On reflector feeds with unidirectional axially symmetric radiation patterns: Their cross-polarization performance and efficiencies

The performance and radiation mechanisms of feed antennas with unidirectional axially symmetric radiation patterns are investigated in detail. Radiating circular apertures, electric-magnetic dipole pairs, and two-element elliptical dipole arrays are discussed. The performance metric considers both the maximum cross-polarization level and the efficiency performance when evaluated on a general reflector system with various focal-length-to-aperture-diameter (F/D) ratios. The axial symmetry and cross-polarization performance are strongly dependent on a complementary or higher-order mode being present. For the three classes of feeds investigated, equivalent performances can be achieved with the maximum cross-polarization levels below -40 dB, and the maximum asymmetry below 0.3 dB. Apertures perform the best in terms of symmetry, and electric-magnetic dipole pairs perform the worst. It is shown that for a two-element dipole array to achieve optimum performance, the dipoles need to be perfectly circular, and the dipole lengths need to be longer than a half-wavelength, to allow for a second-order current mode to coexist with the fundamental mode.