## 2 Antenna Design and Configurations
The CESRR, also known as a sub-wavelength resonant structure, creates a redistribution in current and also electromagnetic coupling effects, which affect the impedance matching positively and can result in a wider BW around the X band. It also functions as an artificial magnetic conductor (AMC)-like structure, which can degrade the surface wave propagation as well as the back radiation, resulting in a higher gain. Moreover, the surface wave reduction also surpasses the ohmic loss, offering improvements in radiation efficiency. Figure 3: The designed stages of layer 3 (Interface 3/6) from stage 1 to stage 7 (S\({}_{1}\)–S\({}_{7}\)). After obtaining the required resonances and resonance modes, it was time to enhance the antenna's performance while maintaining miniaturized dimensions, as this is one of the primary objectives of the antenna for CubeSat communications. It should be noted that the one-port antenna was extended to a two-port MIMO antenna and fed inversely and longitudinally with a 180-degree rotation. Initially, a copper strip connected the two ports, adding another resonance at the higher band, around 22 GHz. However, the impedance bandwidth was reduced at the lower and middle bands. The radiation efficiency was diminished due to surface wave increments after adding this strip and so was the increment in mutual coupling. Therefore, a neutralization network consisting of nested S-shaped and T-shaped slots was etched from this strip. These slots improved the OSB and isolation between the ports (Figure 4). Figure 5 shows the proposed antenna's SCD for each working band. It shows that the antenna's current density peaks around the transmission line and CPW feeding. By increasing the mode and frequency to 5.6 GHz, the SCD extends and reaches its maximum near the large U-shaped slot, reducing as it approaches the end of the patch. This pattern holds for the SCD in the subsequent frequencies and resonant modes, as illustrated in Figure 5c,d, where the highest concentration is observed around the large U-shaped slot and in the proximity of the small U-shaped slots. In addition, Figure 5e,f depict the higher resonance and modes of the SCD and the optimized associated location for them, which should be considered during the design process and when etching them. Figure 4: The reflection coefficient results of each stage of layer 3 (Interface 3/6). Figure 5: SCD for a single element at (**a**) 3.8 GHz, (**b**) 5.6 GHz, (**c**) 7 GHz, (**d**) 9 GHz, (**e**) 22 GHz, and (**f**) 28 GHz. To understand the structure of the antenna and how it functioned during the design procedures, a parametric study was performed, and each parameter was optimized accordingly. For instance, parameters like the patch dimensions--specifically, the patch length (L\({}_{\text{p}}\)) and width (\(W_{\text{p}}\))--the length of the rear ground plane (L\({}_{\text{g3}}\)), and the length of the central S-shaped slot (L3) could significantly affect the antenna's impedance matching and working bands. Following this, varying the patch length directly affects the antenna's fundamental resonant frequency, as increasing L\({}_{\text{p}}\) extends the effective electrical length of the current path, resulting in a lower resonant frequency due to the inverse relationship between resonant frequency and effective length and vice versa. Nevertheless, the coupling effect between the patch and them created some undesired ripples. On the other hand, the width of the patch affected the BW around every resonance, as a wider one enhances the desired BW, and a reduction in that also makes the BW narrower. L\({}_{3}\) also showed some significant changes in BW, especially around the first resonance, by creating some discontinuity in the current passing around it. L\({}_{\text{g3}}\) can also play a crucial role in controlling the lower-frequency bandwidth, as its increment beyond 3 mm adversely affects the impedance matching in the lower frequency band by modifying the ground's reflection phase.