Characteristic Mode Analysis of Antenna Mutual Coupling in the Near Field

Characteristic mode analysis (CMA) has been widely used for guiding antenna design and mitigating antenna mutual coupling in the far field. However, antenna mutual coupling in the near field is still an intractable problem. In this communication, CMA is adopted to analyze and mitigate the near-field coupling between antennas. It is found that more modes are required in CMA as the contributions of higher order modes are more significant in the near-field region. In addition, the modal near-field distributions are proposed to discriminate the unwanted modes along with the modal admittance and eigencurrent distributions. Inductive loads are used to suppress the unwanted modes for mitigating the out-of-band mutual coupling while the in-band radiation property is mostly retained. The design procedures are elaborated through a two-dipole-antenna system, and the effectiveness of the proposed method is verified through fabricated prototypes. Based on the measured results, a significant reduction, more than 10 dB, of mutual coupling in the near field is obtained compared with those without optimization.