# Meta Surface-Based Multiband MIMO Antenna for UAV Communications at mm-Wave and Sub-THz Bands
## 2 Materials and Methods
### Simulation
#### 2.1.1 Design Process of the Antenna with a Single Port
To achieve CP and broadside radiation operating around the 22-25 GHz band for mm-wave 5G and in the 110-130 GHz band for sub-THz communication, two orthogonal modes, TM\({}_{10}\) and TM\({}_{01}\), created by employing transversal and longitudinal slots (with this knowledge that slots are mode perturbation), are incorporated into the antenna design. By adjusting the size parameter of these slots (in terms of length and width), both modes are aligned to possess equal amplitudes and maintain a phase difference of 90\({}^{\circ}\), resulting in favorable CP performance. Explicitly referring to Figure 6b,c, the observed anticlockwise rotation of currents on the patches signifies the realization of the CP wave propagating in the broadside direction (the red and orange color of the current figures. Figure 5: The surface current distribution of the initial design of the antenna at (**a**) 8.5 GHz, (**b**) 25 GHz, and (**c**) 120 GHz. Figure 6: The surface current distribution at (**a**) 8.5 GHz, (**b**) 25 GHz, and (**c**) 120 GHz. Some of the most effective design parameters are presented in Figure 7 to help understand how the proposed antenna can be tuned to different frequencies. Figure 7a presents the tuning of the feed line length (\(L_{f}=L_{f1}+L_{f2}+L_{f3}\)). It shows that, when the length is reduced from 12 mm, all three working bands shift to lower frequencies. The patch length (\(L_{p}\)) increment follows the same trend as the feed line. Conversely, when reduced to less than 4 mm, the other two working bands at mm-wave and THz frequencies are significantly suppressed. The optimization of the ground dimensions (\(L_{g}\), \(W_{g}\)) is depicted in Figure 7c. It also indicates that the ground length follows the same tendency as the feed line. However, reducing the ground width significantly affects impedance matching and creates more unwanted bands when reduced to 2 mm. Last but not least, increasing the patch's width increases the frequency band and shifts them slightly to a lower band, while the opposite happens in the case of a reduction in width. After a parametric study of the single-port antenna, the impedance bandwidth results of the antenna for each step of the design (shown in Figure 2) are depicted in Figure 8. The variation of the reflection coefficient with the frequency illustrates that three modes of resonance are obtained for each stage, and they were enhanced and widened by adding the transversal and longitudinal slots and the three-step feed line. The figure elucidates the wide-band mm-wave antenna's operational mechanism and proves that following the operational principles could offer us the required resonation. The curve exhibited in Figure 8 illustrates the excitation of three resonant modes occurring at approximately 8.5 GHz, 25 GHz, and 120 GHz (black dashed line for the final stage). It should be mentioned that a bit of optimization can achieve the exact resonances. Figure 9 shows that the proposed  
Figure 7: Parametric study of the single element antenna: reflection coefficient results of (**a**) \(L_{f}\), (**b**) \(L_{p}\), (**c**) \(L_{g}\), \(W_{g}\), and (**d**) \(W_{p}\).