Characteristic Mode Formulation for Antennas With Waveguide Port Feeding Structures

In conventional characteristic mode (CM) theory, modal weighted coefficient (MWC) is often adopted to measure the interaction of CMs with external excitations. However, the MWC is only available in simple source models like plane wave incidence and the delta-gap source. In this letter, a CMs formulation is proposed for analyzing the radiation or scattering properties of antennas with practical waveguide port (WP) feedings. Owing to the energy-based derivation, the resultant CMs have a clear physical interpretation and forms an orthogonal set of current basis and far-field basis which can be used for modal decomposition. The following two benefits are obtained with the proposed method: first, it considers the influence of the feeding structures and can measuring the mode excitation level due to the more practical WP excitations; and second, the CMs solved from radiation and scattering problem are consistent, which provide the possibility to optimize the radiation and scattering performance of the antennas with the same set of CMs. Numerical examples are presented to illustrate the accuracy and effectiveness of the proposed CMs method in analysis of antennas with practical WPs.