Editing Phased array (section)

==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="Milligan" /><ref name="Balanis" /><ref name="Stutzman" /><ref name="Lida" /><ref name="Visser" /><ref name="Mazda" />
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.  The computer can alter the phase or signal delay of each antenna element electronically, this results in steering the beam of radio waves to a different direction. 

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".