## 3 Simulation Results and Discussion
### The Design and Optimization of the Proposed Design
Figure 11: **(a)** Surface wave current before deputing DGPS (top view) at 3.89 GHz; (b) Surface wave current after deputing DGPS at 3.89 GHz (top view); (c) Surface wave current after deputing DGPS at 3.89 GHz (front view); (d) Surface wave current on the ground plane before deputing DGPS at 3.89 GHz (counterclockwise); and (e) Surface wave current on the ground after deputing DGPS at 3.89 GHz (co-centric). the new placement of the radiating element at a 0.15\(\lambda\) position which, as a result, enhanced the bandwidth a bit. Simulated analyzed conclusions disclosed that the proposed DRDA MIMO had a return loss of 24.19%, covering the range from 3.57 GHz to 4.68 GHz. The attained S11 bandwidth was about 2.62% more than the bandwidth of Design-2. The previous section explains the two resonance modes in the S11 results because of the novel conformal metal strips. On the other hand, Figure 13 illustrates the CP bandwidth of the proposed design. It has been found that the inclusion of the DGPS has not such a considerable impact on the CP ratio. The simulated circular polarization bandwidth (3 dB) achieved by the proposed design was 22%, which was achieved over the band from \(\sim\)3.55 GHz to 4.40 GHz. The minimum of axial ratio was achieved at 3.88 GHz. The obtained AR bandwidth of 100% occurred in conjunction with impedance-matching bandwidth. By placing the DGPS and radiating elements at their optimum positions, mutual coupling was reduced significantly, as illustrated in Figure 14; while explained earlier;  
Figure 12: Comparison between the return loss of Design-1, Design-2, and the proposed design. Figure 13: Axial ratio of the proposed antenna. mutual coupling occurs either through radiation or surface currents on the ground; by placing both DRs in the optimum position, at 0.15 \(\lambda\), with the working mechanism of the DGPS, these steps overcame the issues which were experienced in Design-1 and Design-2. High MC in the low-frequency band is due to the surface waves coupling between feed elements [29], and based on the simulated result, as depicted in the results, suitable isolation of more than \(-28\) dB was achieved on average (12 dB at 3.65 GHz, 26 dB at 3.89 GHz, 16 dB at 4.85 GHz). Furthermore, Table 5 compares the performances of the proposed antenna with respect to the previous work based on the technique for reducing MC, return loss bandwidth, and the type of antenna. In parallel, the proposed antenna is also compared with other circularly polarized MIMO antennas discussed in the literature. The comparisons are presented in Table 6. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Literature** & **Shape of the Radiating Elements** & **Isolation Technique** & **BW (GHz)** & **Isolation (dB)** \\ \hline Ref [6] & Fractal DRAs & DGS and sliding & 3.89–10.4 & \(\sim\)15 \\ \hline Ref [10] & Fractal DRAs & DGS and sliding & 8.36–12.6 & \(\sim\)18 \\ \hline Ref [21] & Cylindrical DRA & SRR and Meta-surface Shield & 56.6–64.5 & \(\sim\)28 \\ \hline Ref [22] & Rectangular DRA & MPR & 5.10–5.80 & \(\sim\)16 \\ \hline Ref [6] & Rectangular DRA & Sliding & 3.4–4.40 & \(\sim\)24 \\ \hline Proposed Antenna & Rectangular DRA & DGS & 3.35–4.5 & \(\sim\)28.25 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparisons with other MIMO antennas. Figure 14: Comparison of mutual coupling reduction in Design-1, Design-2, and the proposed design.