Improvement of TE11mode coaxial waveguide and horn radiation patterns by incorporation of a radial aperture reactance

The mode coaxial waveguide or horn radiator provides a natural geometry for multiple frequency confocal feeding of a paraboloid reflector antenna system. Its radiation patterns, however, are generally characterized by unequal - and -plane beamwidths and high -plane sidelobe amplitudes. These characteristics become increasingly more obvious for coaxial radiators designed to illuminate reflector systems requiring feeds with reduced fields of view, such as the symmetrical paraboloid reflector antenna with large focal-length-to-diameter ratio or the offset paraboloid reflector antenna. Left uncorrected, these feed characteristics severely limit the available antenna performance. Incorporation of a radial reactive iris in the aperture plane of a mode coaxial radiator has been found to produce rotationally symmetric pattern characteristics with reduced sidelobe levels. This paper describes an approximate technique for determining the radiation patterns of a mode coaxial radiator with a radial aperture reactance. Experimental results are shown that reveal the pattern improvements predicted by the analysis. A multiple frequency coaxial feed that employs radial aperture reactances is capable of providing the aforementioned reflector systems with a high performance capability.