ction of the power which is intercepted by the reflector. As the aperture edge taper in- creases, the spillover decreases and the spillover efficiency increases, whilst the illumination efficiency decreases. There is an optimum combination which corresponds to an edge illumi- nation of about-10dB 3. The phase error efficiency is a measure of the deviation of the feed face front away from spherical and is usually nearly 100%. 4. The crosspolarisation efficiency is a measure of the loss of Figure 17.9 Dual reflector antennas: (a) Cassegrian; (b) energy in the orthogonal component of the polarisation vector. Gregorian For a symmetric reflector no crosspolarisation is introduced by the reflector SO the efficiency is determined by the feed charac- teristics. For good feeds this factor is also nearly 100%. 5. The blockage efficiency is a measure of the portion of the aperture which is blocked by the feed and the feed supports. 17.4.2 Dual symmetric reflector antennas The fields blocked by the feed do not contribute to the radiation SO it is desirable to keep the proportion of the area blocked to The performance of a large reflector antenna can be improved and less than 10% of the total area of the aperture because otherwise the design made more flexible by inserting a sub-reflector into the the sidelobe structure becomes distorted. The feed support system, Figure 17.9. There are two versions, the Cassegrain, where blocking is more complicated because it depends on the shape the subreflector is a convex hyperboloid of revolution placed on the and orientation of the supports (Lamb, 1986). It is electrically inside of the parabola focus, and a Gregorian where a concave desirable to keep the cross-section of the supports small which elliptical subreflector is placed on the outside of the parabola focus. means that a compromise with the mechanical constraints is In symmetric reflectors the Cassegrain is more common because it needed. is more compact, but the electrical performance is similar for both 6. The surface error ef