Modal-S-Matrix Design of Optimum Stepped Ridged and Finned Waveguide Transformers

Optimum stepped transformers from rectangular waveguide to ridged and all-metal finned waveguides are designed with the method of field expansion into eigenmodes, which includes higher order mode interaction between the step discontinuities. Computer-optimized design data are given for a Ku-band ridged waveguide prototype with a ridge width of 1 mm, as well as for Ku-band and E-band finned waveguide transformers with commercially available metal fin thicknesses of 0.19 mm and 0.1 mm, respectively, suitable for metal-etching manufacturing technique. The optimum designs achieve a minimum return loss of about 36 dB, or 34 dB for the whole Ku- or E-band, respectively. The theory is verified by comparison with measured results.