Near-Field Mutual Coupling Suppression in Sub-Wavelength MIMO Janus Antenna

Compact multiple-input multiple-output (MIMO) antenna arrays are critical for wireless communication, but sub-wavelength spacing between elements often leads to strong mutual coupling. Existing decoupling methods, such as adding extra elements or pattern diversity, introduce design complexity or angular-dependent channel variations. In this work, we propose a solution using Janus antennas composed of orthogonal electric and magnetic dipoles with 90-degree phase difference, which exhibit quasiisotropic far-field radiation and directional near-field coupling. Building on our prior two-dimensional waveguide demonstration, we here optimize a three-dimensional active Janus antenna to construct sub-wavelength MIMO arrays. The results show over 25 dB isolation at a center-to-center distance of just 0.12 wavelengths. This work demonstrates the feasibility of highly compact MIMO systems with low coupling, enabling new paradigms in antenna design for future wireless applications.