Applications of the characteristic mode theory to antenna design

In this article discusses the characteristic mode analysis was done to identify and select two eigen-modes of the antenna that are significant (MS > 0.707) and to obtain information on the phase difference of these modes. With this knowledge, antennas with circularly polarized patterns can be designed by combining the two orthogonal characteristic modes with a 90° phase difference. In the wideband antenna case, the MSs of the antenna's characteristic modes were examined to find modes that are significant over a wide bandwidth. Subsequently, the eigen-currents and the eigen-fields of the wideband modes were evaluated to find modes that possess omni-directional radiation patterns, which is an essential property of ultrawideband antennas. In the example of chassis antenna design, two significant chassis modes were excited using the same set of capacitive coupling elements. This was accomplished by understanding the MSs and the eigen-current distributions of the two desired modes, and by placing the capacitive coupling elements at the locations where the magnitudes of the eigen-currents are the weakest for these modes. Loop antennas were mainly used as inductive coupling elements to excite a desired mode of a metallic platform, and a systematic approach to enhance the bandwidth of the platform-based antenna was demonstrated. This approach employed the characteristic mode theory to examine the MSs and the eigen-current distributions of the platform modes, and to calculate the maximum available bandwidth that the desired characteristic mode could offer. The characteristic mode theory can be used as a tool to aid antenna design by allowing engineers to interpret the physical characteristics of the radiator(s).