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[[File:DN-ST-92-02246.JPEG|thumb|The [[Mikoyan MiG-31]] combat aircraft with nose fairing removed, showing its [[Zaslon]] passive electronically scanned array radar antenna. ]]
[[File:Phased array animation with arrow 10frames 371x400px 100ms.gif|thumb|upright=1.2|Animation showing how a passive electronically scanned array works. It consists of an array of antenna elements ''(A)'' powered by a single [[transmitter]] ''(TX)''. The feed current for each antenna 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]], a beam of radio waves travelling in a specific direction θ. 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. The computer can alter the phase shifters to steer the beam to a new direction, very quickly. The velocity of the radio waves is shown slowed down enormously.]]
[[File:DARPA Phased Array 1959.jpg|thumb|DARPA's experimental two-dimensional Electronically Steered Array Radar]]
A '''passive electronically scanned array''' ('''PESA'''), also known as '''passive phased array''', is an antenna in which the beam of radio waves can be electronically steered to point in different directions (that is, a [[phased array]] antenna), in which all the antenna elements are connected to a single [[transmitter]] (such as a [[magnetron]], a [[klystron]] or a [[travelling wave tube]]) and/or [[radio receiver|receiver]].
The largest use of phased arrays is in [[radar]]s{{Citation needed|date=August 2022|reason=Plausible, but still a strong claim that needs authorative sourcing.}}. Most phased array radars in the world are PESA{{Citation needed|date=August 2022|reason=Questionable considering modern (massive) MIMO communications as present in every 4G+ base station.}}. The civilian [[microwave landing system]] uses PESA transmit-only arrays.
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