A Miniaturized Quad-Port Circularly Polarized MIMO Antenna for mmWave Wireless Applications

This paper presents a miniaturized quad-port circularly polarized (CP) multiple-input multiple-output (MIMO) slot antenna with enhanced isolation, gain, and diversity performance for millimeter-wave (mmWave) applications. Each port excites an identical notched rectangular slot via a 50 Ω microstrip line. A simple “+”-shaped slot etched on the ground plane improves inter-element isolation. Additionally, a split ring resonator (SRR)-based reflector, designed at the antenna’s fundamental mode and placed above the array, improves the gain by 1 dBic and further reduces mutual coupling within a compact size of 14×14×0.508 mm (1.31λ0×1.31λ0×0.047λ0), where λ0 is free space wavelength at the operating frequency of 28 GHz. The proposed CP MIMO antenna has an impedance bandwidth (IBW) of 25.80–28.96 GHz, an axial ratio bandwidth (ARBW) of 26.56–28.96 GHz, a gain of 6.0 dBic, isolation of ≤-22 dB, and a radiation efficiency exceeding 94%. Excellent diversity performance is demonstrated through a low envelope correlation coefficient (ECC), high diversity gain (DG), a mean effective gain (MEG), and low channel capacity loss (CCL). An equivalent circuit model of the MIMO antenna is analyzed. Measured and simulated results show strong agreement, confirming the antenna’s suitability for mmWave wireless communications and smart devices.