Analysis of reflector antennas in radio telescopes

Abstract

We present an analysis on the performance of the Cassegrain and Gregorian on-axis, off-axis and offset antennas. In our study, we have adopted the design parameters for the Cassegrain configuration used in the Atacama Large Millimeter Array (ALMA) project. Modifications on the original parameters are made so as to meet the design requirement for the off-axis and offset configurations. To reduce spillover loss in the offset antennas, we have adjusted the angle between the axis of the primary reflector and that of the sub-reflector, so that the feed horn is placed right next to the edge of the primary reflector. This is to allow the offset antennas to receive the highest power at the feed horn. In our study, signal frequency at the high-edge of ALMA band 1, i.e. 45 GHz has been selected. The results obtained from the physical optics simulation show that the radiation characteristics of both Cassegrain and Gregorian antennas are similar. The on-axis designs exhibit the best performance with an aperture efficiency εa as high as 80.34%. This is followed by the offset designs with εa = 80.24% and 80.26% for the Cassegrain and Gregorian configurations, respectively. Due to significant spillover loss at the centre hole of the primary reflector, the off-axis designs give the lowest main lobe and high side lobes. The performance of the off-axis configurations is therefore the poorest, with εa below 75%. Our analysis also shows that the performances of both onaxis Cassegrain and Gregorian antennas are comparable to each other.