# 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
Simultaneously, the voltage on the feed line prompts decreasing surface currents, facilitated by capacitive coupling, to flow from the gap side towards the opposite end of the other patch (also indicated by black arrows in Figure 4). Consequently, if the two patches  
Figure 3: Coupling mechanisms of coplanar proximity feed ((**a**) inductive coupling and (**b**) capacitive coupling). resonate at slightly different frequencies, then their currents appear in phase. Therefore, meticulous adjustment of the patch sizes is essential to achieve CP through this antenna configuration. This adjustment generates the necessary 90\({}^{\circ}\) phase difference between the resonant frequencies of the two patches. In Figure 4's arrangement, left-handed circular polarization (LHCP) is achievable by ensuring the capacitively coupled patch resonates at a higher frequency (25 GHz). On the contrary, to achieve right-handed circular polarization (RHCP), the inductively coupled patch must resonate at a lower frequency (8.5 GHz). It should be mentioned that the black arrows in Figure 4 indicate the current direction of the surface due to the capacitive and inductive effects. Following the previous equations used to define the overall dimensions of the patch, substrate, and ground, the following equations are utilized to obtain the dimensions of the capacitive and inductive patches after using this feeding technique. According to the voltage and current arrows and the previous explanations, two types of coupling occur in this structure at two different frequencies. These are described here. (1) At a frequency, f\({}_{1}\), with guided wavelength \(\lambda_{g1}\), \(l=(n\lambda_{g1})/2\), where \(l\) is the distance from the red dashed line (Figure 3) and \(n\) is an integer. The patch will resonate having this distance and will do so when the \(W_{p}\) (in Figure 2) is half of the first effective wavelength (vertical polarization). (2) At a different frequency, \(f_{2}\), with guided wavelength \(\lambda_{g2}\), \(l=(n\lambda_{g1})/2+\lambda_{g2}/4\), where \(n\) is also an integer, and the patch will resonate when \(L_{p}\) is almost half of the second effective wavelength (horizontal polarization). In these two equations, \(f_{1}\) and \(f_{2}\) are 8.5 GHz and 25 GHz, respectively. After calculating, the actual values of 1 for cases number 1 and 2 are 4.9 mm and 5.8 mm, considering \(n=1\). After achieving resonances at frequencies of 8.5 GHz and 25 GHz and confirming the functionality of the CP concept, both rectangular patches undergo modification by incorporating meshed slots. These slots, comprising transverse and longitudinal slots, serve specific purposes within the antenna system. Following the LWA principles in Figure 1, the transverse slots (Figures 1a and 2b) primarily act as left-handed capacitances in series, while the longitudinal slots (Figures 1b and 2c) alter currents and introduce an additional frequency band spanning 70-75 GHz. Furthermore, these transverse and longitudinal slots improve the impedance BW of the LWA and address inherent OSB issues in leaky-wave antennas. Figure 4: The surface current mechanism of the proposed antenna (the blue and red lines are the current and voltage standing wave amplitude, respectively). These slots play a pivotal role in guiding and facilitating wave propagation, promoting a constructive radiation pattern in the central region of the patch, and enhancing circular polarization. However, incorporating these slots and cuts into the patch has a drawback, resulting in decreased radiation efficiency and gain. Additionally, the radiation pattern tends to lean towards an end-fire orientation, a common characteristic in LWAs. Therefore, transforming from end-fire to broadside radiation directed outward from the drone is vital for D2D, D2G, and D2S. One method to achieve this transformation involves introducing a matching element in the feeding section. A three-step impedance-matching approach is utilized to mitigate the OSB issue.