The radiation pattern and impedance of offset and symmetrical near-field Cassegrainian and Gregorian antennas

Most Cassegrainian and Gregorian antennas have axial symmetry, in which case the subreflector and associated supporting members partially block the aperture. Consequently, relatively high sidelobes appear in the radiation pattern, and a reflection is produced in the transmission line of the feed. These undesirable effects can be largely eliminated using asymmetrical configurations. Here we compare axisymmetrical and offset near-field Cassegrainians and Gregorians; expressions for the reflection coefficient and increase in sidelobe level are given. The offset designs are found to have superior performance in both respects.