## 2 Delta-Stub-Based Wilkinson Power Divider (WPD) Design Configuration and Characterization Results
### Single-Element Fractal Antenna Design
Similarly, the measured \(S_{11}\) (dB) response of the fractal antenna array exhibits a bandwidth of 85 and 182 MHz, around 4.17 and 5.97 GHz, respectively. After the return loss measurements, the 2D and 3D radiation patterns and surface current distribution measurements of the four-element MSFAA are performed to further investigate and understand the behavior of the proposed antenna array for IEEE 802.11 ax WLANs (Wi-Fi 6E) and the upcoming 5G wireless systems. The radiation pattern measurements are performed in a NI-800F anechoic chamber using a Keysight N5230A VNA. Figure 9a-d shows the simulated and measured E- and H-plane 2D radiation patterns of the four-element MSFAA, respectively. In Figure 9a, the simulated and measured E-plane 2D radiation patterns at 4.17 GHz are shown, where the MSFAA exhibits a 3 dB beamwidth of 34.8 and 31 \({}^{\circ}\)C, and a gain of 4.9 and -4.2 dB, respectively. In Figure 9b, the simulated and measured H-plane 2D radiation patterns at 4.17 GHz are shown, where the MSFAA exhibits a 3 dB beamwidth of 36.8 and 32 \({}^{\circ}\)C, and a gain of 5.5 and -5 dB, respectively. Similarly, in Figure 9c, the simulated and measured E-plane 2D radiation patterns are shown at 5.97 GHz, where the MSFAA exhibits a 3 dB beamwidth of 17.5 and 26.2 \({}^{\circ}\)C, and a gain of \(\sim\)10.5 dB and \(\sim\)9.6 dB, respectively. In Figure 9d, the simulated and measured H-plane 2D radiation patterns at 5.97 GHz are shown, where the MSFAA exhibits a 3 dB beamwidth of 17.1 and 28 \({}^{\circ}\)C, and a gain of 10 and \(\sim\)9.8 dB, respectively. It can be observed from Figure 9a-d that the four-element MSFAA is radiating in the boresight at both resonating frequencies. Moreover, the MSFAA exhibits more directional behavior at the high frequency than the low frequency of resonance, and the null is reduced at zero degrees in the E- and H-plane at 4.17 GHz. It is also important to observe here that the radiation pattern is directed away from the antenna with much-reduced back radiation by virtue of the presence of the extended ground plane in the form of an aluminum sheet. This is more prominent at the 5.97 GHz frequency, thus making the proposed fractal antenna array design more suitable to work in the high-efficiency IEEE 802.11 ax WLANs (Wi-Fi 6E) and upcoming 5G wireless systems. After measuring the 2D radiation pattern of the four-element MSFAA, the 3D radiation patterns are also studied at both the oscillating frequencies of 4.17 and 5.97 GHz to further understand the fractal antenna array behavior, and the results are summarized in Figure 10a,b, respectively. It can be observed from the 3D radiation patterns in Figure 10a,b that at 4.17 GHz resonance frequency, the antenna gain is 6.8 dB, but at the 5.97 GHz frequency, the antenna gain increases to 11.85 dB. The results show the directionality of the radiating electromagnetic waves from the proposed design, which is an important requirement for both high-efficiency IEEE 802.11 ax WLANs and the upcoming 5G wireless systems for smart antenna beam-forming and beam-steering technologies [1; 2; 3; 6; 12]. It can be observed from the results in Figure 11a,b that when the antenna array is excited, the surface current distribution is high near to and around the resonating fractal patch antennas and low around the rest of the aluminum sheet, especially at the edges, which is acting as an extended ground plane. The co- and cross-polarization analysis of the four-element MSFAA is then performed. The results of the co- and cross-polarization of the four-element MSFAA for E- and H-plane at 4.17 and 5.97 GHz, respectively, are illustrated in Figure 12a,b. The proposed four-element MSFAA shows E- and H-plane cross-polarization isolation of \(-13\) and \(-17\) dB at 4.17 GHz, respectively, as depicted in Figure 12a. In Figure 12b, it can be observed that E- and H-plane cross-polarization isolation of below \(-10\) dB at 5.97 GHz is achieved.