A Design Method for Mode-Selective Waveguide Couplers in Dual-Polarized Wideband Matched-Feed Antennas

A novel type of multihole directional coupler is presented as well as a design method involving novel procedures. The device couples two orthogonal TE11 modes in a circular waveguide into two orthogonal TE21 modes with specified excitations in a surrounding coaxial waveguide, which has the circular waveguide as the inner conductor. A simple, novel analytical procedure is used to determine azimuthal hole positions, such that each of the input modes couples to only the desired coaxial TE21 mode. The method is applied to a specific coupler which is designed to couple -14 dB to both higher order coaxial modes in the band from 10 to 14 GHz. The suppression of undesired modes is generally better than 40 dB. The coupler is intended for broadband dual-polarized matched feeds, which are able to compensate for the intrinsically high cross polarization of offset single reflector antenna systems.