# 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
This process converts the 50 \(\Omega\) feeding to a 110 \(\Omega\) impedance observed from the patch (Figure 2d). Subsequently, another longitudinal slot is introduced adjacent to the feed line, altering the transmission mode and redirecting surface currents. This alteration aims to create an additional resonance around 120 GHz, further enhancing the antenna's bandwidth. Conceptually, the vertical slots can be envisioned as two impedances connected by a transmission line within the equivalent circuit model, contributing to the antenna's overall performance and characteristics. The structural changes applied to the capacitive coupling mechanism of the antenna play a crucial role in widening the bandwidth for both resonance modes of the antenna across the millimeter-wave and sub-THz frequency bands, extending BW up to 5 GHz. Moreover, incorporating vertical slots and an additional slot, which is capacitively loaded and positioned adjacent to the meshed slots but distanced from the transmission line, serves the purpose of suppressing surface waves around the patches. This suppression significantly contributes to improving the antenna's impedance bandwidth. Notably, the edges neighboring the transmission line are curved to minimize surface waves in the proximity, thereby improving bandwidth at lower frequencies and within the 5G mm-wave spectrum. The principles derived from LWA theories and inductive coupling feeding, explained thoroughly in the previous paragraphs in this section, provided a baseline for establishing the dimensions of patches, slots, and gaps [33; 38]. Since the proposed antenna facilitates multiple operational radiation modes with distinct radiation requirements for UAV-assisted communications, considering the influence of fringing fields and surface current distribution around the patch's perimeter is essential. In addition, a parametric study of the vital parameters of the design provides us with fine-tuning tools to achieve the desired resonant frequency and working BW. (The surface current assessment was performed by CST studio). As can be observed from Figure 5 for the proposed antenna, a strong current density is mainly concentrated on the inner edge of the square patches with the direction along the \(x\)-axis, verifying that the fundamental mode (TM\({}_{10}\)) of the inner patch is activated at 8.5 GHz. Furthermore, the current solid density can be observed around the gap between the patches and the transmission line, showing practical resonance around 8.5 GHz, and the antenna's omnidirectional radiation at its early stage is slightly changing to end-fire radiation when it is steered for frequencies higher than 80 GHz (exp. Figure 5c at 120 GHz). According to the transmission line and coupling proximity model, the antenna's total radiation pattern exhibits a radiation null at the boresight in the vertical plane. This null arises from the self-cancellation of E-field radiation from distinct radiating apertures in the far field. Consequently, an almost omnidirectional radiation pattern can be achieved in the azimuth plane around the 8.5 GHz frequency. As depicted in Figure 6, the current intensity on the outer edges of the patch at 8.5 GHz, 25 GHz, and 120 GHz is considerably strong, and each edge has almost similar amplitude distribution, especially at two higher modes. The orientation of the current direction is very similar to that of the TM11 mode of a square patch. The antenna also employs the TM11 mode of the patch antenna for radiation to achieve omnidirectional at the lower bands. Introducing parasitic slots on the capacitive patch on the transmission line's right side improves this mode's characteristics. The operational mechanism of the TM11 mode is extensively discussed in [39] and is only briefly interpreted here. In addition, it is radiated towards the front and makes an end-fire radiation (90\({}^{\circ}\)) at a higher frequency of around 120 GHz.