Characteristic mode analysis using green's function of arbitrary background

We perform characteristic mode analysis (CMA) for objects immersed in an arbitrarily inhomogeneous and anisotropic background. In addition to the free-space Green's function, our scheme leverages the background Green's function which consists of an extra scattering term to account for the scattering of the background inhomogeneity. The impedance operator of the object is constructed with the technique of numerical Green's function (NGF). With the proposed scheme, CMA is extended to more general applications, which offers better guidance in practical antenna engineering.