tive Antenna Types Fig. 9.13 Types of feed horns for reflector antennas: (a) rectangular or square; (b) small conical; (c) small conical with chokes; (d) conical corrugated; (e) array feed. Of particular interest in horn feed design is the polarization performance, and the quality of a feed is usually expressed by the level of peak cross-polarization. The radiation characteristics of horns are predicted by a two-part process. First, the fields in the aperture are computed from a knowledge of the guided-wave behavior inside the horn. Second, the aperture fields are used to compute the radiated fields. The Fourier transform method has been found to work very well for the case of horns. The main types will now be briefly described. For more details, see studies done by Love (1976) and Love, Rudge, and Olver (1986). Rectangular or Square Horns Rectangular or square horns are the simplest horn type (see Figure 9.13(a)), but they are rarely used as feeds for reflectors because they have very high cross-polarization if the aperture size is not large. Small Conical Horns Small conical horns can have reasonably good cross-polarization performance (see Figure 9.13(b)). They are widely used as prime-focus feeds in small symmetric and offset reflectors. The basic design has an aperture diameter of about one wavelength and is essentially an open-ended circular waveguide propagating a TE11 mode. The radiation pattern can be improved by adding one or more rings or chokes around the aperture (see Figure 9.13(c)). These have the effect of changing the distribution of current on the flange and creating a more symmetric radiation pattern. The theoretical design of the open-ended waveguide is straightforward, but the analysis of the choked version is much more complicated. As a consequence most small feeds are designed empirically with measured data. Multimode Conical Horns Multimode conical horns improve the performance of conical horns by generating a second mode inside the horn in such a manner that the aperture fields are linear