EMSS Antennas (Pty) Ltd., Technopark, Stellenbosch, 7600, South Africa; Department of Electrical and Electronic Engineering, Stellenbosch University, Private Bag X1, Stellenbosch, South Africa
Lehmensiek, R., EMSS Antennas (Pty) Ltd., Technopark, Stellenbosch, 7600, South Africa; De Villiers, D.I.L., Department of Electrical and Electronic Engineering, Stellenbosch University, Private Bag X1, Stellenbosch, South Africa
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. © 1990-2011 IEEE.
