## 5 Circularly Polarized MIMO Antenna Design Approaches  
Circularly polarized (CP) antennas have several significant advantages over linearly polarized antennas. In long-distance communication, circular polarization reduces the losses carried on by polarization mismatch. Additionally, circularly polarized antennas will reduce the spread of multipath propagation delays [68]. CP antennas are rapidly becoming a key component for a wide range of wireless systems, including satellite communications, mobile communications, wireless sensors, radio frequency identification (RFID), wireless power transmission, WLAN [69], wireless personal area networks (WPAN) and global navigation satellite systems (GNSS) [70]. Figure 17 shows some typical design approaches of circularly polarized MIMO antennas. Moreover, some latest designs and methodologies to realize CP MIMO antennas are described in this section. Figure 18 shows a small and compact two-port circularly polarized MIMO antenna with dimensions 24 \(\times\) 24 \(\times\) 1.6 mm\({}^{3}\)[71]. The circularly polarized radiated field of the MIMO antenna is achieved using a unique ground structure implanted with rectangular slots and Z-shaped radiating patches. A meandering U-shaped narrow metallic strip, which provides the good isolation, is placed between the asymmetric Z-shaped radiating components. The impedance bandwidth (3.04-8.11 GHz) of this antenna is 90.94% (Figure 19) and a 3 dB axial ratio bandwidth of 32.10% (4.42-6.11 GHz) is achieved (Figure 20). The ECC and CCL of the reported antenna are found to be 0.004 and 0.32, respectively. Figure 17: Circularly polarized MIMO antennas design approaches. Figure 18: Prototype of circularly polarized MIMO antenna [71]. The circular polarization mechanism can be explained by using the current distribution graph shown in Figure 21. From the figure it is seen that the current density is largely concentrated along the y-axis at time instants t = 0\({}^{\circ}\). The current direction on the radiating patch and on the GP are in opposing directions at t = 90\({}^{\circ}\), but the current flowing on the GP along the x-axis is maximum, and therefore the radiation occurs. With regard to time phase, the present rotation is clockwise, and the radiation is left-hand circular polarization (LHCP) in the +z direction. Furthermore, when the current is rotated in the \(-\)z direction, the current rotates anticlockwise, and the radiation received is right-hand circular polarization (RHCP). The radiation pattern is symmetrical, and the RHCP is 18 dB greater than the LHCP in the boresight direction. As a result, there is excellent cross-polarization rejection. In [72], a broadside circularly polarized T-shaped slot antenna and two end-fire CP antennas are combined to create a three-port MIMO antenna system for WLAN application (5.15-5.35 GHz). Figure 22 shows a T-shaped slot antenna with L-shaped feed line to excite the CP wave in the broadside direction. The parallel arrangement of an electric dipole and magnetic dipole with a 90\({}^{\circ}\) phase shift is used to create the end-fire CP antenna. Figure 19: S–parameters of circularly polarized MIMO antenna [71]. Figure 20: Axial ratio of CP-based MIMO antenna [71]. Figure 22: Fabricated circularly polarized MIMO antenna [72]. Figures 23 and 24 show the S-parameters and axial ratio of the designed CP MIMO antenna. Figure 25 shows that a horizontally polarized wave in the +y direction is produced at \(\omega\)t = 0\({}^{\circ}\) by the electric dipole. While the electric field distribution along the open-ended cavity is robust at time phase \(\omega\)t = 90\({}^{\circ}\), the current distribution on the electric dipole is small. As a result, at \(\omega\)t = 90\({}^{\circ}\), a vertically polarized wave along the +y-direction is excited by the magnetic dipole. The generation of CP waves along the +y-direction is caused by this 90\({}^{\circ}\) phase difference between the electric and magnetic dipoles.