Modal S-matrix design of metal finned waveguide components and its application to transformers and filters

Optimized all-metal E-plane finned waveguide components are designed with the rigorous method of modal field expansion into the ridged eigenmodes, which includes both the higher order mode interaction between the step discontinuities and the finite thickness of the fins. The design, which combines the advantage of constant fin thickness with that of the optimum matching potential of different waveguide inner cross-section dimensions and fin heights, achieves very broadband transformers and evanescent-mode filters with improved performance. Computer optimized data demonstrate the efficiency of the method by typical design examples: compact transformers for WR112, WR42, WR15, and WR12 input waveguides to E-plane finned waveguides, corrugated lowpass filters designed for a cutoff frequency in the waveguide Ku and U bands, and an evanescent-mode bandpass filter with unequal fin heights. The theory is verified by available measurements