}</ref> and a fully integrated 77&nbsp;GHz phased array transceiver with integrated antennas in 2006<ref>{{Cite web|url=http://chic.caltech.edu/wp-content/uploads/2013/05/Aydin_ISSCC_06.pdf|title=A 77GHz 4-Element Phased Array Receiver with On-Chip Dipole Antennas in Silicon|url-status=live|archive-url=https://web.archive.org/web/20180511100234/http://chic.caltech.edu/wp-content/uploads/2013/05/Aydin_ISSCC_06.pdf|archive-date=2018-05-11}}</ref><ref>{{Cite web|url=http://chic.caltech.edu/wp-content/uploads/2006/02/HajimiriArun_ISSCC_77TX_06.pdf|title=A 77GHz Phased-Array Transmitter with Local LO- Path Phase-Shifting in Silicon|url-status=live|archive-url=https://web.archive.org/web/20150909231447/http://chic.caltech.edu/wp-content/uploads/2006/02/HajimiriArun_ISSCC_77TX_06.pdf|archive-date=2015-09-09}}</ref> by the Caltech team. In 2007, [[DARPA]] researchers announced a 16-element phased-array radar antenna which was also integrated with all the necessary circuits on a single silicon chip and operated at 30–50&nbsp;GHz.<ref>[http://ucsdnews.ucsd.edu/newsrel/science/10-07PhasedArrayChipDK-L.asp World’s Most Complex Silicon Phased Array Chip Developed at UC San Diego] {{webarchive|url=https://web.archive.org/web/20071225084723/http://ucsdnews.ucsd.edu/newsrel/science/10-07PhasedArrayChipDK-L.asp |date=2007-12-25 }} in UCSD News (reviewed 2 November 2007)</ref>

The relative [[amplitude]]s of—and constructive and destructive [[Interference (wave propagation)|interference]] effects among—the signals radiated by the individual antennas determine the effective [[radiation pattern]] of the array. A phased array may be used to point a fixed radiation pattern, or to [[wikt:scan|scan]] rapidly in [[azimuth]] or elevation. Simultaneous electrical scanning in both azimuth and elevation was first demonstrated in a phased array antenna at [[Hughes Aircraft Company]], California in 1957.<ref>See Joseph Spradley, "A Volumetric Electrically Scanned Two-Dimensional Microwave Antenna Array," IRE National Convention Record, Part I{{Snd}} Antennas and Propagation; Microwaves, New York: The Institute of Radio Engineers, 1958, 204–212.</ref>

== Applications ==

=== Broadcasting ===
In [[broadcast engineering]], the term 'phased array' has a meaning different from its normal meaning, it means an ordinary [[array antenna]], an array of multiple [[mast radiator]]s designed to radiate a [[directional antenna|directional]] radiation pattern, as opposed to a single mast which radiates an [[omnidirectional antenna|omnidirectional]] pattern. Broadcast phased arrays have fixed radiation patterns and are not 'steered' during operation as are other phased arrays.

Phased arrays are used by many [[AM broadcasting|AM broadcast]] [[radio stations]] to enhance [[signal strength]] and therefore coverage in the [[city of license]], while minimizing [[Interference (communication)|interference]] to other areas. Due to the differences between daytime and nighttime [[ionosphere|ionospheric]] [[radio propagation|propagation]] at [[mediumwave]] frequencies, it is common for AM broadcast stations to change between day ([[groundwave]]) and night ([[skywave]]) radiation patterns by switching the [[phase (waves)|phase]] and power levels supplied to the individual antenna elements ([[mast radiator]]s) daily at [[sunrise]] and [[sunset]]. For [[shortwave]] broadcasts many stations use arrays of horizontal dipoles. A common arrangement uses 16 dipoles in a 4×4 array. Usually this is in front of a wire grid reflector. The phasing is often switchable to allow [[beam steering]] in azimuth and sometimes elevation.

=== Radar ===
Phased arrays were invented for radar tracking of ballistic missiles, and because of their fast tracking abilities phased array radars are widely used in military applications.  For example, because of the rapidity with which the [[Beam steering|beam can be steered]], phased array radars allow a warship to use one [[radar]] system for surface detection and tracking (finding ships), air detection and tracking (finding aircraft and missiles) and missile uplink capabilities. Before using these systems, each [[surface-to-air missile]] in flight required a dedicated [[fire-control radar]], which meant that radar-guided weapons could only engage a small number of simultaneous targets. Phased array systems can be used to control missiles during the mid-course phase of the missile's flight. During the terminal portion of the flight, [[continuous-wave]] fire control directors provide the final guidance to the target. Because the antenna pattern is [[Beam steering|electronically steered]], phased array systems can direct radar beams fast enough to maintain a [[fire-control system|fire control quality]] track on many targets simultaneously while also controlling several in-flight missiles.

[[File:APAR.jpg|thumb|[[Active Phased Array Radar]] mounted on top of [[Sachsen-class frigate|''Sachsen''-class frigate]] F220 ''Hamburg's'' superstructure of the [[German Navy]]]]
The [[AN/SPY-1]] phased array radar, part of the [[Aegis Combat System]] deployed on modern U.S. [[cruisers]] and [[destroyers]], "is able to perform search, track and missile guidance functions simultaneously with a capability of over 100 targets."<ref>{{cite web|title=AEGIS Weapon System MK-7 |publisher=[[Jane's Information Group]] |date=2001-04-25 |url=http://www.janes.com/defence/naval_forces/news/misc/aegis010425.shtml |access-date=10 August 2006 |archive-url=https://web.archive.org/web/20060701055247/http://www.janes.com/defence/naval_forces/news/misc/aegis010425.shtml |archive-date=1 July 2006 |url-status=dead }}.</ref> Likewise, the [[Thales Herakles]] phased array multi-function radar used in service with [[France]] and [[Singapore]] has a track capacity of 200 targets and is able to achieve automatic target detection, confirmation and track initiation in a single scan, while simultaneously providing mid-course guidance updates to the [[MBDA Aster]] missiles launched from the ship.<ref>{{cite journal |last=Scott |first=Richard |date=April 2006 |title=Singapore Moves to Realise Its Formidable Ambitions|journal=Jane's Navy International |volume=111 |issue=4 |pages=42–49}}</ref> The [[German Navy]] and the [[Royal Dutch Navy]] have developed the [[Active Phased Array Radar]] System (APAR). The [[MIM-104 Patriot]] and other ground-based antiaircraft systems use phased array radar for similar benefits.

Phased arrays are used in naval sonar, in active (transmit and receive) and passive (receive only) and hull-mounted and [[towed array sonar]].

{{See also |SAMPSON |Active Phased Array Radar |SMART-L |Active Electronically Scanned Array |Aegis combat system |AN/SPY-1 }}

=== Space probe communication ===
The ''[[MESSENGER]]'' spacecraft was a [[space probe]] mission to the planet [[Mercury (planet)|Mercury]] (2011–2015<ref name=nyt20150430>{{cite news |last=Corum |first=Jonathan |title=Messenger's Collision Course With Mercury |url=https://www.nytimes.com/interactive/2015/04/30/science/space/messenger-collides-with-mercury.html |date=April 30, 2015 |work=[[New York Times]] |access-date=10 May 2015 |url-status=live |archive-url=https://web.archive.org/web/20150510040355/http://www.nytimes.com/interactive/2015/04/30/science/space/messenger-collides-with-mercury.html |archive-date=10 May 2015 }}</ref>). This was the first deep-space mission to use a phased-array antenna for [[telecommunications|communications]]. The radiating elements are [[circular polarization|circularly-polarized]], slotted [[waveguide]]s. The antenna, which uses the [[X band]], used 26 radiative elements and can [[Fault tolerance|gracefully degrade]].<ref>{{cite web|url=http://messenger.jhuapl.edu/the_mission/publications/Wallis_Cheng.2001.pdf |title=Phased-Array Antenna System for the MESSENGER Deep Space Mission |last1=Wallis |first1=Robert E. |last2=Cheng |first2=Sheng |publisher=[[Johns Hopkins University Applied Physics Laboratory]] |access-date=11 May 2015 |archive-url=https://web.archive.org/web/20150518091704/http://messenger.jhuapl.edu/the_mission/publications/Wallis_Cheng.2001.pdf |archive-date=18 May 2015 |url-status=dead }}</ref>

=== Weather research usage ===
[[File:Par installation.jpg|thumb|left|AN/SPY-1A  radar installation at [[National Severe Storms Laboratory]], Norman, Oklahoma. The enclosing [[radome]] provides weather protection.]]
The [[National Severe Storms Laboratory]] has been using a SPY-1A phased array antenna, provided by the US Navy, for weather research at its [[Norman, Oklahoma]] facility since April 23, 2003. It is hoped that research will lead to a better understanding of thunderstorms and tornadoes, eventually leading to increased warning times and enhanced prediction of tornadoes. Current project participants include the National Severe Storms Laboratory and National Weather Service Radar Operations Center, [[Lockheed Martin]], [[United States Navy]], [[University of Oklahoma]] School of Meteorology, School of Electrical and Computer Engineering, and [[Atmospheric Radar Research Center]], Oklahoma State Regents for Higher Education, the [[Federal Aviation Administration]], and Basic Commerce and Industries. The project includes [[research and development]], future [[technology transfer]] and potential deployment of the system throughout the United States. It is expected to take 10 to 15 years to complete and initial construction was approximately $25 million.<ref>[[National Oceanic and Atmospheric Administration]]. [http://www.norman.noaa.gov/publicaffairs/backgrounders/backgrounder_par.html PAR Backgrounder] {{webarchive|url=https://web.archive.org/web/20060509134036/http://www.norman.noaa.gov/publicaffairs/backgrounders/backgrounder_par.html |date=2006-05-09 }}. Accessed 6 April 2006.</ref> A team from Japan's RIKEN Advanced Institute for Computational Science (AICS) has begun experimental work on using phased-array radar with a new algorithm for [[3D NowCasting|instant weather forecasts]].<ref>{{cite journal|last1=Otsuka|first1=Shigenori|last2=Tuerhong|first2=Gulanbaier|last3=Kikuchi|first3=Ryota|last4=Kitano|first4=Yoshikazu|last5=Taniguchi|first5=Yusuke|last6=Ruiz|first6=Juan Jose|last7=Satoh|first7=Shinsuke|last8=Ushio|first8=Tomoo|last9=Miyoshi|first9=Takemasa|title=Precipitation Nowcasting with Three-Dimensional Space–Time Extrapolation of Dense and Frequent Phased-Array Weather Radar Observations|journal=Weather and Forecasting|date=February 2016|volume=31|issue=1|pages=329–340|doi=10.1175/WAF-D-15-0063.1|bibcode=2016WtFor..31..329O}}</ref>

=== Optics ===
{{Main|Phased-array optics}}
Within the visible or infrared spectrum of electromagnetic waves it is possible to construct [[phased-array optics|optical phased arrays]]. They are used in wavelength multiplexers and filters for telecommunication purposes,<ref>P. D. Trinh, S. Yegnanarayanan, F. Coppinger and B. Jalali [http://www.ee.ucla.edu/~oecs/comp_pub/intr_opt/Optics23.pdf Silicon-on-Insulator (SOI) Phased-Array Wavelength Multi/Demultiplexer with Extremely Low-Polarization Sensitivity] {{webarchive|url=https://web.archive.org/web/20051208105830/http://www.ee.ucla.edu/~oecs/comp_pub/intr_opt/Optics23.pdf |date=2005-12-08 }}, ''IEEE Photonics Technology Letters'', Vol. 9, No. 7, July 1997</ref> laser [[beam steering]], and holography. [[Synthetic array heterodyne detection]] is an efficient method for [[multiplexing]] an entire phased array onto a single element [[photodetector]]. The dynamic beam forming in an optical phased array transmitter can be used to electronically raster or vector scan images without using lenses or mechanically moving parts in a lensless projector.<ref name=":0">{{Cite web|url=http://authors.library.caltech.edu/60779/1/06886570.pdf|title=Electronic Two-Dimensional Beam Steering for Integrated Optical Phased Arrays|archive-url=https://web.archive.org/web/20170809130907/http://authors.library.caltech.edu/60779/1/06886570.pdf|archive-date=2017-08-09|url-status=live}}</ref> Optical phased array receivers have been demonstrated to be able to act as lensless cameras by selectively looking at different directions.<ref name=":1">{{Cite web|url=http://chic.caltech.edu/wp-content/uploads/2017/03/Cleo_2017_2D_OPA_V7.pdf|title=An 8x8 Heterodyne Lens-less OPA Camera|archive-url=https://web.archive.org/web/20170713050602/http://chic.caltech.edu/wp-content/uploads/2017/03/Cleo_2017_2D_OPA_V7.pdf|archive-date=2017-07-13|url-status=live}}</ref><ref name=":2">{{Cite web|url=http://chic.caltech.edu/wp-content/uploads/2016/06/CLEO_SI-2016-STu3G.3.pdf|title=A One-Dimensional Heterodyne Lens-Free OPA Camera|archive-url=https://web.archive.org/web/20170722055717/http://chic.caltech.edu/wp-content/uploads/2016/06/CLEO_SI-2016-STu3G.3.pdf|archive-date=2017-07-22|url-status=live}}</ref>

=== Satellite broadband internet transceivers ===
[[Starlink]] is a [[low Earth orbit]] [[satellite constellation]] that is under construction {{As of|2021|lc=y}}. It is designed to provide broadband internet connectivity to consumers; the user terminals of the system will use phased array antennas.<ref>
{{cite AV media |people=Elon Musk, Mike Suffradini |date=7 July 2015 |title=ISSRDC 2015{{Snd}} A Conversation with Elon Musk (2015.7.7) |medium=video |url=https://www.youtube.com/watch?v=ZmEg95wPiVU |access-date=2015-12-30 |time=46:45–50:40 }}</ref>

=== Radio-frequency identification (RFID) ===
By 2014, phased array antennas were integrated into [[RFID]] systems to increase the area of coverage of a single system by 100% to {{convert|76200|sqm|sqft|abbr=on}} while still using traditional passive [[UHF]] tags.<ref>{{cite web |url=http://www.mojix.com/pdf/Mojix_STAR_System.pdf |title=Mojix Star System |access-date=24 October 2014 |url-status=dead |archive-url=https://web.archive.org/web/20110516020744/http://www.mojix.com/pdf/Mojix_STAR_System.pdf |archive-date=16 May 2011 }}</ref>

=== Human-machine interfaces (HMI) ===
A phased array of acoustic transducers, denominated airborne ultrasound tactile display (AUTD), was developed in 2008 at the University of Tokyo's Shinoda Lab to induce tactile feedback.<ref>{{cite web |archive-url=https://web.archive.org/web/20090318064419/http://www.alab.t.u-tokyo.ac.jp/~siggraph/08/Tactile/SIGGRAPH08-Tactile.html |archive-date=18 March 2009|title=Airborne Ultrasound Tactile Display|url=http://www.alab.t.u-tokyo.ac.jp/~siggraph/08/Tactile/SIGGRAPH08-Tactile.html}} SIGGRAPH 2008, Airborne Ultrasound Tactile Display</ref> This system was demonstrated to enable a user to interactively manipulate virtual holographic objects.<ref>{{cite web|url=http://www.alab.t.u-tokyo.ac.jp/~siggraph/09/TouchableHolography/SIGGRAPH09-TH.html |title=Touchable Holography |access-date=2009-08-22 |url-status=dead |archive-url=https://web.archive.org/web/20090831054307/http://www.alab.t.u-tokyo.ac.jp/~siggraph/09/TouchableHolography/SIGGRAPH09-TH.html |archive-date=2009-08-31 }} SIGGRAPH 2009, Touchable holography</ref>

=== Radio astronomy ===
Phased Array Feeds (PAF)<ref>{{Cite journal|last1=Hay|first1=S.G.|last2=O’Sullivan|first2=J.D.|date=2008|title=Analysis of common-mode effects in a dual-polarized planar connected-array antenna|journal=Radio Science|volume=43|issue=6|pages=RS6S04|doi=10.1029/2007RS003798|bibcode=2008RaSc...43.6S04H|doi-access=free}}</ref> have recently been used at the focus of [[radio telescope]]s to provide many beams, giving the radio telescope a very wide [[Angle of view|field of view]]. Three examples are the [[Australian Square Kilometre Array Pathfinder|ASKAP]] telescope in [[Australia]], the Apertif upgrade to the [[Westerbork Synthesis Radio Telescope]] in The [[Netherlands]], and the Florida Space Institute in the United States .

==Critical theory and arithmetic==
[[File:Array frame.svg|thumb|398x398px|Coordinate frame of phased array used in calculation of array factor, directivity, and gain.]]

=== Array factor ===
The total [[directivity]] of a phased array will be a result of the gain of the individual array elements, and the directivity due their positioning in an array. This latter component is closely tied (but not equal to<ref name=":3">{{Cite web |title=Antenna Arrays: A Computational Approach |url=https://ieeexplore.ieee.org/book/5599319 |access-date=2023-05-20 |website=[[IEEE]]}}</ref>) to the [[array factor]].<ref name=":4">{{cite book
 | last  = Balanis
 | first = Constantine A. 
 | title  = Antenna Theory: Analysis and Design, 4th Ed.
 | publisher = John Wiley & Sons
 | date   = 2015
 | url    = https://books.google.com/books?id=PTFcCwAAQBAJ&q=%22phased%20array%22&pg=PA303
 | isbn   = 978-1119178989
 }}</ref>{{page needed|date=July 2023}}<ref name=":3" /> In a (rectangular) planar phased array, of dimensions <math>M\times N</math>, with inter-element spacing <math>d_{x}</math> and <math>d_{y}</math>, respectively, the array factor can be calculated accordingly<ref name="Balanis" /><ref name=":4" />{{page needed|date=July 2023}}:[[File:Phase array sweep.webm|thumb|Radiation pattern of phased array containing 7 emitters spaced a quarter wavelength apart, showing the beam switching direction.  The phase shift between adjacent emitters is switched from 45 degrees to −45 degrees|398x398px]]<math display="block">AF=\sum_{n=1}^{N}I_{n1}\left[\sum_{m=1}^{M}I_{m1}\mathrm{e}^{j\left(m-1\right)\left(kd_{x}\sin\theta\cos\phi+\beta_{x}\right)}\right]\mathrm{e}^{j\left(n-1\right)\left(kd_{y}\sin\theta\sin\phi+\beta_{y}\right)}</math>

Here, <math>\theta</math> and <math>\phi</math> are the directions which we are taking the array factor in, in the coordinate frame depicted to the right. The factors <math>\beta_{x}</math> and <math>\beta_{y}</math> are the ''progressive phase shift'' that is used to steer the beam electronically. The factors <math>I_{n1}</math> and <math>I_{m1}</math> are the excitation coefficients of the individual elements.

Beam steering is indicated in the same coordinate frame, however the direction of steering is indicated with <math>\theta_{0}</math> and <math>\phi_{0}</math>, which is used in calculation of progressive phase:

<math>\beta_{x}=-kd_{x}\sin\theta_{0}\cos\phi_{0}</math>

<math>\beta_{y}=-kd_{y}\sin\theta_{0}\sin\phi_{0}</math>

In all above equations, the value <math>k</math> describes the [[wavenumber]] of the frequency used in transmission.

These equations can be solved to predict the nulls, main lobe, and grating lobes of the array. Referring to the exponents in the array factor equation, we can say that major and grating lobes will occur at integer <math>m,n=0,1,2,\dots</math>  solutions to the following equations:<ref name="Balanis" /><ref name=":4" />{{page needed|date=July 2023}}

<math>kd_{x}\sin\theta\cos\phi+\beta_{x}=\pm2m\pi</math>

<math>kd_{y}\sin\theta\sin\phi+\beta_{y}=\pm2n\pi</math>

=== Worked example ===
It is common in engineering to provide phased array <math>AF</math> values in [[Decibel|decibels]] through <math>AF_{dB}=10\log_{10}AF</math>. Recalling the complex exponential in the array factor equation above, often, what is ''really'' meant by array factor is the magnitude of the summed [[phasor]] produced at the end of array factor calculation. With this, we can produce the following equation:<math display="block">AF_{dB}=10\log_{10}\left\|\sum_{n=1}^{N}I_{1n}\left[\sum_{m=1}^{M}I_{m1}\mathrm{e}^{j\left(m-1\right)\left(kd_{x}\sin\theta\cos\phi+\beta_{x}\right)}\right]\mathrm{e}^{j\left(n-1\right)\left(kd_{y}\sin\theta\sin\phi+\beta_{y}\right)}\right\|</math>For the ease of visualization, we will analyze array factor given an input ''azimuth and elevation'', which we will map to the array frame <math>\theta</math> and <math>\phi</math> through the following conversion:

<math>\theta=\arccos\left(\cos\left(\theta_{az}\right)\sin\left(\theta_{el}\right)\right)</math>

<math>\phi=\arctan2\left(\sin\left(\theta_{el}\right),\sin\left(\theta_{az}\cos\left(\theta_{el}\right)\right)\right)</math>

This represents a coordinate frame whose <math>\mathbf{x}</math> axis is aligned with the array <math>\mathbf{z}</math> axis, and whose <math>\mathbf{y}</math> axis is aligned with the array <math>\mathbf{x}</math> axis.

If we consider a <math>16\times16</math> phased array, this process provides the following values for <math>AF_{dB}</math>, when steering to bore-sight (<math>\theta_{0}=0^{\circ}</math>,<math>\phi_{0}=0^{\circ}</math>):
{|
|[[File:16x16 0.250 lambda spacing planar array factor.png|frameless|400x400px]]
|[[File:16x16 0p500 lambda spacing planar array factor.png|frameless|400x400px]]
|-
|[[File:16x16 1p0 lambda spacing planar array factor.png|frameless|400x400px]]
|[[File:16x16 2p0 lambda spacing planar array factor.png|frameless|400x400px]]
|}
These values have been clipped to have a minimum <math>AF</math> of -50 dB, however, in reality, null points in the array factor pattern will have values significantly smaller than this.

== Different types of phased arrays ==
{{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.

== See also ==
{{portal|Electronics}}
{{colbegin}}
* [[Aperture synthesis]]
* [[History of smart antennas]]
* [[Huygens–Fresnel principle]]
* [[Interferometric synthetic-aperture radar]]
* [[Inverse synthetic-aperture radar]]
* [[Multi-user MIMO]]
* [[Optical heterodyne detection]]
* [[Radar MASINT]]
* [[Reconfigurable antenna]]
* [[Side-scan sonar]]
* [[Single-frequency network]]
* [[Smart antenna]]
* [[Standard linear array]]
* [[Synthetic-aperture radar]]
* [[Synthetic aperture sonar]]
* [[Synthetically thinned aperture radar]]
* [[Thinned-array curse]]
* [[Wave field synthesis]]
{{colend}}

==References==
{{Reflist}}

==External links==
{{Commons category|Phased arrays}}
* [https://web.archive.org/web/20120204094857/http://www.nssl.noaa.gov/par/ Radar Research and Development - Phased Array Radar]—[[National Severe Storms Laboratory]] 
* [http://www.harpoonhq.com/waypoint/articles/Article_044.pdf Shipboard Phased Array Radars] {{Webarchive|url=https://web.archive.org/web/20171118103754/http://www.harpoonhq.com/waypoint/articles/Article_044.pdf |date=2017-11-18 }}
* [https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870018450_1987018450.pdf NASA Report: MMICs For Multiple Scanning Beam Antennas for Space Applications]
* [http://www.radartutorial.eu/06.antennas/Phased%20Array%20Antenna.en.html Principle of Phased Array]
* [http://www.sengpielaudio.com/TonyFaulknerPhasedArray06.htm 'Phased Array' microphone system of Tony Faulkner]
* [http://sharif.edu/~fakharzadeh/Tutorial%201%20Phased%20ArrayF.pdf  Principles of Phased Array systems] - Tutorial 1

{{Antenna Types}}

{{DEFAULTSORT:Phased Array}}
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