# Slotted Circular-Patch MIMO Antenna for 5G Applications at Sub-6 GHz
## 3 A 2-Port MIMO System Using Two Flipped Parallel 1 \(\times\) 2 Arrays Antenna  
### Design of (1 \(\times\) 2) Antenna Array Arrangement  
Higher antenna gain is necessary due to the longer operating wavelengths in sub-6 GHz compared to higher frequency bands like mm-wave and terahertz. Antenna gain significantly affects coverage and signal strength performance. To enhance gain, a two-element circular-shaped antenna array has been designed. The two-element array can potentially double the gain in dB compared to a single antenna. A common feeding line is utilized to power the radiating elements. Figure 9a,b illustrate the geometric top and back views of the antenna. The design's input impedance is around 50 ohms at 3.5 GHz and 5.8 GHz, as shown in Figure 9c.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Ref. No.** & **Area** & **Bandwidth** & **Max Gain** & **Max Efficiency \%** \\ \hline
[30] & 50 \(\times\) 50 & 2.89–4.07 & 2.6 & - \\  & 5.1–6.19 & 2.6 & - \\ \hline
[31] & 30 \(\times\) 29 & 2.38–2.77 & 1.42 & 74 \\ \hline
[32] & 40 \(\times\) 40 & 2.36–2.74 & 0.74 & 46 \\  & 5.14–6.18 & 2.3 & 46 \\ \hline Proposed single antenna & 30 \(\times\) 46 & 2.96–7.1 & 4.3 & 84 \\ \hline \end{tabular}
\end{table}
Table 2: Comparison of the proposed single design with previous studies.  
Figure 9: Simulated design: (**a**) the 1 \(\times\) 2 array’s top side, (**b**) the back side, and (**c**) input impedance over frequency.  
The design is composed of a 1 \(\times\) 2 array based on defective circular patches linked by a transformer using a power splitter. Table 3 provides the actual design's dimensions, which has an overall size of 66 \(\times\) 60 \(\times\) 1.57 mm\({}^{3}\).  
The simulated S11 of the suggested 1 \(\times\) 2 array of circular antennas is exhibited in Figure 10. The simulated design supports a frequency band of 2.59-6.1 GHz. The S11 is less than \(-10\) dB over the whole operational range. The minimal value measured for S11 is \(-33\) dB at 4 GHz. Figure 11 illustrates the changes in gain across the frequency band. The simulated gain starts from 3.1 dBi at 2 GHz and, increasing gradually, reaches 8.16 dBi at 6.4 GHz as a peak gain. Within the frequency band from 3 GHz to 6 GHz, the gain grows from 4.2 dBi to 8 dBi. At frequencies of 3.5 GHz and 5.8 GHz, the circular-shaped antenna exhibits gain of 5 dBi and 8.5 dBi, respectively.  
Figure 11: The simulated gain of the 1 \(\times\) 2 design.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Parameter** & **Substrate Width** & **Substrate Length** & \(\mathbf{W_{f}}\) & \(\mathbf{L_{F}}\) & \(\mathbf{W_{1}}\) & \(\mathbf{W_{2}}\) & \(\mathbf{L_{1}}\) & \(\mathbf{L_{2}}\) & \(\mathbf{W_{g}}\) & \(\mathbf{L_{g}}\) \\ \hline (mm) & 66 & 60 & 4.21 & 10 & 10 & 13.04 & 13 & 11 & 66 & 20.89 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Dimensions of the 1 \(\times\) 2 design.  
Figure 10: The simulated S11 of the 1 \(\times\) 2 design.  
Figure 12 shows a radiation pattern of simulated electric fields at the sub-6 GHz band. Figure 12a shows that, at frequency 3.5 GHz, the antenna has a figure-eight radiation pattern, indicating that it radiates strongly in two opposite directions. Its main lobe gain is 4.73 dBi, directed at 184\({}^{\circ}\). The 3 dB beamwidth is 84.4\({}^{\circ}\), which means the antenna has a relatively wide beam. Figure 12b shows that, at frequency 5.8 GHz, the main lobe magnitude is 8 dBi at 175\({}^{\circ}\), which is almost directly along the broadside direction (180\({}^{\circ}\)). This indicates symmetric radiation and good alignment. The main lobe gain is quite high, with strong directionality and efficient radiation. The 3 dB beamwidth, measures 62.5\({}^{\circ}\), which is narrower than the beamwidth at 3.5 GHz (84.4\({}^{\circ}\)). This means that the radiation is more precise and directive at 5.8 GHz.