e 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 satel- lites or transportable earth stations. 17.4.4 Horn feeds for reflector antennas A reflector antenna consists of the reflector plus the horn feed at the geometric focus of the reflector. Thus the correct choice and Figure 17.10 Offset reflector antennas design of the feed is an important part of the design of the total The limiting factor to obtaining high efficiency in a standard parabola is the amplitude taper across the aperture due to the feed pattern and the space loss in the parabola (i.e. the illumination efficiency). By shaping the surfaces of a dual reflector antenna it is possible to increase the efficiency and produce a more uniform illumination across the aperture. A well known method to produce a high efficiency Cassegrain symmetric reflector antenna is due to Galindo and Williams (Galindo, 1964; Williams, 1965). It is a geometric optics technique in which the shape of the subreflector is altered to redistribute the energy more uniformly over the aperture. Then the shape of the main reflector is modified to refocus the energy and create a uniform phase across the aperture. After this process the reflector surfaces are no longer parabolic and hyper- bolic. The method works well for