Power gain of cassegrain antennas with small gridded hyperboloidal subreflectors

A Cassegrain antenna consisting of a solid paraboloid, a gridded hyperboloid and a symmetrically driven directed dipole feed is analysed. Moment method solutions are given for the input impedance and distributions of scattered electric fields obtained from gridded hyperboloids of circular and elliptical apertures with maximum dimensions of 2.3¿¿ and 2.9¿¿. It is shown that the distributions osf these fields are significantly different in the E and H plances osf each hyperboloid, and that apertures osf elliptical construction reflect the feed energy more efficiently into the forward region of the subreflector. The analysis is completed by solving the solid paraboloid problem using physical optics, and then calculating the total electric field radiated in the forward direction of the Cassegrain antenna. It is found that, for the range of positions tested, maximum power density is achieved when an axially defocused gridded hyperboloid of elliptical aperture is used together with a shift of the feed towards the hyperboloid vertex. This optimum situation offers a 12.5% increase in forward-radiated power density over that obtained from an ordinary front-fed paraboloid.