Offset reflectors

In recent years the growth in communication systems has led to a tightening in the radiation pattern specifications and the consequent need to produce reflectors with low far-out sidelobes. Symmetric reflectors cannot be made to have low sidelobes because of the inherent limitations caused by scattering from the feed and feed supports. This blockage loss can be entirely eliminated with the offset reflector, Figure 17.10, which consists of a portion of a parabola chosen so that the feed is outside the area subtended by the aperture of the reflector. The projected aperture is circular, though the edge of the reflector will be elliptical. The removal of the blockage loss also means that smaller reflector antennas can be made efficient which has led to their widespread use as DBS receiving antennas.

In addition to the unblocked aperture, the offset reflector has other advantages, (Rudge, 1986, Page 185; Rahmat-Samii, 1986). The reaction of the reflector upon the primary feed can be reduced to a very low order so that the feed VSWR is essentially independent of the reflector. Compared to a symmetric paraboloid, the offset con­figuration makes a larger F/D ratio possible which in turn enables a higher performance feed to be used. The removal of the feed from the aperture gives greater flexibility to use an array of feeds to produce multiple beams or shaped beams.

The offset reflector antenna also has some disadvantages. It is much more difficult to analyse and design due to the offset geometry and it is only with the advent of powerful computers that this has become feasible. The lack of symmetry in the reflector means that when a linearly polarised feed is used, a cross-polarised component is generated by the reflector surface. When circular polarisation is used, a cross-polarised component does not occur but the offset surface causes the beam to be 'squinted' from the electrical bore-sight. Lastly the construction of the offset reflector is more difficult. However if the reflectors are made by fibreglass moulding this is not really significant. Also the structural shape can be put to good use because it is convenient for deployable configurations on satel­lites or transportable earth stations.