Optimum digital beam scanning with directive array elements

Array antennas for digital beamforming are often composed of directive elements. These elements may consist of compact antennas, or, may be realised by summing up elementary antennas. In any case, directive antennas require a certain aperture according to their beamwidth, and their phase centres are separated by more than half a wavelength. This leads to grating effects when the beam is scanned digitally. We present theoretical results on digital beamforming and solutions to this grating problem obtained from different optimisation problems. The efficiency of this digital processing is sensitive against the underlying subarray configuration. We consider here the case of a linear antenna which is typical for SAR applications. One result is that partially overlapping subarrays (or directive elements) outperform fully overlapping subarrays (double layer subarrays).