THE FAR SIDELOBES AND NOISE TEMPERATURE OF A SMALL PARABOLOIDAL ANTENNA USED FOR RADIO ASTRONOMY

Measurements have been made of the radiation pattern of a symmetrical, prime-focus paraboloidal antenna which is used as a radio telescope at 1420 MHz. A transmitter was placed on a nearby hilltop, and the test antenna, used as a receiver, was driven through the range of directions permitted by its mounting; about 55% of the radiation pattern was accessible to measurement. The main beam, near sidelobes, and spillover lobes have been measured, and the conical sidelobes generated by scattering from the feed support struts are clearly seen. The effective temperature of the ground at 1420 MHz has been determined by radiometry. By convolving the radiation pattern with the ground temperature distribution the antenna temperature has been calculated and the contribution from various parts of the radiation pattern estimated. Of a measured total antenna temperature of 26.8 K with the antenna pointing at the zenith, the following contributions can be accounted for: cosmic microwave background (2.7 K), galactic emission (1.0 K), atmospheric emission (2.0 K), direct spillover from the ground into the feed (8.0 K), leakage through the reflector mesh (5.9 K), and diffraction around the reflector rim (0.6 K). It is concluded that ground radiation scattered from the feed support struts into the aperture is a significant contributor to antenna noise, and 5.8 +/- 2.5 K has been attributed to this cause. Of the total noise from the ground, 1.1 K is contributed by hills surrounding the site; most of this enters the antenna through the sidelobes generated by the feed support struts. On a completely flat site, strut scattering would contribute about 4.7 K.