A unified multimodal variational formulation is described for the characterization of uniaxial waveguide discontinuities. A variational form is obtained by using a self-adjoint susceptance operator which is defined in terms of two eigenmode sets corresponding to the two constituent waveguides. Utilizing the similarity between the field and network theory allows the use of the accessible and localized modes, resulting in a final admittance matrix that is small in size. This formulation leads to an appreciable reduction in the computation time and computer memory space. thereby facilitating the use of the optimization process on personal computers and workstations. Examples for both homogeneous and inhomogeneous waveguide discontinuity problems are given, showing good agreement with the experiments. The design of an evanescent-mode ridge-waveguide filter is discussed.
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