7.5 LOW-SIDELOBE PHASED ARRAYS

Low sidelobes have long been of interest to antenna designers. This interest has been heightened by the jamming which threatens most military radars. The requirement for low sidelobes for clutter rejection in the AWACS radar resulted in technology which now supports sidelobe levels of more than 50 dB below the main-beam peak.78 ^,79 The price that must be paid to achieve these low sidelobes includes (1) a reduction in gain, (2) an increase in beamwidth, (3) increased tolerance control, (4) increased cost, and (5) the need to operate in an environment free from obstructions that can readily increase the sidelobes.80 In spite of these drawbacks, the trend to low-sidelobe antennas has accelerated since low sidelobes provide an excellent counter to electronic countermeasures (ECM).

Antenna sidelobes can be controlled by the aperture amplitude distribution. For phased arrays the amplitude of each element may be controlled individually, and therefore good sidelobe control can be achieved. The process of designing a low-sidelobe antenna can be considered in two parts:

1. Choose the correct illumination function to achieve the desired design (error-free) sidelobes.

2. Control the phase and amplitude errors which are the contributors to the random sidelobes.

Of the two it is the control of errors which fundamentally limits sidelobe performance. The effects of illumination function and errors are discussed below.