What are Electronically Steerable Antennas or ESA?

An Electronically Steerable Antenna or ESA is an electronically-controlled array of antenna elements that can be electronically steered to point in different directions without moving the antenna itself. These antennas are usually available in the form of a flat module with several small identical antennas, each one capable of transmitting and receiving. Electronically steering antennas are also called phased array antennas because of their working principle.

How do Electronically Steered Antennas Operate?

In array antennas, the RF signal from the transmitter can be fed to the individual antenna elements with a calculated phase, which results in the signal from the separate antenna elements adding together (constructively) to increase the radiation in a desired direction and destructively to suppress radiation in undesired directions. The signal from the transmitter is fed to the antennas through devices called phase shifters, that are electronically controlled to alter the phase, thus steering the beam of radio waves in different directions. This is the basic principle of Electronically Steering Antennas.

As shown in the diagram an ESA consists of an array of antenna elements (A) powered by a transmitter (TX). The feed current for each antenna passes through a phase shifter (φ) controlled by a computer/circuit (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 traveling 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.

Types of Electronically Steered Antennas (Phase Array Antennas)

Passive Electronically Scanned Array (PESA): 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 receiver. They are the most common type of phased array.

Active Electronically Scanned Array (AESA): 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 which creates the phase shifting required to electronically steer the antenna beam. Active arrays are more advanced, second-generation antennas that are usually used in military applications. Unlike PESAs they can radiate several beams of radio waves at multiple frequencies in different directions simultaneously.

Digital Beamforming (DBF): A digital beamforming (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 method allows for multiple simultaneous antenna beams to be formed.

Hybrid Beamforming: A hybrid beamforming phased array is a combination of an AESA and digital beamforming phased array. It uses subarrays that are active phased arrays and are combined to form the full array. Each subarray has its own digital receiver/exciter. This method allows clusters of simultaneous beams to be created.

Conformal Antenna: A conformal antenna 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 positions 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.

Electronically Steerable Antennas were originally developed for use in military radar systems, to steer a beam of radio waves quickly across the sky to detect planes and missiles. They are now widely used and have spread to civilian applications such as 5G MIMO for cell phones. The steering capabilities of these antennas are extremely useful in applications where large antennas are used like broadcast stations (radio, TV etc.), RADAR, space communications etc. In such scenarios, using a large antenna means they also need huge and powerful mechanisms to be able to turn them in the right direction. Electronically steering antennas eliminate such a need making it easier to build large antennas. Among small devices, electronically steering antennas are used in satellite communication transceivers and RFID devices to increase their area of coverage by steering their antennas in the direction of maximum signal strength.

Electronically Steerable Antennas are also called Phased Array Antennas. Click here to learn about Phased Array Antennas.