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The theory of characteristic modes (TCM) is presented for arbitrarily shaped 3-D structures including perfect electric conductors (PECs) and homogeneous penetrable objects. It is shown that by properly expressing the weighting operator of the generalized eigenvalue equation in terms of the integral operators related to the radiated fields, TCM can be formulated directly for the surface integral equation formulation of the problem. This avoids symmetrization or other modifications of the equations. The eigenvalues are shown to be related to radiated, reactive, and dissipated power, and the corresponding far-field patterns are orthogonal. The previously introduced TCM formulations for PEC are special cases of this generalized TCM.

Resonances, i.e., extrema of the eigenvalues of characteristic modes for closed perfectly conducting objects are investigated. The characteristic modal solutions based on the electric, magnetic, and combined field integral operators (EFIO, MFIO, and CFIO) are studied and compared with analytical solutions for a sphere. All these formulations are found to capture both external (radiating) and internal (cavity) resonances predicted by the analytical expressions. At the internal resonances, the eigenvalues obtained with the EFIO- and MFIO-based approaches are not correct, and the corresponding modes are nonunique. These solutions also exhibit a strong duality between the electric (TM) and magnetic (TE) type modes. A connection is found between the external and internal resonances and the condition numbers of the matrices. The modal expansion of the CFIO-based solution is correct, even though it also experiences the nonuniqueness of the EFIO- and MFIO-based solutions.

The creation of a magnetic boundary condition into the plane of symmetry in a radiating ground plane of a portable device is proposed to enhance its radiation efficiency and bandwidth. This magnetic boundary condition is achieved by exciting the antenna through symmetrical feeding. A novel antenna consisting of a symmetrical folded dipole is presented in order to exemplify how a broad bandwidth and very good radiation properties can be obtained with the proposed technique. The Theory of Characteristic Modes is used to analyze and compare a folded dipole with the symmetrical folded dipole. This study shows how the magnetic boundary condition favors excitation of antenna modes in a broad bandwidth while avoiding excitation of transmission line modes. Two different prototype antennas, a monopole and a symmetrical folded dipole with an LC-balun, were implemented and measured. The measured -6 dB impedance bandwidth of the monopole extends from 0.95 GHz to 2.15 GHz. In turn, the bandwidth of the symmetrical folded dipole extends from 0.90 GHz to 1.96 GHz. The average of the measured total efficiencies at the aforementioned bandwidth is -1.6 dB for both prototypes. The measured maximum total gain at 0.95 GHz is 2.6 dBi for the monopole and 1.9 dBi for the symmetrical folded dipole. © 2011 IEEE., This work was supported by the Finnish Funding Agency for Technology and Innovation, Nokia Devices Oulu, and Pulse Finland. The work of M. Sonkki was supported by the Universidad Politecnica de Valencia and COST IC0603 ASSIST.