Robust and efficient model-order reduction for lossless microwave structures using the impedance formulation of the finite element method

In the lossless case, immittance formulations of the finite element method lead to real-valued system matrices. This reduces memory consumption and solution times. However, their solutions may get corrupted by interior resonances. The scattering formulation, in contrast, is inherently stable, but the resulting matrices are always complex-valued. This article presents a model-order reduction technique for lossless microwave structures that combines the low computational costs of the impedance formulation with the high robustness of the scattering approach. It extends the authors' previous work to structures fed by inhomogeneous waveguides.