The most important use in antennas for a hybrid-mode waveguide or horn is as a feed for a reflector, so we begin this chapter by considering, in general terms, how the feed influences antenna performance. Fig. 2.1 shows a photograph of a horn feed in a Cassegrain antenna forming part of a satellite-communication earth station while Fig. 2.2 shows a dielectric-cone feed in an antenna for a similar application. Although different in appearance and construction, the corrugated horn and dielectric-cone feed support nearly identical hybrid fields and in this chapter we shall use the term hybrid-mode feed to cover both types.