09-24-2021-1026 - phased array electronically scanned array

 In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving the antennas.^{[1][2][3][4][5][6][7][8]}

In a simple array antenna, the radio frequency current from the transmitter is fed to the individual antenna elements that are arranged in a planar or linear configuration with a differential phaserelationship so that the energy from the separate elements add together to increase the far-field power in a desired direction and suppress radiation in undesired directions. The power in the undesired direction is a function of phase accuracy and the relative amplitude of the elements across the array aperture. For example, a uniform power distribution across a phased array will exhibit a sidelobe level that's down 13dB from the main beam. In a phased array, the power from the transmitter is fed to the radiating elements through devices called phase shifters, 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 high gain, phased arrays are mainly practical at the high frequency end of the radio spectrum, in the 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 acoustic transducers are used in medical ultrasound imaging scanners (phased array ultrasonics), oil and gas prospecting (reflection seismology), and military sonar systems.

The term "phased array" is also used to a lesser extent for unsteered array antennas in which the phase of the feed power and thus the radiation pattern of the antenna array is fixed.^[6][9] For example, AM broadcast radio antennas consisting of multiple mast radiators fed so as to create a specific radiation pattern are also called "phased arrays".

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 (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 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 in this diagram.

https://en.wikipedia.org/wiki/Phased_array