# Meta Surface-Based Multiband MIMO Antenna for UAV Communications at mm-Wave and Sub-THz Bands
## 2 Materials and Methods
### Simulation
#### 2.1.2 The Proposed MIMO LWA and Incorporation of Neutralization Network and MTS Cells  
Upon achieving the results from the initial single-port antenna setup, the design expanded into an MIMO antenna system with two ports. The objective was to broaden the overall bandwidth of the antenna, improve the bit rate transmission, and alleviate the path loss and fading effects prevalent in MIMO communication, especially in D2D and D2G communications. However, introducing an additional antenna increased mutual coupling between the two ports, undermining the desired isolation in MIMO communications. Figure 8: Reflection coefficient results of the proposed single-port antenna for each stage. Figure 9: S-parameters results of the MIMO antenna without the neutralization tape and metasurface elements. Various techniques were explored to enhance isolation between the ports. These included implementing a neutralization network, utilizing shorting posts, or increasing the physical separation between the ports, which unavoidably increases the antenna's profile. A narrow copper tape was introduced between the antennas to tackle this challenge. Subsequently, sixteen periodic H-slots (comprising three different scaling factors and sizes) were carved from this copper tape. This alteration served multiple purposes: enhancing isolation between the ports, diminishing surface waves around the CPW feeding grounds, and improving the antenna's impedance bandwidth. Introducing these H-slots inherently endowed the antenna with multiband characteristics. It also enhances the impedance BW level at the working bands and incorporates more poles across the broad operating bandwidth encompassing the millimeter-wave and sub-THz regions. Figure 10 shows a perspective view of the MIMO antenna, showing each layer and its associated materials. For further investigation, electric field distribution on the radiating elements in the three main desired resonance frequencies and modes and other poles in the working BW are illustrated in Figure 11. Figure 11 also shows the electric field variation of the antenna without the MTS cells at the back. It also indicates how the antenna works in the desired modes and frequency bands when port one is active. It can be seen that the two rectangular slots and feed lines at their centers are responsible for 8.5 GHz resonance. In contrast, in the 25 and 75 GHz, mostly the transversal and longitudinal slots at the corners and the gap between the feed line and patches create resonances, and almost no surface wave is concentrated around the slots. Therefore, according to the electric field magnitude, when one port is excited, the other one is completely isolated; this means that the mutual coupling between two elements is low, which is one of the critical challenges in MIMO antennas. Furthermore, the dominant modes can be seen with the determined antenna length and width, and other lower modes can be achieved by using these slots and the three-step matching feed line. In addition, strong fields are observed around the edges of the patches and in proximity to the substrate's edge, indicating that this resonance is associated with the 8.5 GHz and its mode of TM\({}_{11}\). Additionally, the intensity of the electric field resembles that of a patch-loaded monopolar antenna, suggesting that the patch TM\({}_{11}\) mode introduces the second resonance. Lastly, examining the electric field in Figure 11d,e reveals that the mode related to the 115 GHz and 120 GHz resonances corresponds to the leaky wave mode. It is distributed throughout the leaky wave's transmission line, capacitive patch, and inductive patch based on the characteristic of a higher mode. This observation confirms that the resonant mode at 120 GHz primarily involves the TM\({}_{22}\) mode. Figure 10: A perspective view of the MIMO antenna showcasing each layer and their materials. Following the characterization of the antenna across three antenna modes, it undergoes loading with MTS elements.