Systematic Antenna Design Using the Theory of Characteristic Modes
Abstract: 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.
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