Electronic Beam Steering
Phased array antennas revolutionized radar by replacing mechanical dish rotation with electronic beam steering. By controlling the phase of each radiating element, the combined beam can be pointed in any direction within microseconds — thousands of times faster than a rotating antenna. This enables modern radars to track hundreds of targets simultaneously, interleave search and track modes, and rapidly adapt to changing threats.
Array Factor and Pattern Synthesis
The radiation pattern of a phased array is the product of the individual element pattern and the array factor — a function determined by the number of elements, their spacing, and their excitation weights. For a uniform linear array, the array factor produces a sinc-like pattern with a main beam whose width scales inversely with the array length (Nd). Side lobes at -13.3 dB for uniform weighting can be suppressed using amplitude tapering at the cost of slightly broader beamwidth.
Grating Lobes and Element Spacing
The most critical design parameter is element spacing relative to wavelength. When d > λ/2, the array factor has additional maxima (grating lobes) at angles where the path difference between elements is a full wavelength. These spurious beams waste power and create ghost targets. The constraint d ≤ λ/(1 + |sin(θ_max)|) ensures no grating lobes within the scan volume, effectively requiring d ≤ λ/2 for ±90° scanning.
Modern Phased Array Systems
Today's active electronically scanned arrays (AESAs) integrate a transmit/receive module at each element, enabling per-element amplitude and phase control, waveform diversity, and graceful degradation if modules fail. Systems like the AN/SPY-6, AN/APG-81, and Patriot radar use thousands of elements to achieve simultaneous multi-function operation — tracking, searching, missile guidance, and electronic warfare — all from a single aperture.