Study of efficiencies and phase centers of broadband log-periodic feeds for large offset dual-reflector antennas using formulas for bodies of revolution (BOR/sub 1/ extraction)

The Allen Telescope Array (ATA) is a new instrument being built by the SETI institute. It is an array of offset Gregorian reflector antennas with a very large bandwidth, covering 0.5 GHz to 11 GHz. The feed consists of four log-periodic arms, together forming a pyramid. In the center of the pyramid, a metallic pyramid is located, holding low noise amplifiers and cryogenics. We present a method for analyzing dual reflector systems based on BOR/sub 1/-extraction and use it to analyze the performance of the ATA feed in the offset Gregorian dual reflector. With a good corrugated feed in a paraboloid or Cassegrain antenna, the spillover efficiency is typically around -0.5 dB for the subtended angle giving the highest efficiency. The spillover efficiency for the ATA feed is around -1 dB, which is caused by high sidelobes. In order to increase the performance of the system, the sidelobes should be reduced. We have found that 0.5 dB gain can be accomplished by changing the shape of the ground pyramid in the center of the feed to a cone. This reduces the spillover and increases the BOR/sub 1/ efficiency. With optimized placement of the feed in the reflector, the reduction of phase efficiency due to phase center movement is better than -1 dB. Considering the bandwidth of the feed, this is very good.