A new technique of complex impedance matrix calculation based on finite element method (FEM), in contrast to the established use of Method of Moment (MoM) is proposed. The infinite solution domain is truncated with the introduction of a fictitious surface-S f. For the field solution inside the surface-Sf the applied formulation is based on the finite element technique, being able to model inhomogeneous arbitrary shaped radiating 3D structures. The field solution in the semi-infinite domain outside S f is expressed by an in principle infinite series expansion of spherical harmonics. The two solutions are bind together by enforcing the "exact" field continuity conditions strictly following a vector Dirichlet to Neumann map formalism. The overall procedure ends up to a final system with sole unknown the surface current distribution flowing over the metallic surfaces. This system defines the impedance matrix [Z] and it is formulated and solved as a characteristic mode eigenproblem. Namely, separating the complex impedance matrix into a real [R] and imaginary [X] part, yields a real eigenproblem of the form [X][I] = λ[R] is formulated and solved.
