# Compact Four-Port Circularly Polarized MIMO X-Band DRA
## 4 Four-Port CP MIMO DRA  
Analysis of a two-port CP MIMO antenna revealed that a four-port CP MIMO DRA could also be implemented as shown in Figure 9. As the use of DGS improves antenna performance, it was applied to the final antenna structure. The four antennas shared the same ground to reduce the antenna size into a compact 30 \(\times\) 30 mm\({}^{2}\). The symmetric antenna structure provided similar results at Ports 1 and 3, and Ports 2 and 4. The location of the DR was selected so that the proposed antenna could maintain the high isolation between the ports, and the phase difference between the generated orthogonal modes remained intact. The structure of a two-port antenna was repeated by maintaining the separation of around a quarter wavelength, so that the desired results could be obtained without changing the basic nature of radiation of the single-port antenna. Figure 8: (**a**) S\({}_{11}\), (**b**) AR, and (**c**) S\({}_{21}\) parameter with separation between the DR antennas with a slot in the ground. Figure 9b shows the fabricated prototype of the CP MIMO DRA. Figure 10 shows the electric field distribution in the antenna structure without and with DGS. The field distribution shows the coupling between the adjacent radiating DR elements in a case when four ports were applied. Figure 10 also shows that there was high coupling in the case of usage of the full ground structure. The usage of DGS improved the isolation between ports. The color bar shows that the red field did not appear in the second slot area when DGS was applied. Especially at lower frequencies, this coupling was stronger in the case of a four-port structure in comparison to the two-port structure shown in Figure 6. At higher frequencies, the spacing between radiating elements became comparable to the half-wavelength; hence, the isolation was not much affected. The S-parameter response in these two cases was expected to be as usual, so it is not reported in this manuscript. The field distribution is only reported at lower frequencies and could be assumed at higher frequencies. The four-port antenna contained four radiating elements. The effect of horizontal separation between the radiating elements of the antenna having two ports was observed earlier in this manuscript (Figure 8). The effect of vertical separation being represented by Y is shown in Figure 11a-e. The variation in Y did not significantly affect the reflection coefficient. The variation in Y may affect the phase difference between the generated orthogonal modes desired for generating the CP response; thus, the AR response was varied. The increment in Y enhanced the isolation between radiating elements, which were separated by Y but not by X, as expected. The port parameters were measured using vector network analyzer ZVA 67 to validate the simulated results, as shown in Figure 12. The variation in the S parameter response at each port was due to the fabrication tolerances. A 10 dB bandwidth of 38% (8.5-12.5 GHz) was obtained from the antenna. The isolation between ports is illustrated in Figure 11e. The measured isolation results had a level higher than 20 dB in the operating band. The simulated and measured results of 3 dB AR are displayed in Figure 13. The antenna had a bandwidth of 10.41% (9.1-10.1 GHz) and 9.3% (9.2-10.1 GHz), respectively. Figure 9: Proposed four-port DRA layout: (**a**) 2D front/back view; (**b**) photo of the fabricated model. Figure 11: _Cont._  
Figure 10: E-field distribution in four-port antenna with full and partial ground structure at 9.53 GHz. The mismatching between the two results was due to fabrication tolerances, but it was still at a good level. The advantage of the proposed CP MIMO DRA is that it provides the CP response at all individual ports. Figure 14 illustrates the simulated 3D patterns of the MIMO DRA at each port at 9 and 11 GHz. As the antenna operated with the fundamental mode, the broadside radiation was obtained.