ficiency is a measure of the deviations of systems. the aperture wavefront from a plane wave due to surface distor- The advantages of the dual reflectors are: tions on the parabolic surface. Assuming that the errors are small and randomly distributed with a root mean square (r.m.s.) 1. The feed is in a more convenient location. surface error, the efficiency is given by Equation 17.20. This is 2. Higher performance feeds can be used because the subtended a function of frequency and falls-off rapidly above a certain angle is such that wide aperture diameter feeds are needed. value which means that the upper frequency for which a reflec- 3. Spillover past the subreflector is directed at the sky which tor can be used is always given by the surface errors. The effect reduces the noise temperature. on the radiation pattern of random surface errors is to fill in the 4. The depth of focus and field of view are larger. nulls and to scatter energy in all directions SO that the far out sidelobes are uniformly raised. The study of the radiation characteristics and the efficiency of dual reflectors is similar to that for the prime focus reflector. Analysis of the radiation patterns depends partly on the size of the ns = exp -(*) subreflector. If it is small then physical optics or GTD must be used on the subreflector. The main reflector is usually large SO geometric (17.20) optics is adequate. 17/10 Antennas 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 con- figuration 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 multipl