Design of a feed-reflector configuration to optimize performance in the presence of large central blockage

In the case of an existing antenna system, an additional antenna may be required for an auxilliary application. Once a primary system has been designed and developed, available area is at a premium, for applications such as on a spacecraft. Commonly, auxilliary antenna systems would not be considered due to large blockage resulting from existing hardware. It is preferred that the blockage be kept to a minimum in order to minimize the deleterious effects on the gain and sidelobe level of the antenna. Normally, the size and blockage of the auxilliary system are constrained by the parameters of the primary system and are not variable. In a typical reflector antenna configuration, the feed illuminates the reflector with a given edge taper. This type of reflector illumination results in the majority of the energy in the central region, where the blockage effects are the greatest. In the case where large central blockage is unavoidable it would be desirable to have a feed illumination pattern that maximizes illumination in the outer periphery of the reflector and minimizes that in the central region; i.e. a doughnut or toroidal shaped pattern is required. The authors show sample current distributions on reflectors illuminated with helices operating in the first and second modes. They compare the calculated gain losses for reflectors with first and second mode helix feeds against increasing central blockage. This data shows that the illumination efficiency is comparable for both cases, but as blockage is increased, the gain loss is decreased when a second mode helix feed is used.