A Low-Profile Circularly Polarized Metasurface MIMO Antenna with Enhanced Axial-Ratio Bandwidth for IoT Applications

This study develops and experimentally evaluates a low-profile four-port circularly polarized metasurface MIMO antenna for fixed or sectorized wireless links in the 6 GHz band. The design begins with an aperture-coupled single element incorporating a 4×4 slotted metasurface and is subsequently extended to a 2×2 MIMO configuration. Mutual coupling between the closely spaced elements is controlled by a hybrid decoupling arrangement comprising metamaterial unit cells and slots etched in the common ground plane. The closely spaced metasurface resonances broaden the circular-polarization response, while the hybrid MTM and DGS decoupling structure suppresses complementary coupling paths between adjacent elements. A fabricated prototype was characterized to verify the simulated performance. The measured −10 dB impedance band extends from 5.56 to 7.85 GHz, corresponding to 34.15%, while the measured 3 dB axial-ratio band covers 5.60–7.13 GHz, corresponding to 24.03%. Across the operating region, the isolation exceeds 23 dB, and the maximum measured gain reaches 8.4 dBic. The MIMO characteristics include an envelope correlation coefficient below 0.0015, diversity gain above 9.98 dB, mean effective gain below −3 dB, and total active reflection coefficient below −10 dB. The measured results demonstrate that the proposed configuration combines wide circular-polarization bandwidth, low interelement correlation, high gain, and effective port isolation for fixed or sectorized IoT and Wi-Fi 6E applications.