From dish to array
A mechanical radar has one transmitter, one receiver, one antenna, one beam. The antenna rotates to scan. Each rotation takes seconds. An AESA radar has thousands of independent transmit-receive modules — the F-35's APG-81 has about 1,600. Each one is a tiny radar. By controlling the phase of each module, the combined beam can be steered electronically in microseconds, with no moving parts.
Why this changes everything
An AESA can search one part of the sky while tracking a target in another, while jamming a third, while painting a high-resolution synthetic aperture map of the ground. It can switch between modes faster than the enemy can react. Older mechanical radars had to choose one job at a time.
Reliability and stealth interaction
With 1,600 modules, losing 100 of them in combat only degrades performance by 6%. A mechanical radar with a damaged motor is dead. AESA beams can also be 'low probability of intercept' — frequency-hopping pseudorandom waveforms that look like background noise to enemy radar warning receivers. A stealth aircraft with AESA can light up its own radar without giving away its position.
Why everyone wants one
AESA is now standard on Western fighters (F-22, F-35, F-15EX, Eurofighter Tranche 3, Rafale, Gripen E), Russian Su-57 and Chinese J-20. Even the most modern naval and ground radars (Aegis SPY-6, Patriot LTAMDS) are AESA. The mechanical dish is becoming a museum piece.