GB818447A - Microwave antenna feed for circular polarization - Google Patents

Abstract

818,447. Aerials; R.F. couplings. BENDIX AVIATION CORPORATION. Oct. 25, 1957 [Oct. 31, 1956], No. 33418/57. Class 40 (7). [Also in Group XL (b)] The invention relates to an aerial feeder and aerial arrangement for the transmission and reception of circularly polarized waves. The system is such that received waves of one sense of rotation are accepted and those of the opposite sense rejected. As shown, the input is from a rectangular guide 1 carrying energy of TE 01 mode to the system. The guide 1 is coupled by a transition unit of three elliptical sections 3, 4 and 5, which progressively approach circularity, to a circular guide 6. Propagation in guide 6 is in the TE 11 mode with the electric vector vertical. Rods 7 extending diametrically and horizontally across the guide a quarter wavelength apart act as mode aligners and the polarization is such that no energy is fed to shunt arm 10 terminating in a dissipative load 11 placed opposite a capacitive pin 12. The guide 10 and pin 12 are close to the output end of guide 6 which feeds into a further and rotatable circular guide 13 containing a dielectric polarizer 13, having deep V-grooves 15 with bevelled edges in its ends. This guide may be positioned so that its output is either circularly or horizontally polarized. Following the guide 13 is a square guide 16 in the top and bottom walls of which are fixed ridges 17 whose inmost surfaces are arcs of a circle. Where the guide 16 receives circularly polarized energy this is converted into two modes, one vertically and one horizontally polarized and the purpose of the ridges is to compensate for the unequal phase shift caused by the unequal side dimensions of the horn which is designed to have different beam-widths in azimuth and elevation. The ridges are dimensioned to ensure that the transmitted wave is circularly polarized. Reflections in the horn will be elliptically polarized and that component which results in vertically polarized energy will be passed back down waveguide 1 whilst that which results in horizontally polarized energy in guide 6 will be reflected at 7 and absorbed in the shunt arm 11. This enables satisfactory results to be obtained over a broad band of frequencies. The horn 21 has capacitive pins at its end section to match it for horizontal polarization. It is designed to have a pattern which remains the same in both the horizontal and vertical planes regardless of the polarization of the energy fed to it. This is done by the introduction of fins 22 on the sidewalls of the horn which change the aperture as a function of polarization by acting as waveguides beyond cut-off when the electric field is parallel to them.