Theoretical performance of prime-focus paraboloids using cylindrical hybrid-mode feeds

Abstract

An expression is derived for the radiation pattern of a hybrid-mode field distributed over a circular aperture. The hybrid mode consists of a combination of TM_m and TE_m components (where m ≥ 1) and is the type of wave which propagates in a cylindrical waveguide with an internal lossless anisotropic surface. The requirements for pattern symmetry and linear polarisation are then derived, and the form of the radiation pattern when the waveguide boundary consists of either circumferential or longitudinal slots is discussed. The advantages of using hybrid modes, instead of TM and TE modes, in a smooth pipe for optimising the performance of a paraboloid are considered. The principle of synthetising a desired paraboloid-aperture distribution by the superposition of the beams radiating from a hybrid-mode feed at the prime focus is discussed. The parameters of the hybrid modes in the waveguide, and its diameter, are optimised to obtain maximum aperture efficiency for a given number of modes by a field-matching technique at the feed aperture. Efficiencies for feeds supporting up to five hybrid modes are calculated for a paraboloid semiangle of 63°. These values are compared with those derived by field matching over the paraboloid aperture. The procedure for optimising the figure of merit of 1- and 2-hybrid-mode feeds is then considered, together with the effect of changing the mode-power ratio in the case of the 2-mode feed. Finally, the effect of frequency on the aperture efficiency and figure of merit of the 1- and 2-mode feeds is discussed.