Design of antennas for wireless communication systems has attracted increasing interest during last years. The main objective of this Thesis is to propose a general design procedure of antennas for wireless communications systems, which provides a physical insight into the design process. To accomplish this objective, a method based on a modal decomposition of the current on the surface of the conducting structure will be proposed. Modes have the advantage to provide physical insight into the radiating behavior of the antenna, as well as useful information for the optimization of the antenna geometry and the selection of the optimum feeding mechanism and its location.
A review of different modal methods, as well as the most important parameters to deal with when working with modal solutions, will be made. A method to obtain closed-form expressions for the modal surface currents on open planar conducting objects will be investigated. As it will be discussed, planar objects with canonical shapes can be interpreted in most cases as a deformation of three-dimensional objects whose surfaces coincide with any of the curvilinear reference coordinate systems. Consequently, closed-form expressions for vector modes in a circular conducting disk and an infinite planar conducting strip will be obtained. These functions will be proposed to be used as entire-domain basis functions in more complex problems including these planar surfaces.
Current modes defined from vector wave functions are of complex nature, what sometimes makes them difficult to use for design purposes. In contrast, the Theory of Characteristic Modes provides a decomposition of the total current in the surface of any arbitrary conducting body in a set of real modes, whose radiation patterns are orthogonal over both the source region and the sphere at infinity. 
The Thesis will apply the Theory of Characteristic Modes to antenna design. Investigations will mainly focus on the effect of the feeding configuration and location on the modal excitation. The objective is to provide final prototypes of antennas to be used in real wireless systems, so as to prove the utility of the proposed method in a real design process. The method will hence be applied to antennas for different applications: Wideband antennas, antennas for UWB systems, antennas for Multiple-Input Multiple-Output (MIMO) systems and antennas for handheld devices.