Offset spherical reflector antenna with low cross-polarisation

In view of the emphasis now attached to low sidelobe envelopes for satellite communications, an increasing importance has been given to dual offset antennas for small Earth stations. The above mentioned offset geometry is particularly interesting because it avoids subreflector and feed blockages which are responsible for rising sidelobes. The spillover effects in the offset plane can be minimised by aligning the plane of offset orthogonally to the geostationary orbit. Attracted by the costs characteristics of the spherical reflector, the authors present a few modifications in the geometry initially considered by P. Ramanujan et al. (see IEE Proc. H, vol.134, no.2, p.199-204 (1987)). These modifications lead to designs with a higher efficiency, a larger clearance from the sidelobe specifications, and a significant reduction on the farfield cross-polarisation. To illustrate this the authors use the Gregorian configuration