An Experimental Study of Near-Field Cassegrainian Antennas

Large microwave antennas of high efficiency and low noise are desirable in radio astronomy, in tracking of space probes and in satellite communications. In all of these cases, convenient access to the associated electronic equipment is also a desirable feature. The horn reflector 1,2 is an antenna which provides this access and also admirably satisfies the electrical requirements. Nevertheless, it is of interest to * P a r t of this material was presented to the URSI in Washington, D. C. (May, 1962). 2677 2f>92 T H E BELT, SYSTEM TECHNICAL J O U R N A L , N O V E M B E R 1964 examine other types of microwave antennas of more favorable ratio of geometrical aperture to total size with a view to improvement of their electrical performance toward that of the horn reflector. The purpose of this study is twofold: to evaluate the near-field Cassegrain as a microwave antenna, and to compare its noise performance with that of other antennas. Actually, two 16-foot diameter paraboloids have been tested, one with an f/D ratio of 0.375 and the other of 0.25. Measurements of antenna noise temperature, gain, and radiation patterns were made at a frequency of 6 gc using various feeding arrangements on both of these main reflectors. Most paraboloids have relatively low aperture efficiencies and exhibit poor noise performance. For example, paraboloids fed by a horn at the focal point typically have intrinsic (back lobe) noise temperatures of 20 or 30 degrees Kelvin, 3 4 whereas the equivalent noise for the horn reflector is about 2°K.