Abstract 


The main goal of this thesis is to show how the Theory of Characteristic Modes can be systematically applied to design wire and planar antennas. Through numerous examples, it will be demonstrated that in contrast with other classical design methods, the Theory of Characteristic Modes brings insight into the physical phenomena taking place on an antenna. 

Examples will be presented in order to demonstrate that an in depth knowledge of the radiating mechanisms of very basic antennas helps to design novel antennas on a clear and rational basis. It will be also explained how the information given by characteristic modes can be used for the selection of the most suitable shape for the radiating element, as well as for the choice of an optimum feeding arrangement to maximize the impedance bandwidth.

The Theory of Characteristic Modes was first formulated by Garbacz in 1968, and later refined by Harrington and Mautz in 1971. Traditionally, characteristic modes have been applied to antenna shape synthesis, and control of obstacle scattering by reactive loading. However, at present, the Theory of Characteristic Modes has practically fallen into disuse, in spite of the fact that it leads to modal solutions, which are particularly useful in problems involving analysis, synthesis and optimization of antennas and scatterers.
Characteristic modes are real current modes that correspond with the eigenvectors of a particular weighted eigenvalue equation that involves the generalized impedance matrix of the body. Thus, characteristic modes can be computed numerically for conducting bodies of arbitrary shape, and since they form a set of orthogonal functions, they can be used to expand the total current on the surface of the body. However, what makes characteristic modes really attractive for antenna design is the physical insight they bring into the radiating phenomena taking place in the antenna. 
Associated to each characteristic mode there is an eigenvalue whose magnitude provides information about the resonant frequency and radiating characteristics of modes. Additionally, since characteristic modes are computed in the absence of any kind of excitation, they only depend on the shape and size of the conducting object. Hence, antenna design using characteristic modes can be performed in a controlled way following two steps:

-Firstly, the shape and size of the radiating element are optimized on the base of the information provided by eigenvalues. 
-Next, studying the current distribution of modes an optimum feeding arrangement is chosen so that the desired mode or modes may be excited, in order to obtain a specific radiating behaviour.