Ground radiation scattered from feed support struts: A significant source of noise in paraboloidal antennas

Abstract

The struts supporting the feed or subreflector of a symmetrical paraboloidal antenna generate sidelobes around cones of wide opening angle by scattering energy from the plane wave leaving the reflector. On the basis of simple assumptions about these “scatter cone” sidelobes, an approximate formula is derived to predict their level relative to the main beam; it is tested against published measurements. The noise added to the antenna when the scatter cone sidelobes receive radiation from the ground is calculated, and this mechanism is seen as a significant contributor to antenna noise. The use of struts of triangular cross section in place of circular ones redirects sidelobe energy away from the back hemisphere of the radiation pattern to the front hemisphere. For most antenna pointings these sidelobes will not strike the ground, and antenna noise temperature is likely to be reduced. Radiometric measurements at 1420 MHz have been made using a 9‐m antenna equipped with struts of various cross sections and sizes. These measurements have been used to isolate the strut contribution to antenna noise, and have verified that this contribution can be reduced by using triangular struts. The strut contribution to antenna noise is calculated as a function of zenith angle of the main beam of the antenna. Triangular struts are superior to circular ones at all main‐beam zenith angles. When three struts are used, the upright Y configuration of the tripod is better than the inverted Y.