Characteristic Mode Equations for Impedance Surfaces

Characteristic mode formalism is presented for closed surfaces modeled with the impedance boundary condition. The characteristic equations are expressed in terms of the electric, magnetic, and combined field integral operators (EFIO, MFIO, and CFIO). Similarly as for perfectly conducting (PEC) surfaces, the modal solutions based on EFIO (MFIO) are corrupted by spurious solutions, as the frequency coincides with the internal resonance frequency of a cavity with PEC (perfect magnetic conductor) walls, whereas the CFIO-based approach gives physically correct modal solutions.