## 3 Results and Discussion  
In this research, the various antenna structures were simulated to identify the optimized results for the MIMO antenna. We have investigated the five types of antenna structures. The proposed antenna and its various sub-designs (Types), as shown in Table 1, were numerically investigated using the HFSS Electromagnetic simulator tool. These antennas were simulated by considering 7.5 THz of the resonating frequency. Figures 2 and 3 show the variation in S parameters for the different port excitation of Type 1 (2 \(\times\) 2) MIMO design. Figure 2a,b shows the variation in the S parameters while exciting port 1 and port 2 for 1-30 THz of the frequency band. Similarly, Figure 3a,b shows the variation in S parameters for port 3 and port 4 excitation. The detailed comparative results are shown in Table 2 for different port excitations. In all the port excitation conditions, three operating bands were observed, with a wide band of accepted return loss. In port 1 excitation, the maximum bandwidth (S\({}_{11}\) \(<\) \(-\)10 dB) was observed at 10.02 THz, with maximum return loss on band 3 with \(-\)23.44 dB, while the minimum bandwidth observed in port 1 excitation was 0.63 THz. In port 2 excitation, the maximum operating bandwidth (S\({}_{22}\) \(<\) \(-\)10 dB) was observed at 9.12 THz, with \(-\)22.55 dB of the minimum return loss. Similarly, in port 3 and port 4 excitation, the maximum bandwidth was observed at (S\({}_{33}\) \(<\)\(-\)10 dB) 10.86 THz and (S\({}_{44}\) \(<\)\(-\)10 dB) 10.96 THz, with \(-\)26.59 dB and \(-\)18.85 dB of the return loss, respectively. The directivity with co-polarization and cross-polarization is shown in Figure 4a. Figure 2: Calculated S-parameters for the Type-1 antenna (2 \(\times\) 2 structure) by considering (a) port-1 and (b) port-2 as exciting ports. Similarly, the three-dimensional directivity is shown in Figure 4b. To derive the results, only port 4 of the Type 1 antenna is excited while the other port excitation is considered zero. It can be observed from Figure 4 that co- and cross-polarization both provide the bidirectional high gain response in single-port excitation conditions. Figure 4b shows that the maximum gain is 6.32 dB in this single-port excitation condition. We have also calculated the effect in return loss, gain, and directivity for the other antennas, as shown in Table A1. These antennas' return loss and directivity are shown in Figures A1-A3. These results have helped us to choose the correct configuration of 2 \(\times\) 2. While calculating the results of the 1 \(\times\) 2, we found the results of an oscillator instead of an antenna, where return loss S11 \(>\) 0 in some of the cases. This band is generally observed between 15 and \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Port** & **Number of** & \multicolumn{2}{c}{**Minimum and**} & \multicolumn{1}{c}{**Bandwidth (THz)**} & \multicolumn{1}{c}{**Maximum Return Loss (dB)**} \\ \hline \multirow{3}{*}{Port 1} & 1 & 3.05–11.01 & 7.96 & \(-\)15.31 \\  & 2 & 15.85–16.47 & 0.63 & \(-\)12.49 \\  & 3 & 19.98–30 & 10.02 & \(-\)23.44 \\ \hline \multirow{3}{*}{Port 2} & 1 & 2.26–10.04 & 7.78 & \(-\)28.67 \\  & 2 & 15.63–16.61 & 0.98 & \(-\)15.38 \\  & 3 & 18.84–27.96 & 9.12 & \(-\)22.55 \\ \hline \multirow{3}{*}{Port 3} & 1 & 3.51–11.43 & 7.92 & \(-\)14.13 \\  & 2 & 15.82–16.51 & 0.69 & \(-\)13.19 \\  & 3 & 19.14–30 & 10.86 & \(-\)26.59 \\ \hline \multirow{3}{*}{Port 4} & 1 & 3.68–10.52 & 6.84 & \(-\)13.29 \\  & 2 & 15.88–16.24 & 0.36 & \(-\)11.93 \\ \cline{1-1}  & 3 & 19.04–30 & 10.96 & \(-\)18.85 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparative analysis of type 1 antenna for individual port excitation. Figure 3: Calculated S-parameters for the Type-1 antenna (2 \(\times\) 2 structure) by considering (a) port-3 and (b) port-4 as exciting ports. 20 THz in type 2 antennas. These results are not considered as standard antenna operation. Hence, the placement of the MIMO antenna in 1 \(\times\) 2 mode is not suitable for wideband operation.