Computer-Aided Analysis of Cassegrain Antennas

Abstract

A method of analyzing, in detail, the performance of symmetrical Cassegrain antennas has been developed that uses a digital computer efficiently. For a specified antenna geometry and feed excitation, the program will compute and graphically display the amplitude and phase illumination of the subreflector, main reflector, and far-field pattern. These results may be used to optimize antenna performance by changing parameters and observing the effect. Analysis of a Cassegrain antenna with a near-field conical horn feed is discussed as an application of the method. Because the radiation characteristics of the horn are determined by the horn flare angle rather than the horn aperture, broadband performance is obtained. It was indeed found that a 50 per cent bandwidth is achieved with a dual mode TE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</inf> –TM <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</inf> mode feed, provided the proper phase relationship between the modes can be maintained over the band. For dual mode excitation an aperture efficiency of 70% and a noise temperature due to the power loss at the sub and main reflectors of less than 6.5$dGK was obtained. For a single mode feed (TE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</inf> ), there was a degradation in the E-plane side lobe levels and a corresponding 10$dGK increase in noise temperature. Excitation in the TM <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</inf> mode was also examined for angle-error sensing purposes. Also, the antenna can be used with reasonable efficiency well below the design frequency in which case it functions as a far-field fed Cassegrain antenna.