A septet beam forming network for reflector multiple-beam antennas

The problem of multiple feeds for a reflector (or lens) multiple-beam antenna (MBA) is a long-standing one in the field of microwave antennas. The dilemma is that such a system has either good beam-to-beam crossover or reasonable aperture efficiency, not both. When the feed radiators are small to be placed close enough together to obtain good beam crossover level, the aperture efficiency is poor due to excessive spillover loss. If the spillover loss is cut down using large feed radiators, the beams are too far apart, causing too much gain variation within the coverage. The problem can be overcome using the feed sharing technique. The technique requires a beam forming network (BFN) to share feeds between beams. This paper describes a BFN for a septet MBA, i.e., each beam of the MBA is produced by a 7-feed cluster and two adjacent beams share four feeds. The unique circuit topology of the BFN leads to a reliable, compact, lightweight, and low cost implementation. A breadboard model was fabricated using flight qualified multiple-layer stripline process. The test results verified the design concept and demonstrated the excellent performance of the design.