## 5 Circularly Polarized MIMO Antenna Design Approaches
Similar effects are also seen for time phases at \(\omega\)t = 180\({}^{\circ}\) and \(\omega\)t = 270\({}^{\circ}\)[72]. Figure 23: S–parameters of three-port CP MIMO antenna [72]. Figure 24: Simulated and measured axial ratio of CP MIMO antenna [72]. Figure 25: Surface current distribution in 3-port MIMO antenna [72]. In [73], the MIMO antenna uses an array of periodic metallic plates and two square patches with distinct truncated corners for polarization variety. The antenna emits RHCP and LHCP waves when corners of the patches are truncated properly. DRA proves to be a better alternative in terms of CP and impedance bandwidth [74; 75; 76; 77; 78; 79; 80; 81]. To achieve wideband AR performance, 45\({}^{\circ}\) partial truncations at two opposing corners of the dielectric resonator are paired with an optimal ratio cross-ring coupling slot. To connect the feeding structure, glue is used to attach the DRA to the PCB. Small triangular supports are added to the bottom of the DR to allow it to be mounted on the PCB without causing any problems [74]. In [75], two cylindrical dielectric resonators and two symmetrical modified Y-shaped microstrip-printed lines are used to create a MIMO antenna. Two orthogonal modes are generated using a Y-shaped printed line. As a result of the feeding arrangement, CP waves are generated. To improve the isolation, two metallic strips and a slit on the GP are employed. In [76], as a consequence of the MIMO antenna orientation, CP radiation is achieved. To make the structure simple for obtaining CP radiation, an L-shaped DR is employed. The antenna components are 19 mm apart when placed side by side. In [77], a pair of self-complementary slot-based DGS is utilized to reduce mutual coupling between the antenna elements, and L-shaped DR is used to achieve CP radiation with the help of a common coaxial probe feeding network. In [78], by integrating the parasitic patch and incorporating the diagonal location of DRAs, isolation between the radiators is improved. Because of the proper implementation of the parasitic patch, CP is attained. In [82], the edge-to-edge spacing between the two patches of MIMO design is 12.2 mm for the best results. The basic square patch with square cut at the corner provides circular polarization. Moreover, impedance matching, high isolation and better AR bandwidth are achieved by creating a slot in the ground plane. In [83], to achieve high gain and desired impedance bandwidth, the MIMO design includes four CP pentagonal microstrip components with a suspended substrate of 6 mm height and five loaded slits. The center-to-center distance of the elements is taken as 0.655\(\lambda_{0}\). In [84], a circularly polarized MIMO antenna having four G-shaped components is reported in which two demonstrate LHCP and other two show RHCP. In the G-shaped element, a vertical line strip achieves 90\({}^{\circ}\) phase change for circular polarization and the I-shaped strip on the GP achieves an identical voltage level. In [85], for high isolation, three grounded stubs and a mirrored F-shaped DGS are used, and circular polarization is achieved using simple offset feeding. In [86; 87], the usage of a ring-shaped DRA provides the increased bandwidth. By converting rectangular to Z-shaped slots, circular polarization is obtained in both working bands. Two diagonal slotted square patches are designed in [88] to improve the bandwidth. Furthermore, to eliminate mutual coupling, four parasitic elements are used. In [89], a broadband 2 \(\times\) 1 CP array antenna with a mutual coupling of less than \(-\)25 dB and a 0.3 edge-to-edge spacing is reported and a parasitic line patch is added to the array to improve the isolation between the radiating elements. The line patch width is increased to expand AR Bandwidth. In [90], a MIMO antenna is reported consisting of two port T-shaped radiators and a rectangular-shaped GP which helps to create circular polarization.