Design Guidelines for the Excitation of Characteristic Modes in Slotted Planar Structures

This paper provides some design guidelines for the excitation of broadband slotted planar antennas, addressing key issues such as coupling, symmetries and multiple feeding. The theory of characteristic modes (CMs) is used to identify the collection of current modes that exists on these structures, dealing to a valuable understanding of the radiating mechanisms and allowing a more controlled design process. Modal analysis of a circular aperture cut on a finite square ground plane is presented in order to demonstrate that an optimum choice of the feeding mechanism can be made according to the current distribution of the desired modes. Based on the information yielded by this modal analysis, a capacitive-coupled dual-feed circular aperture antenna is presented. This antenna takes advantage of the symmetries of the characteristic currents by making use of multiple feeding, in order to excite only some particular modes. Usage of commonly-fed and differentially-fed configurations enables an increased control of the excitation of modes in the structure. CM analysis of the antenna including the feeding structures is presented, showing the influence of the feeding lines in the performance of this type of antennas. A prototype of the antenna has been fabricated and measured. Simulated and measured results are presented, being in good agreement.