A radar detects an aircraft by sending out a high-frequency radio wave and then measuring the time it returns to understand the distance and direction to a target. If you have a slotted planar antenna like the APG-66/68 or the APQ-159, the HF radio wave is directed to search a region in the sky by mechanically moving the antenna array up and down or back and forth.
In an electronically scanned phased array antenna, the HF signal is directed in a different manner. Instead of a configuration where the beam is transmitted through a fixed array that is steered mechanically, beam steering is performed via electrically controlled phased shift elements. To steer the beam from location to location, the signal is shifted by manipulating the phase of the signal from each radiating element. When the sinusoidal waves are transmitted out, as Mark Hickle demonstrates using water droplets in his video, “they (waves) constructively and destructively interfere with each other”, cancelling each other out. In doing so, the signal is amplified by constructive interference while also being sharpened by destructive interference. Also read What is the Difference Between a Passive Electronically Scanned Array (PESA) and an Active Electronically Scanned Array (AESA) Radar?
An electronically controlled phased array where all the antenna elements are connected to a single transmitter/receiver is a passive phased array, or Passive Electronically Scanned Array (PESA). An array where each antenna element is connected to its own transmitter/receiver and is controlled by a computer is an active phased array, or Active Electronically Scanned Array (AESA).
To change the direction of the beam in a phased array antenna without using a mechanical device like the gimbal of the APG-66/68, the antenna elements are fed an equally spaced signal to form a narrow beam. If you add a time delay to the signals sent from each of the antenna elements, it will electronically steer beam. In Mark Hickle’s video below, “Phased Array Antennas”, he uses water droplets to demonstrate this constructive and destructive interference that allows for this beam steering.
We encourage you to pass this video along to your colleagues, friends, and your classroom. If you enjoyed this, we also invite you to view Silas Gibbs’ video, “What Makes an AESA Radar Better?” If you have questions, please email us.
Phased Array Antennas