Planar Four-Port Dual Circularly-Polarized MIMO Antenna for Sub-6 GHz Band

In this work, two configurations (Config.-A and Config.-B) of planar four-port dual circularly-polarized (CP) multiple-input-multiple-output (MIMO) antenna are proposed for sub-6 GHz (3.4-3.8 GHz) applications. The MIMO antennas are designed on the FR-4 substrate with size of $60\times 60\times 1.6$ mm³. Each radiating element (unit cell) of the MIMO antenna is comprised of a microstrip feed line and an open slot ground plane integrated with two rectangular arms for realizing circular polarization. An I-shaped metallic strip is used in the proposed four-port MIMO Config.-A and Config.-B antennas for obtaining common voltage level in the ground plane. Both MIMO antenna configurations exhibit good impedance matching in the band of interest. In the MIMO Config.-A antenna, all ports provide the same CP band, while in the Config.-B antenna the ports-1/-2 and -3/-4 radiate at different CP bands. The proposed MIMO Config.-A and Config.-B antennas support both types of polarization (left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP)). With the Config.-B antenna, the maximum 3-dB axial-ratio (AR) beamwidths of 118.9° and 92.7° are obtained at 3.536 GHz in the xz- and yz-planes, respectively. Furthermore, the I-shaped strip helps in improving 3-dB AR beamwidth in the MIMO Config.-B antenna. Inter-element isolation greater than 19 dB and envelope correlation coefficient (ECC) less than 0.12 (far-field) are achieved between the ports of the fabricated MIMO antenna prototype.