Pattern Shifting and Size Control in Offset Reflector Antennas with Microstrip Array as Matched Feed

In this paper, details of a dual-layered microstrip based matched feed array for an offset reflector are presented. The top layer of the proposed feed consists of 9 circular microstrip antennas operating in TM
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mode arranged in a centered circular array configuration. The second layer contains a circular microstrip antenna operating in TM
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mode and located below the central element of the circular array. The excitation ratio between the higher order mode and dominant mode patches is varied to achieve a low cross-polar level at the asymmetric plane by using the concept of conjugate field matching. The effect of circular array radius on the reflector pattern of the offset reflector with focal length to diameter (F/D) ratio equal to 0.7 is investigated to find the optimum radius of the circular array. An investigation has been done to show that by exciting selective array antenna elements, beamwidth and beam shift reconfigurability in the reflector pattern can be achieved, while maintaining the cross-polar level below –38 dB. A maximum beam shift of 0.4° (approximately 31% of 3 dB beamwidth) from the principal axis has been achieved in the reflector pattern.