## 2 Delta-Stub-Based Wilkinson Power Divider (WPD) Design Configuration and Characterization Results
### Single-Element Fractal Antenna Design
Four-Element (2 \(\times\) 2 Configuration) Minkowski-Sierpinski Fractal Antenna Array (MSFAA) Design and Characterization  
The key demand for the IEEE 802.11 ax WLANs (Wi-Fi 6E), currently deployed 5G wireless systems, and next-generation wireless technologies is to have high-gain, directional, and narrow beam-width antenna arrays. Therefore, a four-element (2 \(\times\) 2 configuration) MSFAA has been designed, fabricated, and realized using four single-element antennas, as shown in Figure 7a,b. Therefore, initially, the four-element MSFAA is simulated using CST Microwave Studio, and the results are summarized in Figure 8.  
Then, the antenna array is realized, and the prototype is illustrated in Figure 7a,b, which shows the hybrid implementation of the MSFAA. Figure 7a shows the front, and \begin{table}
\begin{tabular}{c c} \hline \hline
**Antenna Dimensions** & **Value (Unit: mm)** \\ \hline \(L_{I}\) & 38.5 \\ \hline \(W_{I}\) & 25 \\ \hline \(L_{3}\) & 7 \\ \hline \(L_{4}\) & 2.5 \\ \hline \(L_{5}\) & 0.8 \\ \hline \(L_{6}\) & 7.35 \\ \hline \(L_{7}\) & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Detailed dimensions of single-element combined Minkowski-Sierpinski carpet fractal antenna structure. Figure 7b shows the back view of the four-element MSFAA prototype with detailed dimensions and connection configurations, respectively. An aluminum sheet of 240 \(\times\) 240 mm is used to hold the antennas with the help of screws, as shown in Figure 7b. The aluminum sheet with a thickness of 2 mm also acts as an extended ground plane for the four-element MSFAA, as shown in Figure 7a,b. The 1 \(\times\) 2 WPDs presented previously are used as a part of the antenna array feeding network to feed equal power to the four elements of the MSFAA. It can be seen in Figure 7a,b that we used small RF co-axial cables to make the connection between the main MSFAA feed point and the three 1 \(\times\) 2 WPDs. In order to reduce the grating and radiation side lobes, the spacing between the center of one antenna and another is a key factor while designing the antenna array. Therefore, the spacing between the antenna elements (center-to-center spacing) is kept at 0.7 \(\lambda_{0}\), i.e., 75 mm for the MSFAA. It can be seen from Figure 7b that co-axial cables, SMA connectors, and adapters are used to connect the three 1 \(\times\) 2 WPDs to the individual antenna element feeds. Figure 4: Simulated \(S_{11}\) (dB) response of the single\(-\)element Minkowski\(-\)Sierpinski fractal antenna using low\(-\)end FR4 and high\(-\)end Rogers TMM4 substrates. Figure 5: _Cont._  
Figure 5: Simulated E\(-\)plane 2D radiation pattern of single-element antenna using (**a**) FR4 and (**b**) TMM4 substrates at 4.17 and 5.97 GHz, respectively, and simulated H\(-\)plane 2D radiation pattern of single\(-\)element antenna using (**c**) FR4 and (**d**) TMM4 substrates at 4.17 and 5.97 GHz, respectively. Figure 6: Shows (**a**) fabricated prototype of single-element combined Minkowski–Sierpinski fractal antenna using FR4 substrate, and (**b**) S-parameter measurement setup showing the AUT connected to Keysight VNA for \(S_{11}\) (dB) measurement. Figure 8: Shows the simulated and measured return loss \(S_{II}\) (dB) of the four\(-\)element MSFAA. Figure 7: Shows (**a**) front view of a fabricated prototype of four-element MSFAA with dimensions, and (**b**) back view highlighting the four-element MSFAA feed network using WPDs and co-axial cables. that the four-element MSFAA is operating at the two desired frequency bands for IEEE 802.11 ax WLANs (Wi-Fi 6E), and 5G wireless systems operation with very good bandwidth and an \(S_{11}\) (dB) response of more than \(-17\) and \(-25\)dB is achieved at 4.17 and 5.97 GHz, respectively. It can also be observed from Figure 8 that the 2 \(\times\) 2 configuration MSFAA has the simulated bandwidth of 96 and 165 MHz, around 4.17 and 5.97 GHz, respectively.