Short backfire antenna for Doppler sensing

The design of a circular short backfire antenna fed from a standard X-band waveguide (WR 90/WG 16) for use at 10.5 GHz is described. Waveguide-fed short backfire antennas have the advantage of high gain per unit length. The designs discussed here have 2(lambda)-diameter circular apertures and a best gain figure of 14.5 dB with an overall length of the dielectric insert of 1.1(lambda). An aperture transforming principle for this antenna is discussed.