## 3 Results and Discussions  
The structure presented in Figure 1 was simulated using the FEM-based HFSS tool, and the results are given in Figures 2-8 for S-parameters, gain, and directivity. Initially, both designs were compared for their S-parameters, and the best design was optimized for different parameters. The S-parameter results are presented in Figure 3 for both techniques. The CSRR metasurface design results are presented in Figure 3a. The blue line is S11, and the orange line is S21. The results of the CSRR metasurface design show that it has two clear bands visible, with the first band having a frequency range of 4.42 to 4.87 GHz, corresponding to a bandwidth of 450 MHz. The second band has a frequency range of 8.11 to 8.98 GHz, corresponding to a bandwidth of 870 MHz and a maximum S11 of \(-\)25 dB. The highest isolation of 55 dB is achieved for the first band. The circular disk design results are shown in Figure 3b. The results of the circular disk design show that it has two clear bands visible, with the first band having a frequency range of 5.28 to 5.77 GHz, corresponding to the bandwidth of 500 MHz, but the highest S11 is only \(-\)14 dB with an isolation of 29 dB. The second band has a frequency range of 8.67 to 11.68 GHz, corresponding to a bandwidth of 3 GHz. \begin{table}
\begin{tabular}{c c c} \hline \hline Length—35 mm & Width—25 mm & D—10 mm \\ \hline DGW—8 mm & D1—8 mm & D2—6 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Dimensions of presented design. Figure 2: The equivalent model of the proposed CSRR circuit. Figure 3: S-parameter results for two designs. (**a**) Complementary split-ring resonator metamaterial design. (**b**) Circular disk design. High isolation is available in complementary split-ring resonator metamaterial design with 55 dB. Two bands are available in design for C and X bands. Figure 4: S-parameter results for different ground widths. Ground width is changed over 6 to 16 mm, with a step size of 2 mm. (**a**) S\({}_{11}\) (**b**) S\({}_{21}\). The results show that for the ground width of 8 mm, both S\({}_{11}\) and S\({}_{21}\) have good results. The multiband response with good isolation is visible for 8 mm, ground width. Figure 5: S-parameter results for different inner ring diameters. The inner ring diameter varies from 4 to 7 mm with a 1 mm step size. (**a**) S\({}_{11}\) (**b**) S\({}_{21}\). The results show that the inner ring diameter of 6 mm leads to good results. The multiband response with good isolation is visible for a 6 mm inner ring diameter. Figure 6: S-parameter results for different outer ring diameters. The inner ring diameter changed from 8 to 9 mm with a step size of 1 mm. Results show that an outer ring diameter of 8 mm shows good results. The multiband response with good isolation is visible for an 8 mm inner ring diameter. Figure 7: Gain polar plots for both designs. (**a**) Complementary SRR design and (**b**) circular disk design. The complementary split ring resonator design has a higher gain of 5.38 dB. The ground plane width becomes defective by etching a piece of the ground plane width, which enhances the structure's resonance characteristic. The result of dimension variations in the ground plane width is investigated in Figure 4. The variation is considered over 6 to 16 mm with a step size of 2 mm. The ground plane width variation from the plot clearly shows that the \(\mathrm{S}_{11}\) results of the 8 mm ground plane width and 16 mm ground plane width have good values. Still, the isolation for both widths shows that the 16 mm results are in isolation of about 15 dB or less. Thus, we have selected an 8 mm ground plane width for these two MIMO antenna designs on the importance of the \(\mathrm{S}\)-parameters and the number of bands and their applications. The inner ring of the complementary split ring resonator is varied, and the variation in the results is observed in Figure 5. The variation in \(\mathrm{S}\)-parameter results is observed from 4 to 7 mm with a 1 mm step size.