## 2 Antenna Design and Configurations
A longer ground causes destructive interference at specific frequencies, reducing efficiency and the bandwidth. On the other hand, lowering it improves coupling between the radiating patch and the substrate, improving the BW around the first and third resonances. The optimized dimensions of the antenna, as determined by a parametric study, are presented in Table 2. It should be mentioned that the optimization process was performed using Computer Simulation Technology (CST) studio suite and the choice of the genetic algorithm in the CST itself. Following a brief analysis of some of the antenna's parameters in Interface 3/6 and their optimization, the S-parameter outcomes were evaluated at each phase of incorporating additional layers into the antenna. Figure 6 illustrates the reflection and transmission coefficient results after adding each layer (adding layers 1, 2, and 4-6). It reveals that the three resonance modes and corresponding bands are achieved around 3.8 GHz, 7.5 GHz, and 22 GHz in the absence of slots and a ground plane on the back, and after cutting the slots, the BW was enhanced around 8 GHz. Moreover, adding a ground and the CESRR at the back shifted the whole band to a higher band, and another band was created around 12 GHz. Adding layer 1 (Interface 1/6) of a MTS, as shown in Figure 2a, enhances  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Parameters** & **Values (mm)** & **Parameters** & **Values (mm)** & **Parameters** & **Values (mm)** & **Parameters** & **Values (mm)** \\ \hline \(W_{s1}\) & 40.00 & A & 45\({}^{\circ}\) & \(L_{s3}\) & 30.00 & \(b_{3}\) & 18.00 \\ \(L_{s1}\) & 30.00 & \(S_{1}\) & 0.25 & \(L_{g3}\) & 3.00 & \(L_{p}\) & 31.00 \\ \(t_{1}\) & 0.25 & \(S_{2}\) & 0.30 & \(W_{g3}\) & 12.00 & \(L_{g1}\) & 4.25 \\ \(Wthe impedance bandwidth and radiation performance, such as gains and efficiency. It should be mentioned that coffee bean MTSs are so named due to their resemblance to coffee bean shapes, and they are a specialized type of MTS, characterized by their unique geometry, looking like an elliptical curve with a slot at the middle. These structures are utilized to manipulate electromagnetic (EM) waves with high precision, offering distinct advantages over traditional antenna designs. They can tailor electromagnetic responses, such as controlling the polarization and phase. Their design enables subwavelength-scale structures, allowing for compact and lightweight antennas (not too strict, based on the \(\lambda/2\) rules). They can control the phase, amplitude, and polarization of EM waves through the arrangement and orientation of their unit cells. They can also support dual or multi-resonant points, facilitating wideband performance (e.g., 13-20 GHz or 14-25 GHz in some designs) suitable for multi-band applications. In addition, they have the ability to convert from LP to CP. Considering all these features, four unit cells make an array of a flower-like structure formed by rotating the cells by 45 degrees. Their elliptical shape, slot geometry, and their rotation create a symmetric phase gradient control that enhances constructive radiation toward the broadside direction, resulting in higher directivity and gains as compared to square or circular shapes. This structure also minimizes the side lobes and focuses energy on the main lobe [35]. A layer of air with a thickness of 5.4 mm (layer 2, as Interface 2/6 is not depicted in Figure 2) exists between layer 1 and layer 3 to further enhance the gain and efficiency. It should be noted that, considering electromagnetic wave theories, this air gap (with relative permittivity close to 1) acts as a transitional medium that minimizes the impedance mismatch between the antenna substrate and free space, improving the impedance BW and working BW. It also preserves the phase velocity of radiated waves and decreases possible power dissipation through substrate losses so that higher radiation efficiency can be achieved by directing more energy into free space rather than being absorbed or trapped within the structure.