Multi-Modal Measurement of Waveguide Orthomode Transducers for Quad-Ridge Horn-Feeds

A measurement technique is presented that characterises the full multi-modal S-matrix describing an orthomode transducer, using only one-port coaxial measurements and a sliding waveguide short. The method uses a combination of optimisation techniques, with multiple initial parameter sets, to extract the multi-modal S-parameters from the one-port coaxial measurements for a set of accurate offset shorts at the waveguide port. A cross-validation of the extracted parameters, using a validation measurement set, forms an integral part of the procedure. The method is validated using a standard coaxial to WR90 waveguide transition, and then applied successfully to a quad-ridged waveguide orthomode transducer developed for the future SKA Band A. It is shown that the S-parameters for four quad-ridged waveguide modes can be extracted over a frequency range of multiple octaves.