# Slotted Circular-Patch MIMO Antenna for 5G Applications at Sub-6 GHz
## 2 Evolution of Single Antenna Element
### Performance Analysis  
The functionality of a defective circular-shaped antenna is evaluated by comparing its performance at each iteration after making changes. All design iterations maintain a partial ground on the backside. Figure 5 illustrates the adjustments to the top geometry. Figure 5a shows the traditional solid circular patch structure of the proposed antenna. The second iteration, represented in Figure 5b, includes a slot in a circular shape that is introduced at the center of the patch. In the third iteration, depicted in Figure 5c, the slot is shifted downward by 4 mm from the center. Figure 6 highlights the effects of these changes on the S11 for the three iterations. The S11 curve progression demonstrates that the single antenna design achieves optimal performance, maintaining simulated S11 values below \(-10\) dB across a frequency range from 2.73 GHz to 6.3 GHz. This high gain is notably achieved with the circular slot antenna design featuring the shifted slot at \(-4\) mm. Figure 4: The E-plane: (**a**) measured and simulated at 3.5 GHz; (**b**) at 5.8 GHz, and (**c**) radiation pattern and gain setup. The S11 values for the first and second iterations--representing the solid patch and the central slot, respectively--show that S11 approaches the \(-10\) dB threshold between 4 GHz and 5.2 GHz. In contrast, the third iteration with the lower shifted slot exhibits improved stability, reaching values as low as \(-40\) dB and providing excellent coverage at the operational frequency band. The effectiveness of the proposed antenna is influenced by various factors, and a parametric analysis has been performed to fine-tune its design. The position of the slot on the patch is vital for energy coupling, current distribution, and radiation efficiency. When the slot is near the feed line, it is closer to the high current region, allowing more energy to couple directly into the slot. This enhances interaction with the input excitation and directs current toward the patch edges, resulting in stronger radiation and better S11 performance with lower reflection. In contrast, a centered slot is farther from the feed, reducing coupling efficiency and potentially distorting mode symmetry, which can lead to unwanted mode excitation. In Figure 7a, the antenna features a central circular slot, which creates a highly symmetrical current distribution around both the slot and the patch. However, the current intensity is low and diffused, particularly around the edges of the slot, leading to weak coupling, low radiation efficiency, and potentially narrow bandwidth. In Figure 7b, the circular slot is shifted downward, bringing it closer to the feed point. This adjustment leads to stronger current localization, especially around the lower section of the patch near the slot. As a result, there is better excitation, stronger resonance, and higher efficiency  
Figure 5: Design development for (**a**) the \(1\)st, (**b**) the \(2\)nd, and (**c**) the \(3\)rd iteration. Figure 6: S11 for (1) the \(1\)st, (2) the \(2\)nd, and (3) the \(3\)rd iteration. compared to the central-slot design. The enhanced current strength in critical areas makes the patch with a shifted down slot the superior design, particularly for improving gain or bandwidth S11. In Figure 8a, the impact of the radius of the circular centered slot (r) on the antenna's impedance matching and its reflection coefficient response (S11) is depicted. Adjusting the slot radius from 3 mm to 7 mm enhances both impedance matching and stability. The optimal radius is 5 mm, which achieves an S11 of \(-40\) dB, while a radius of 3 mm results in an S11 of \(-25\) dB. In contrast, a radius of 7 mm yields an S11 of \(-21\) dB, and at 5 GHz, S11 is close to \(-10\) dB. Figure 8b illustrates how the S11 curve is influenced by shifting the slot's center about 4 mm upward and \(-4\) mm downward from the patch's exact center.