Microstrip feeds for prime focus fed reflector antennas

The use of microstrip radiators as feeds for reflector antennas is of great current interest, as they can result in reduced feed volume and weight, and such aspects are important in a number of applications, including satellite systems. In the paper the performance characteristics, and in particular the radiation pattern symmetry and cross-polarisation, together with bandwidth and efficiency, of such feeds are quantified. Both singlepatch and small-array feeds suitable for prime focus fed circularly symmetric reflectors with a range of F/D ratios are examined. The crosspolarisation level of the reflector is found to be between ¿ 25 and ¿ 30 dB and is limited mainly by unwanted feed radiation and substrate surface wave scattering and, when these are well controlled, by the fact that a thin microstrip radiator is primarily a magnetic source and thus deviates from the balanced magnetic and electric Huygen source used in conventional feeds. This latter limit, together with the reduced bandwidth, stem directly from the great physical advantage of microstrip, namely its small size. Application is thus foreseen when the feed volume or weight are constrained system parameters.