Fast S-domain modeling of rectangular waveguides with radially symmetric metal insets

This paper describes the application of the boundary integral-resonant-mode expansion (BI-RME) method to the modeling of rectangular waveguides with metal insets. It extends to more complicated radially symmetric insets, a method recently introduced by the authors, for the simple case of a cylindric post. In this extension, a self-consistent new theory is presented, which fully exploits the peculiarities of the considered class of structures, thus straightforwardly leading to the system equations. The efficiency of the BI-RME method, already demonstrated in the wide-band modeling of arbitrarily shaped waveguide components, is further enhanced in the particular application considered in this paper because the currents on the waveguide walls are not involved in the calculation. For this reason, the method-of-moments discretization of the field equations leads to a mathematical model of order much smaller than in the general BI-RME approach and in other boundary integral methods. Due to the state-space formulation of this model, a wide-band representation of the generalized admittance matrix of the structure is easily found in the form of a pole expansion in the S-domain by the calculation of a reasonably small number of eigensolutions of a matrix eigenvalue problem. The method is very fast and reliable, and permits the realization of a very efficient software, well suited for inclusion in computer-aided design tools for microwave circuit design. Some examples show the efficiency of the method, including the application to multiple and slanting insets and to the modeling of an evanescent-mode filter.