The hat feed: A dual-mode rear-radiating waveguide antenna having low cross polarization

The hat antenna is a new self-supported rear-radiating waveguide feed, related to existing feeds known as splashplate and waveguide cup antennas. An analytical model for the radiation pattern of the hat antenna, a model which includes the axial waveguide itself is presented. This model shows that by exciting the feed with two modes it is possible to use the waveguide constructively as one of the dominant radiating parts of the feed instead of having to live with it as an undesirable blockage effect. Thereby aperture efficiencies up to 81 percent and cross-polar sidelobes down to -30 dB are available. The results are confirmed by measurements on a practical model, but the bandwidth is narrow.