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The design procedures for interdigital and comb-line filters are summarized and the theory of these filters extended to include various aspects of design and performance which are not covered in the literature. The basic design equations are modified to give unified sets of expressions relating the resonator capacitances directly to the filter specification, and the theory of these filters is extended to include group delay, high-power breakdown effects and precise centre-frequency synthesis. In addition, the factors contributing to in-band dissipation loss are considered, and a method of predicting this loss in terms of the unloaded-Q of a slab-line resonator is given. Finally, a method of suppressing unwanted harmonic responses is outlined.