Design of Inline Waveguide Filters With Frequency-Variant Couplings Producing Transmission Zeros

A general design procedure for inline waveguide filters with frequency-variant couplings (FVCs) is presented. The goal of the design is the introduction of transmission zeros (TZs) in the filter response. The filter design starts with the synthesis of a low-pass prototype with FVC, adopting a method that recently appeared in the literature. The low-pass prototype is then denormalized to obtain an equivalent circuit of the filter suitable for waveguide implementation. The last step of the design consists of the dimensioning of the physical structure of the filter, including the frequency-dependent couplings, by exploiting the equivalence with the synthesized equivalent circuit. The design procedure can be applied independently on the specific structure implementing the FVCs. In the case of narrowband filters, the resulting structure may not require any further refinements by means of numerical optimization based on full-wave modeling. A design example illustrates the details of the procedure implementation. The validation of the procedure is presented through the design and fabrication of a substrate integrated waveguide (SIW) filter with two coincident TZs.