Analysis of beam deflection characteristics of offset parabolic antennas by beam mode expansion

In parabolic antennas with a defocused primary radiator, performance degrad ation, such as gain degradation and increased sidelobe levels, arises due t o aberration in the aperture. In this paper, the aberration produced in the aperture is determined by beam mode expansion, and it is shown that variou s aberrations can be represented by simple expressions by considering the d ominant beam mode. Using the derived expression for the aberrations, a form ula for the beam deflection characteristics is derived for the co-polarizat ion component by the aperture distribution method. A simple expression is d erived for the gain degradation under the condition that FID is large and t he edge level is low. The result obtained by the proposed method is compare d by numerical examples to that of the strict current distribution method, and the validity of the proposed method is demonstrated. (C) 1999 Scripts T echnica, Electron Comm Jpn Pt 1, 82(9): 39-50, 1999.