Focal fields in reflector antennas and associated array feed synthesis for high effiecinecy multi-beam performances

The paper describes status results of investigations into focal-field distributions in reflector antennas with a symmetrical and with an off-set geometry. For the symmetrical case, the 25-m radio-telescope antenna as included in the Westerbork synthesis array [1] has been considered. For the offset geometry, a working scenario has been taken comparable to the
Large Deployable Antenna (LDA) as currently being realised by Alenia [2]. Grid configurations are proposed for the focal plane arrays, which -when properly excited - provide high efficient beam patterns. This is important for off-axis beams, for which the focal field distributions are deformed. The approach to investigate array-fed reflector antennas ‘the other way around’ as compared to the usual adopted synthesis of reflector antennas, gives an insight into the actual field distributions in the focal region for reflector antennas as considered here, namely a front-fed and an offset
geometry. This is well applicable for varies type of dual reflector antennas. It supports the understanding of sizing and allocating the excitations of the set of active array elements. Together with a coupling theorem, derived from Maxwell’s equations (reciprocity related, known under different names like Lorentz-reciprocity theorem or Robieux,, etc) one can
arrive at a suitable focal plane array configuration, which performs optimum. A status of the ongoing work is presented.