Passive electronically scanned array: Difference between revisions

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[[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. 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 to the antenna. The velocity of the radio waves is shown slowed down enormously.]]

A '''passive electronically scanned array''' (PESA), also known as '''passive phased array''', is a [[phased array]] antenna, that is an antenna in which the beam of radio waves can be electronically steered to point in different directions, 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]]. This contrasts with an [[active electronically scanned array]] (AESA) antenna, which has a separate [[transmitter]] and/or [[radio receiver|receiver]] unit for each antenna element, all controlled by a computer. AESA is a more advanced, sophisticated versatile second-generation version of the original PESA phased array technology.

The largest use of phased arrays is in [[radar]]s. Most phased array radars in the world are PESA. The civilian [[Microwavemicrowave landing system]] uses PESA transmit-only arrays.

Radar systems generally work by connecting an antenna to a powerful radio transmitter to emit a short pulse of signal. The transmitter is then disconnected and the antenna is connected to a sensitive receiver which amplifies any echos from target objects. By measuring the time it takes for the signal to return, the radar receiver can determine the distance to the object. The receiver then sends the resulting output to a [[Radar display|display of some sort]]. The transmitter elements were typically [[klystron tube]]s or [[magnetron]]s, which are suitable for amplifying or generating a narrow range of frequencies to high power levels. To scan a portion of the sky, the radar antenna must be physically moved to point in different directions.