PMCHWT-Based Characteristic Mode Formulations for Material Bodies

By studying solutions of various forms of the Poggio-Miller-Chang-Harrington-Wu-Tsai (PMCHWT)-based characteristic mode (CM) formulations, we have found that the proper choice of the weighting operator of the generalized eigenvalue equation is crucial in developing formulations that are immune from spurious solutions. Spurious modes of the Chang-Harrington surface formulation for CMs of lossless material bodies are shown to be solutions of a reverse problem where the material parameters of the background and the object are swapped. These unwanted modes can be avoided by removing the integral operators of the interior of the object from the weighting operator of the generalized eigenvalue equation. We also illustrate that to obtain physically meaningful eigenvalues with orthogonal far-field patterns, it is necessary to define that weighting operator so that it connects the eigensolutions to radiated power.