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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.

Metasurfaces are used to design mantle cloaks based on scattering cancellation which involves surface impedance analysis. Theory of characteristic modes is a good tool for analyzing the scattering of a metallic object because it is based on surface impedance and it brings physical insight into the radiation properties of a structure without an excitation source. The theory of characteristic modes is applied in the analysis of a cylindrical cloaking metasurface used to cloak an LTE antenna to prevent it from interfering with a UMTS antenna when both antennas are co-located on the same site. The result provides a physical insight into the principle and design of mantle cloaks. It shows that degenerate resonant modes of the cloak are used to compensate for the degenerate non-resonant modes of the antenna. There is a symmetry in the modal surface current distribution of the cloak and the direction of the fundamental modal current distribution of the cloak is at 90° to that of the UMTS antenna. Also, the UMTS antenna has a better impedance matching after cloaking.

This paper describes the application of the theory of characteristic modes to excite a conducting cylinder representing the chassis of a rocket. Mode excitation is achieved by cutting H-shaped slots on the cylinder at specific locations where the maximum of modal current distribution occurs. The L-matching network is designed to match the impedance of the slots to the input coaxial cable. Finally, the proposed concept is verified during manufacturing and measurement. It is shown that the measured results are in excellent agreement with the simulation.

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 day Faraday moved a magnet in and out of a wire loop and detected the time-varying magnetic field, the first wireless transmitter / receiver system was created and the world was changed forever. However, it took almost fifty years for Heinrich Hertz to use Maxwell's equations and Faraday's insights in his professorship at Karlsruhe to create the first electromagnetic wireless communication system using a spark gap dipole transmitter and a loop antenna-based receiver. This simple system utilized the first non-optical human designed electromagnetic antenna, and since then, businesses, researchers, doctoral candidates, and hobbyists have been trying to determine the best way to design antennas for a variety of different applications. In almost all situations, antennas are designed using either intuition, closed-form equations, or information which can be obtained from Maxwell's equations and a set of boundary conditions. This thesis combines these three design techniques into one by using the Theory of Characteristic Modes (TCM). This theory allows for physics-based electromagnetic insights of an object to be obtained and combined with closed-form equations for all real media, limiting the overall design space, and allowing an engineer's intuition to be focused on an area with greater importance to antenna performance. TCM is a unique amalgamation of many different theoretical concepts including Maxwell's equations, Sturm-liouville eigenvalue decomposition, Poynting's theorem, and in practical applications the Method of Moments (MoM). TCM was developed first by Garbacz in 1965 and then popularized by Harrington and Mautz in 1971. Many great researchers have put years of effort and hard work into advancing and popularizing TCM, and this thesis would not exist without the advances provided by these great women and men. The research that led to the initial idea of this thesis was based around the development of multiple-input multiple-output (MIMO) antennas for hand held devices, as this type of design environment is challenging due to the electrical size of the device and the limited real-estate available. As TCM is uniquely suited for analyzing electrically compact systems which require orthogonal modes of radiation, it was a perfect candidate for studying how it can be better applied to this type of application.The research contained within this thesis, as well as the articles published during the time of this doctoral study, analyze the practical and theoretical applications of TCM and present a set of theoretical proofs, which explain some of the shortcomings pertaining to MoM-based TCM analysis of dielectric or magnetic objects, and provide some solutions to many of these problems. Furthermore, a unique antenna design methodology was developed which allows for electrically compact MIMO terminal antennas to be designed in a fundamentally new way. As TCM provides a unique set of excitation-free attributes, as well as a set of orthogonal surface currents and far-fields, which are determined only by the object's shape and material. These orthogonal attributes can be used to determine how the object's characteristic modes (CMs) relate to a set of closed-form equations. Using the knowledge gained from each CM, and how the CMs link to these equations, small object alterations can be defined and used to adapt and feed the object, creating single or multiple optimized antennas from the object.

The symmetric PMCHWT method of moments (MoM) impedance matrix allows for the characteristic modes (CMs) of a structure containing dielectric to be found. Recently, this impedance matrix was proven to provide non-real solutions which can be attributed to the MoM internal resonance problem. These internal resonances can be removed through different CM post-processing techniques. However, these studies focus on dielectric structures, whereas the majority of antennas utilize electric conductors as the radiators. As such, dielectrics are often neglected to simplify the CM analysis and hence the problem of internal resonance is overlooked. This work explores the extent of the internal resonance problem in mixed conductor-dielectric structures. The results reveal that the problem is severe even when the structures only contain small amounts of dielectric materials. Moreover, the significant impact of dielectrics on CMs reveals that dielectrics should be included in CM analysis to ensure high accuracy.

Characteristic mode analysis has traditionally been constrained to problems which utilize only perfect electric conductors (PEC). Through forced symmetry of a method of moments surface integral equation and newly proposed post-processing, characteristic modes can be solved for any material in a computationally efficient manner. As an example, the characteristic modes are solved for a mobile terminal consisting of both PEC and dielectric materials.

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.

Duisburg, Essen, Univ., Diss., 2007 WIRELESS COMMUNICATIONS have undergone a drastic development in the past decade and have also been an important motivator for small antenna research. Introduction of new communications systems with wider system bands, evolution from single-band to multiband standard and a significant decrease in average of the terminal size are on the roadmap of the industry. All this combined with strict limitation due to the protection of mobile devices users from radiation exposure has created needs for adequate antenna solutions and better understanding of small antennas on small complex platforms. As answer to these great challenges, a purposeful investigation of a new concept for small antennas development on small platforms is proposed in this thesis. The performance of a small antenna mounted on a small conductive object, such as the chassis of a mobile phone is strongly affected by the size and shape of the object as well as the position of the small antenna on it. For a better understanding of the radiation properties of the radiating system constituted by a small antenna and the conductive chassis of a mobile phone device, this thesis conceptually considers the small antenna and the conductive chassis as two distinctive entities and separately investigates the radiation properties of the metal chassis. The idea here is that from the knowledge of the chassis radiation properties, one easily identifies the optimal location of the small antenna on the chassis and achieves a design of the small antenna to optimally meet some given requirements. Investigation of the conductive chassis radiation properties is based on the theory of characteristic modes. In fact, the theory of characteristic modes is a powerful analytical concept comparable to what modal analysis represents for waveguide circuits. At a frequency of interest this concept provides an insight on the contribution of different characteristic modes in the overall chassis radiation properties, which inexorably leads to the knowledge of the dominant characteristic mode. Resonant characteristic modes which dominates the radiation properties at their respective frequencies assign their radiation properties expressed in terms of resonance frequency and radiation quality factor to the conductive chassis. In the frame of this work, a numerical evaluation of characteristic modes on the surface of conductive chassis of mobile phones is proposed. This has been made possible through the solution of the resulting generalized eigenvalue problem representing the radiating system and leading to characteristic modes. For the resolution of the resulting generalized eigenvalue problem, an algorithm based on the QZ-decomposition is conceived and a corresponding software is developed. As application, results on the radiation properties as well as optimal location of the small antenna (considered as an exciter or coupler) to achieve an effective coupling with the chassis is presented for generic bar-type and folder-type phones. For the latter two cases of operation have been distinguished: the closed and open states. A simplified version of the theory of characteristic modes for the analysis of chassis radiation properties is presented in this thesis. It is based on the far-field radiation properties of a small radiator in free-space. The radiation vector is approximated by moments of different orders depending on the shape of the chassis. This practical approach operates by post-processing output data from standard commercial electromagnetic softwares. The purposeful utilization and the tuning of chassis modes for the design of multiband radiation characteristics on generic bar-type and folder-type geometries is demonstrated.