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UBC Theses and Dissertations

The receiving pattern of a paraboloidal antenna used in radio astronomy Hanson, Bradley Everon

Abstract

This thesis is a performance study of the paraboloidal antenna of the Dominion Radio Astrophysical Observatory and is an assessment of the quantity and distribution of radiation reaching the receiver input from all directions. This study deals first with the ideal reflector for which the radiation pattern is computed for small off-axis angles and for wide-angle radiation. The latter calculations make use of the stationary-phase principle in evaluating the radiation integrals. It has been found that the half-power beam-width is slightly more than 0.5° and the first side-lobe is at least 30 db down. In the following chapter, the surface imperfections of the reflector are considered, in addition to the radiation reaching the feed from the ground. The aperture field is divided into a number of zones perturbed slightly in phase so as to approximate the slowly-varying roughness of the reflector. The resulting increase in side-lobe level is then not only related to the surface tolerance, but to the average size of each zone. The radiation reaching the feed from the ground due to spillover, transmission through the reflector mesh and holes, and reflector surface loss, contributes about 16°K to the equivalent noise temperature of the antenna. The hollow dielectric spars supporting the feed horn are considered and are treated first as being infinite in length where the necessary boundary conditions are applied. The concept of scattering in cones about the cylinder axis is also developed. Then, for the finite cylinder, radiation is assumed to result from the same scattering width. Experimental studies are carried out and with Cassiopeia A as a source, the shape of the main beam is found to agree with the theoretical result, but the level of the first side-lobe is higher than expected. This discrepancy is believed to be due to reflector distortion. The sun is used as a source for detection of spar scattering and the presence of scattering cones is confirmed. An absolute temperature calibration is carried out with a resulting figure 27°K for the antenna pointed at the zenith. This temperature is measured at the input of a Dicke switch and is consistent with the theoretical 16°K presented to the input of the feed horn.

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