on (GTD) (James, 1986). Chapter 9 Antennas Reflector Antenna Gain The gain of a reflector antenna can be calculated from equation 9.53, where n is the efficiency of the reflector. (9.53) The total efficiency is the product of six factors: Illumination efficiency is the gain loss due to nonuniform aperture illumination. Spillover efficiency is the gain loss caused by energy from the feed that radiates outside the solid angle subtended by Oo. Called the spillover, this is the fraction of the power that is intercepted by the reflector. As the aperture edge taper increases, the spillover decreases and the spillover efficiency increases, while the illumination efficiency decreases. There is an optimum combination that corresponds to an edge illumination of about -10 dB. Phase error efficiency is a measure of the deviation of the feed face front away from spherical and is usually nearly 100 percent. Cross-polarization efficiency is a measure of the loss of energy in the orthogonal component of the polarization vector. For a symmetric reflector no cross-polarization is introduced by the reflector, SO efficiency is determined by the feed characteristics. For good feeds this factor is also nearly 100 percent. Blockage efficiency is a measure of the portion of the aperture that is blocked by the feed and the feed supports. 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 less than 10 percent of the total area of the aperture; otherwise, the sidelobe structure becomes distorted. The feed support blocking is more complicated because it depends on the shape and orientation of the supports (Lamb & Olver, 1986). It is electrically desirable to keep the cross-section of the supports small, which means that a compromise with mechanical constraints is needed. Surface error efficiency is a measure of the deviations of the aperture wavefront from a plane wave due to surface distortions on the parabolic surface. Assuming that the errors are small and r