## 2 Design of the Six-Element MIMO Antenna Structure
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Design**} & \multicolumn{2}{c}{**Minimum**} & \multirow{2}{*}{**Peak Gain**} & \multirow{2}{*}{\(f_{min}\) (GHz)**} & \multirow{2}{*}{\(f_{max}\) (GHz)} & \multirow{2}{*}{\(\dfrac{\text{Bandwidth}}{\text{(GHz)}}\)} \\  & **Return Loss** & & & & \\  & **(dB)** & & & & \\ \hline \multirow{2}{*}{Design 1} & \(-34.51\) & 9.01 & 2.03 & 9.91 & 7.88 \\  & \(-15.84\) & 12.17 & 11.78 & 16.95 & 5.17 \\  & \(-16.11\) & 12.64 & 19.54 & 25 & 5.46 \\ \hline \multirow{2}{*}{Design 2} & \(-25.95\) & 21.72 & 4.61 & 9 & 4.39 \\  & \(-15.12\) & 5.27 & 11.88 & 14.35 & 2.47 \\  & \(-15.48\) & 5.184 & 14.95 & 16.33 & 1.38 \\  & \(-22.49\) & 16.28 & 18.13 & 22.3 & 4.17 \\ \hline \multirow{2}{*}{Design 3} & \(-30.26\) & 18.92 & 5.45 & 9.24 & 3.79 \\  & \(-28.20\) & 16.58 & 14.4 & 16.58 & 2.09 \\  & \(-23.70\) & 14.56 & 17.45 & 25 & 7.55 \\ \hline \multirow{2}{*}{Design 4} & \(-35.80\) & 10.23 & 5.42 & 9.57 & 4.15 \\  & \(-17.93\) & 10.01 & 11.82 & 14.98 & 3.16 \\  & \(-16.16\) & 13.13 & 15.94 & 17.33 & 1.39 \\  & \(-28.49\) & 17 & 21.96 & 25 & 3.04 \\ \hline \multirow{2}{*}{Design 5} & \(-32.74\) & 14.74 & 2.92 & 9.81 & 6.89 \\  & \(-15.83\) & 4.42 & 11.78 & 13.68 & 1.9 \\  & \(-19.68\) & 28.23 & 14.32 & 25 & 10.68 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Simulation based derived results and comparative analysis for the different antenna designs. Figure 2: Numerically calculated reflectance coefficients for the different proposed MIMO antenna designs: (**a**) Design 1, (**b**) Design 2, (**c**) Design 3, (**d**) Design 4, and (**e**) Design 5. The results are presented in terms of the return loss parameters, such as S\({}_{11}\), S\({}_{22}\), S\({}_{33}\), S\({}_{44}\), S\({}_{55}\), and S\({}_{66}\). A dotted line is put for the reference to identify the reflectance \(<\!-10\) dB. We have fabricated the proposed structures, and the measurement results for the proposed structure are shown in Figure 4. Similar to the results, we have presented the S parameter results of the proposed structure for all the antenna designs. We have observed the variation in the measured results as compared to the simulations. The variation in both results is due to the connector and the loss. Gain values from simulation and measurement are also showing discrepancies. It could be because of dielectric losses, radiation inefficiency, reflection, and diffraction of waves in a real-world environment. The values of the S parameters and their associated frequency spectrum are shown in Table 4. We have observed that Design 1, after measurement results, offers three operating bands of 4.77 GHz, 3.84 GHz, and 6.1 GHz. The minimum return loss in Design 1 is observed to be \(-24.82\) dB in the first band of operations. In Design 2, we have observed the maximum bandwidth of 5.9 GHz in the last band of 18.68 to 24.58 GHz with a return loss of \(-26.50\) dB. Similarly, we have observed maximum bandwidths of 7.55 GHz, 4.77 GHz, and 5.95 GHz for designs 3, 4, and 5, respectively. We observed the maximum band of 5.95 GHz in the last design with return loss values of \(-27.40\) dB. Overall, the structure of the six-element antenna with split ring resonator and rectangular ground patch offers the maximum bandwidth and minimum return loss for both simulated and measurement results. We have also observed in Design 3 and Design 4 that the number of resonating bands is increasing. In Design 3, the measurement results show 53% of the band of overall frequency converted to radiation mode. In Design 4, the number of bands increases, but overall, 47% of the bandwidth is converted to radiation mode. Out of all the structures measured, the maximum band of 68% converted to radiation conditions is for Design 5. Figure 3: Numerically calculated reflectance coefficients for the different proposed MIMO antenna Design 5. The results are presented in terms of the S parameters, such as S\({}_{ij}\), where i, j = 1, 2, 3, 4, 5, and 6 for indication of specific reflection coefficient at port.