## 3 Evolution Process and Parametric Study
### Evolution Process
However, the dipole resonates at around 6.4 GHz, and it encounters a mismatching issue, as indicated by the REF 4 curve in Figure 7. To address this issue and to tune the resonant frequency toward 5.5 GHz, REF5 is evolved by adding extra inductance through a balun strip that connects the two arms of the structure (REF5 is shown in Figure 5). This decreases the operating frequency of Port 5 down to 4.8 GHz but with a very low matching efficiency, as shown by the blue curve in Figure 7.  
Finally, the Port 5 Mw antenna (the proposed antenna in Figure 5) is optimized by tuning the gap width between the top cavity pair. Its resonant frequency was shifted to 5.5 GHz, as shown in Figure 7, where a 200 MHz 10-dB impedance bandwidth is covered around 5.5 GHz. On the other hand, the MMw MIMO antenna, represented by the four-element MIMO QMSIW antenna, successfully covers the three targeted bands: 28 GHz, 38 GHz, and 0.13 THz, as shown in the reflection scattering parameters in Figure 8a,b. Both figures (Figure 8a, Figure 8b) also show that the achieved isolation levels are higher than 18 dB, 25 dB, and 22 dB for the 28 GHz, 38 GHz, and 0.13 THz, respectively. the operation at 38 GHz and 0.13 THz, respectively. Figure 9b displays a full-wavelength current mode on Slot 1 at 38 GHz; two half-wave current cycles are around the contour of the slot. Figure 9c shows a 2\(\lambda\) current mode on Slot 2 at 0.13 THz; four half-wave current cycles are around the contour of the slot. Additionally, Figure 9 provides extra physical insight regarding the isolation level between antenna ports. It can be seen that as the Port 1 antenna is excited while others are terminated with a 50 Ohms matched load, a very low mutual coupling current is induced in the other antenna ports (Port 2, Port 3, Port 4, and Port 5). This can be seen at the three operating MMw bands (28 GHz in Figure 9a, 38 GHz in Figure 9b, and 0.13 THz in Figure 9c). This low level of coupling is due to two reasons: (1) the presence of metal vias between adjacent QMSIW cavities prevents the current from flowing between adjacent elements, and (2) polarization diversity is employed between adjacent antenna elements, which are arranged orthogonally to each other. The proposed antenna structure's reliability has been verified through the extraction of a simulated vector surface current distribution at 5.5 GHz from a 3D simulation model in CST Microwave Studio, as depicted in \[W_{equ}=\frac{C}{f_{TE101}\sqrt{2e_{r}}} \tag{2}\]0 The shared-aperture antenna (the entire SIW structure) exhibits a behavior similar to a conventional strip antenna when it is excited at low resonant frequencies (Mw frequencies). The Port 5 antenna has an  
\[W_{equ}=\frac{C}{f_{TE101}\sqrt{2e_{r}}} \tag{2}\]1 Optimized simulated scattering parameters of the proposed MMw MIMO antenna: (**a**) at 28 GHz and 38 GHz, and (**b**) at 0.13 THz. \[W_{equ}=\frac{C}{f_{TE101}\sqrt{2e_{r}}} \tag{2}\]2 Evolution of the simulated reflection coefficient of Port 5 at Sub-6 GHz band (5.5 GHz). open-loop structure that resonates at its half-wave length resonant current mode, as shown in \[W_{equ}=\frac{C}{f_{TE101}\sqrt{2e_{r}}} \tag{2}\]3 Two current nulls are at the open edges and one peaks at the middle of the open loop. Furthermore, the excited current mode is concentrated near the inner contour of the open loop, this prevents other MMw ports (Port 1, Port 2, Port 3, and Port 4) from being significantly impacted by the mutual coupling. As a result, no external isolation circuits are required at the 5.5 GHz, 28 GHz, 38 GHz, and 0.13 THz frequencies. \[W_{equ}=\frac{C}{f_{TE101}\sqrt{2e_{r}}} \tag{2}\]4 Simulated surface current vector distribution at 5.5 GHz due to Port 5 antenna. \[W_{equ}=\frac{C}{f_{TE101}\sqrt{2e_{r}}} \tag{2}\]5 Simulated surface current distributions at **(a)** 28 GHz, **(b)** 38 GHz, **(c)** 0.13 THz.