Computational analysis and verifications of characteristic modes in real materials

Despite its long history, the Theory of Characteristic Modes has only been utilized in antenna design for perfect electric conductors. This is due to computational problems associated with dielectrics and magnetic materials. In particular, the symmetric form of the PMCHWT surface formulation for the Method of Moments (MoM) solves for both external (real) and internal (non-real) resonances of a structure. The external resonances are the characteristic modes, whereas the internal resonances are not. This article proposes a new post-processing method capable of providing unique and real characteristic modes in all physical mediums, including lossy magnetic and dielectric materials. The method removes the internal resonances of a structure by defining a minimum radiated power, which is found through utilizing the physical bounds of the structure. The characteristic modes found using the proposed method are verified through the use of a MoM volume formulation, time domain antenna simulations, and experiments involving multiple antenna prototypes.