Aperture efficiency of Itapetinga 45-ft antenna at λ = 3.3 mm

Abstract

The aperture efficiency at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\lambda = 3.3</tex> mm of the 45-ft Itapetinga reflector was found to be of about 25 percent. This radiometric result suggests an equivalent surface accuracy of less than 0.25 mm, root sum square (rss) assuming a Gaussian distribution of errors, which is much smaller than the actual rss estimated for the dish (i.e., 0.37 mm, rss) and, represents a 5 dB improvement in gain. This result confirms that non-Gaussian errors across a smooth surface have negligible effects on the aperture efficiency. It is demonstrated that there is little effect on gain produced by error correlation regions that are small compared to the antenna diameter and that antenna efficiency may be predicted from mechanical measurements when the nature of such errors across the surface is know. It is possible to predict that relatively simple mechanical readjustments of the reflector and subreflector may raise its efficiency to 30 percent.