Optimized low-insertion-loss millimetre-wave fin-line and metal insert filters

Low passband insertion-loss is achieved (1) by large-gap fin-lines, by which the high-Q potential increasing with gap-width is fully utilized, and (2) by pure metal inserts mounted in the E-plane of rectangular waveguides requiring no supporting dielectrics. This design combines the advantages of low-cost etching techniques and the low-loss performance of usual waveguide circuits. The theory described includes both higher-order mode interaction of the discontinuities and the finite thickness of dielectrics, metal fins as well as inserts. An optimizing computer program varies the filter parameters for a given number of resonators until the insertion loss yields a minimum in passband and an optimum in stopband. Data for optimized X-, Ka-, V-, E-, and W-band filters are given. Measurements verify the described theory. Measured minimum pass-band insertion losses are 0.3, 0.7, 1.5 dB for the fin-line filter for midband frequencies of about 12, 34, 75 GHz, and for the metal insert filter 0.1, 0.6, 0.5, and 0.7 dB at 12, 33, 63, and 76 GHz, respectively.