Series-Slot-Fed Circularly Polarized Multiple-Input–Multiple-Output Antenna Array Enabling Circular Polarization Diversity for 5G 28 GHz Indoor Applications

In this article, a four-element circularly polarized series-slot-fed multiple–input-multiple-output (MIMO) antenna array with circular polarization (CP) diversity is presented. The proposed design utilizes a combination of 45° inclined slots and a straight microstrip line feeding technique. The two antennas are designed to operate with the opposite sense of CP. CP is achieved by placing a patch of just about square dimensions on the top of the V-slot with orthogonal but unequal arms. The unequal arm of the slot degenerates the fundamental mode at a slightly different frequency with a 90° phase difference occurring at the center frequency of each arm. Proper positioning of the patch on the top of the slot ensures excitation of CP. For realizing a MIMO antenna with a different sense of polarization, both the slots and the patches are flipped with respect to the feedline. Following optimization at the full-wave level of description, the antenna is prototyped and validated experimentally. The realizable antenna footprint is 20 mm $\times27.7$ mm. The simulated and measured results indicate that the proposed antenna features a wide impedance bandwidth ( $\vert \text{S}_{11}\vert dB) from 26.9 to 30.7 GHz, 3 dB axial ratio bandwidth from 27.31 to 29.65 GHz and the peak realized gain of 11.86 dBic. Moreover, it exhibits a low level of in-band isolation $\vert \text{S}_{21}\vert > -37$ dB and an almost negligible envelope correlation coefficient (ECC), less than 0.00025. The highly directional beams, high gain, and compact size of the proposed antenna permit implementation of multiple antennas for indoor applications in the 28 GHz band.