## 3 Results and Discussion
Three-dimensional directivity with co- and cross-polarization for port excitation conditions of (P1, P2, P3, P4): (**a**) (1, 0, 1, 1), (**c**) (0, 1, 1, 1), and (**e**) (1, 1, 1, 1). Two-dimensional directivity with co- and cross-polarization for port excitation conditions of (P1, P2, P3, P4): (**b**) (1, 0, 1, 1), (**d**) (0, 1, 1, 1), and (**f**) (1, 1, 1). co- (green line) and cross-polarization (red line). In Equation (4), M denotes the total number of ports in the MIMO design, while \(\eta_{i,\text{ rad}}\) denotes the radiation efficiency of the existing MIMO design structure. The value of MEG should be \(-3\) dB to achieve better diversity performance at each port of the device. Making the difference between the two ports around 0 dB is also required. Figure 10e shows the values of MEG \(<-3\) dB for all three operating modes. TARC is the optimum measure to describe radiation performance and frequency response when used in more than one port. It is the square root of total reflected power divided by total incident power in a MIMO system. TARC measures how effective a MIMO system refracts light. When calculating TARC, mutual coupling and random signal pairings between ports on the network are considered. Equation (5) shows how reflected and incident waves describe it. Equation (6) in terms of S parameters, as stated in [26], may also be used to calculate this. Figure 10c shows the variation in the TARC of the proposed MIMO antenna. Acquired data has been deemed suitable for intended applications in terms of THz band MIMO performance. \[\Gamma_{a}^{t}=\frac{\sqrt{\sum_{j}^{M}\left|b_{j}\right|^{2}}}{\sum_{j}^{M} \left|a_{j}\right|^{2}} \tag{5}\]  
\[\Gamma_{a}^{t}=\sqrt{\frac{\left|S_{11}+S_{12}e^{i\theta}\right|^{2}+\left|S_{ 21}+S_{22}e^{i\theta}\right|^{2}}{2}} \tag{6}\]  
\[C_{\text{loss}}=-log_{2}\;det\left(a^{R}\right) \tag{7}\]  
Figure 3:0 Different calculated parameters for the \(2\times 2\) MIMO antenna structure: (**a**) ECC, (**b**) CCL, (**c**) TARC, (**d**) DG, and (**e**) MEG. \[a^{R}=\left( \begin{array}{cc}\rho_{11}&\rho_{12}\\ \rho_{21}&\rho_{22}\end{array} \right) \tag{8}\]  
\[\rho_{ii}=1-\left(\left|S_{ii}\right|^{2}+\left|S_{ij}\right|^{2}\right)\text{, \ and\ }\rho_{ij}=-\left(s_{ii}^{*}S_{ij}+s_{ij}^{*}S_{ij}\right)\text{,\ where\ }i,j=1\text{ or }2 \tag{9}\]  
Another essential characteristic to consider while evaluating the MIMO performance of the planned THz antenna is the CCL. The channel capacity loss defines the maximum rate at which information may be sent without substantial loss. The rate should be less than 0.5 bits/s/Hz to demonstrate lossless information transfer in a well-designed MIMO system. It is possible to calculate the CCL parameter using Equations (7)-(9) presented in [26]. This CCL limit is also satisfied as per the results shown in Figure 9(b). Table 2 shows the comparative analysis between the proposed structure and previously published structure in terms of dimension, gain, and material used for designing the antenna. It can be observed from the comparative analysis structure that the proposed antenna provides the ultra-wideband of 10.96 THz, with an approximate average gain of 7 dB. This design also provides better values of the MIMO parameters CCL, DG, Gain, TARC, ECC, and MEG. Comparative analysis of proposed antenna in terms of gain, dimension, bandwidth and substrate material with previously published results is shown in Table 3.  
Figure 11 shows the calculated impedance for the all-port antenna, with a frequency range impedance variation of 9-80 \(\Omega\) at 1-10 THz. Similarly, an impedance variation of 25-110 \(\Omega\) was observed at 15-20 THz. Therefore, we can apply the antenna's feeding using multiple methods that offer impedance variation through various parameters. For example, the sub-millimeter wave antenna feeding using a diagonal horn structure can be one of the methods to feed these types of antennas [37; 38].