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

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... 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.