## 2 Antenna Design and Analysis
### Antenna Array Configuration  
Antenna arrays typically provide increased gain compared to a single element, making them suitable for meeting the requirements of 5G technology [16]. A monopole antenna array is a configuration of multiple monopole antennas arranged next to each other. When signals from multiple antennas are combined, they experience constructive or destructive interference depending on their relative phases. The constructive interference results in an increase in signal strength, effectively enhancing the gain of the array in the desired \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Dimension (mm)** & **Parameter** & **Dimension (mm)** \\ \hline Ws & 25.7 & Ls & 50 \\ W & 11 & L & 17 \\ Wf & 3.5 & Lf & 13.5 \\ Wf1 & 1.6 & Lf1 & 12 \\ Lg & 22 & Lr & 7 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Optimal parameters for the final design of the single antenna. Figure 4: Reflection coefficient variations with different ground plane sizes. direction, while destructive interference is used to suppress energy from undesired directions [17]. Thus, to increase the antenna gain, the single element was converted into a linear array consisting of two elements as depicted in Figure 5. The feeding network was designed and optimized to achieve the desired performance. Initially, the monopole antennas were matched to 75 ohm transmission lines. Afterwards, the feed networks of the 2 \(\times\) 1 antenna array were combined using a T-junction, enabling the entire array to be matched to a 50 ohm transmission line. While the single element has wider band characteristics, the array antenna exhibits dual-band behavior as depicted in Figure 6. According to the \(-10\) dB criteria, the antenna operates within a dual band with bandwidths of 1.71 GHz and 1.63 GHz, spanning from 2 to 3.71 GHz and 5.9 to 7.54 GHz, respectively. The spacing between patches in an antenna array plays a pivotal role in influencing its dual-band behavior, as demonstrated in the analysis depicted in Figure 7. Systematic variation in the spacing (D) from 20 mm to 50 mm was conducted to optimize the antenna  
Figure 5: A 2 \(\times\) 1 antenna array: (**a**) front; (**b**) back. Figure 6: Simulated and measured reflection coefficient (S11) of the array antenna. performance across the WiMAX band (2.5 GHz), 5G sub-6 GHz band (n78), and 5G sub-7 GHz band (n96). At smaller spacing values (e.g., D = 20 mm), limited coverage and compromised performance in the desired frequency bands were observed. This is attributed to the strong mutual coupling between adjacent patches, which affects the antenna's impedance  
Figure 7: Effect of patch spacing on (**a**) reflection coefficient; (**b**) gain. and radiation characteristics, thereby limiting its ability to resonate effectively in the intended bands. Increasing the spacing (up to D = 40 mm) resulted in a notable improvement. At this particular spacing, the intended bands of WiMAX and 5G New Radio (NR) n78 and n96 are covered. Optimal spacing contributed to enhanced isolation between elements, mitigating mutual coupling effects and allowing for independent radiation characteristics in each band. This led to improved resonance and gain across the desired frequency ranges, facilitating reliable operation in both WiMAX and 5G bands, as demonstrated in Figure 7a,b. In the frequency range of 5.9 GHz to 7.5 GHz, the free-space path loss tends to be higher compared to that at lower frequencies. The distance D = 40 mm corresponds to 1\(\lambda\) at the highest frequency of 7.5 GHz. Selecting this value ensures that grating lobes will not form across the upper band (5.9-7.5 GHz) for a broadside array configuration. The wavelengths within this range are close to 1\(\lambda\) at 7.5 GHz, allowing the gain of the antenna system to increase due to the array factor. An average gain improvement of 3 dBi was obtained for both operating bands with a peak gain of 6.47 dBi, which is consistent with the theoretical approach [18].