Four-Port Wideband Circularly Polarized MIMO Antenna Covering WLAN, WiMAX, and 5G NR n77/n78/n79 Bands

Abstract

This article describes a compact four-port multiple-input-multiple-output (MIMO) antenna that has a broad axial ratio bandwidth (ARBW) for use in WLAN/WiMAX/sub-6 GHz 5G bands: n77 (3.3–4.2 GHz), n78 (3.3–3.8 GHz), and n79 (4.7 GHz) (4.4–5 GHz). The radiating element of the MIMO antenna is a polygon, and a modified ground structure is used to achieve both size reduction and bandwidth enhancement. A folded microstrip line is incorporated at the center of the ground plane to achieve circular polarization. Furthermore, the circular polarization bandwidth is enhanced by using horizontal and vertical parasitic strips on the opposite side of the ground plane. The presented quad-port MIMO antenna utilizes interelement slots to reduce electromagnetic interference (mutual coupling) and improve directional gain. The antenna design is implemented on the 1.6 mm thick FR-4 dielectric substrate with a relative permittivity of 4.4, occupying a total area of 0.25λ₀ × 0.48λ₀. The projected design achieves a measured impedance bandwidth of 2.4–6.0 GHz. It demonstrates circular polarization characteristics with a measured 3-dB ARBW of 3.3–4.1 GHz. Exceptional diversity performance is confirmed by an isolation level exceeding 18 dB, an envelope correlation coefficient below 0.005, and a diversity gain greater than 9.95 dB. These metrics validate the antenna's suitability for sub-6 GHz 5G, WLAN, and dual-band ISM implementations.