A Circular-Polarized MIMO Antenna With Parasitic Phase Control and Cross-Loop Ground Decoupling

This letter introduces a compact four-port circularly polarized (CP) multiple-input multiple-output (MIMO) antenna planned for sub-6 GHz 5G and WLAN applications. The radiating element is a gap-perturbed circularly polarized (GPCP) patch, engineered to ensure steady CP operation within the frequency band by accommodating two orthogonal degenerate modes with a quadrature phase difference. A cross-loop defective ground structure (CL-DGS) is employed beneath the substrate to reduce surface-wave currents and create a stable ground-current channel, thereby improving mutual coupling (MC) and bandwidth stability. The axial-ratio (AR) bandwidth and CP purity are enhanced by carefully coupling each radiator to a sequential phase parasitic (SPP) element, which causes regulated phase progression among nearby elements. The suggested arrangement maintains a 3-dB AR bandwidth of 5.34-6.32 GHz while achieving |S11|23 dB. The antenna achieves a gain of 6.8 dBic, >88% radiation efficiency, channel capacity loss (CCL) < 0.4 bps/Hz, and an envelope correlation coefficient (ECC) < 0.01, confirming its suitability for high-performance CP MIMO sub-6 GHz wireless systems.