A computer-aided design is introduced for E-plane filters with single and double metal inserts within a reduced-width or increased-width waveguide section, respectively. This design achieves filter performances with high peak attenuation in a broad second stopband. The theory described includes both the higher-order mode interaction of all discontinuities and the finite thickness of the inserts. The step-wall discontinuity effect is included in the optimisation process as an additional

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... to be optimised. design examples demonstrate the good stopband attenuation behaviour of this type of filters. Moreover, this design only requires thin strips, which are very appropriate for technical realisation.