Applying band-pass filter techniques to the design of small-aperture evanescent-mode waveguide antennas

A design methodology is presented which applies band-pass filter principles to form wideband small-aperture evanescent-mode waveguide antenna designs. This approach permits matching of the complex antenna aperture admittance of an evanescent-mode open-ended waveguide to a real impedance generator, and thereby to an SMA coaxial feeding probe. A reflection coefficient of less than -10 dB was obtained over a measured bandwidth of 15%, from 2.30-2.68 GHz, in 2.72 GHz cutoff waveguide. Capacitive posts and evanescent-mode waveguide sections are used to form the resonators/coupling elements of the antenna's filtering sections. Measured realised gain variation from 3.5-4.7 dBi is observed across the bandwidth. The antenna's maximum aperture dimension is <0.5 lambda(0) at the upper frequency in the bandwidth and so it is suitable for use in a wide angle scanning phased array.