iguration 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 analyze and design because of the offset geometry, and it is only with the advent of power- ful computers that this has become feasible. The lack of symmetry in the reflector means that when a linearly polarized feed is used, a cross-polarized component is generated by the reflector surface. When circular polarization is used, a cross-polarized component does not occur but the offset surface causes the beam to be "squinted" from the electrical bore- sight. Last, the construction of the offset reflector is more difficult. However, if the reflectors are made by fiberglass molding, 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. 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 design of the feed are important parts of the design of the entire reflector antenna. High-performance feeds are necessary to achieve high- performance antennas. The diameter of the feed in wavelengths will be determined by the angle subtended by the reflector at the feed. A prime focus reflector with an F/D between 0.25 and 0.5 will have a subtended half angle of between 90° and 53°. Application of the general rule that beamwidth is approximately equal to the inverse of the normalized aper- ture diameter shows that this means a feed with an aperture diameter of between about one and three wavelengths. Dual reflectors (Cassegrain or Gregorian) and offset reflectors have subtended angles between 30° and 7°, leading to feed diameters of between 3 and 10 wavelengths. Reflec