System aspects of planar array antennas in airborne air defence radars

Airborne air-defence radars are required to satisfy two general operational requirements namely airborne early warning (AEW), which necessitates surveillance over 360° of azimuth, and airborne intercept (AI) which requires coverage of most of the hemisphere ahead of the aircraft. Mechanically-scanned antennas can utilize either reflector or array types, but some form of array of individual radiating elements is necessary to implement electronic scanning. While array antennas can, in principle, have a curved radiating surface (usually termed conformal) virtually all array antennas used in radar have constrained the radiating elements to a single plane; hence the description planar array. The system considerations which apply to the choice of planar array antennas in airborne radar are examined, first for mechanically-scanned then for electronically-scanned antennas