Monopulse Operation with Continuously Variable Beamwidth by Antenna Defocusing

Monopulse tracking operation with continuously variable beamwidth may be obtained by antenna defocusing. To demonstrate this property, antenna patterns (including phase) of a square aperture for large amounts of defocusing have been computed by diffraction theory for the case of highly tapered illumination in both the sum and difference antenna modes. From an examination of the computed patterns over a 10 to 1 beamwidth range, it is shown that with the increasing defocusing, the patterns in the sum and difference modes exhibit smoothly increasing width. Some change of shape is noted, but the pattern amplitudes and phase relationships are still suitable for on-axis tracking or off-axis location by monopulse techniques. To achieve monopulse operation with the defocused patterns two modifications to ordinary monopulse receivers are necessary. A phase shifter, inserted in the difference or sum channel and ganged to the amount of defocusing, is needed to cancel the phase variation with defocusing. In addition, amplitude correction is required to keep the error-signal slope constant.