An Alternative Surface-Integral-Equation-Based Sub-Structure Characteristic Mode Formulation for Lossy Composite Structures

The sub-structure characteristic modes (CMs) can be used to describe the localized modal behavior of antenna systems, giving us more degrees of freedom to explore the resonance characteristics of antennas. Due to this attractive property, the sub-structure CM analysis is gaining more and more research interest in antenna engineering. On the other hand, handling the loss and spurious-mode issues in the sub-structure CM formulations for lossy composite structures is still a challenging task and further studies are needed. In this article, following the generalized theory of CM, a novel surface-integral-equation (SIE)-based sub-structure CM formulation is developed. In the proposed sub-structure CM equation, a new sub-structure-oriented right-hand side weighting matrix is strictly formulated, which is exclusively related to the radiated power. Eigen solutions of the CM equation can clearly separate contributions of the radiated, dissipated, and reactive powers of full structure, giving important physical insights into the radiation-and-loss mechanisms of antennas whose working modes are desired to be confined to the designated part. Two numerical examples, the Yagi–Uda dielectric resonator antenna (DRA) with and without loss, and the chassis of mobile device with a human hand, are presented to validate the effectiveness of the proposed formulation.