Abstract
The near-field Cassegrainian antenna is a double-reflector system that employs, in its simplest form, confocal paraboloids. Unlike the standard Cassegrain which employs a hyperboloidal subreflector illuminated by a spherical wave, the near-field device is fed by a uniform phase front. Experimental data on noise performance, gain, and radiation patterns have been obtained at a frequency of 6 gc using two 16-foot paraboloids (focal length-to-diameter ratios of 0.375 and 0.25) in both standard and near-field configurations. Using the shallow antenna, zenith noise temperatures of 10°K and 6°K were obtained for the standard and near-field systems, respectively; at an elevation angle of 10° the antenna temperatures were 50°K and 20°K. Using the deep secondary reflector, zenith noise temperatures of 4°K were obtained for both configurations; at 10° above the horizon, however, the standard Cassegrain has an antenna temperature of 30°K and the near-field device 13°K. In all cases, the antenna efficiencies are not far above 50 per cent. Discussion of noise produced by various methods of mounting subreflectors is included. Since noise produced by transmission lines and antenna environment is closely related to these experiments, it is discussed in detail in appendices.
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