## 3 Simulation and Measurement Results  
To validate the simulation results, a prototype antenna was fabricated on a copper layer of 0.003 mm thickness, glazed on the FR4 substrate with an \(\varepsilon_{r}\) of 4.3 and a height of 1.6 mm, as depicted in Figure 1b. Later, an experiment was set up to determine the impedance and radiation characteristics of the antenna with an E5071C network analyzer. The numerical and experimental values of \(\left|S_{11}\right|\) are shown in Figure 10. It can be observed that both types of values follow a common trend, and the most corresponding numerical and experimental values do not deviate much from each other. The deviation was likely to be from a minor difference in the numerical and experimental setup: a 50-\(\Omega\) SMA connector in the experimental setup was not taken into account in the simulation [21]. The reason was that we wanted to focus only on the simulated properties of the antenna itself, not any other external components, as well as to reduce simulation time. The second resonant frequency was increased with the inclusion of SMA, whereas the first resonant frequency variation was less due to the smaller reflection at the low frequency. The simulated \(\left|S_{11}\right|\) results were better than \(-10\) dB over the frequency ranges of 2-2.56 GHz/4.42-6.82 GHz/13.26-17.70 GHz, and the measured \(\left|S_{11}\right|\) results were better than \(-10\) dB over the frequency ranges of 2.10-2.70 GHz/4.82-6.10 GHz/12.73-18 GHz. In addition, the proposed antenna with the two-arm ILSS also provided the common trend of both simulated and measured \(\left|S_{11}\right|<-10\) dB over the frequency ranges of 1.92-2.69 GHz/4.67-6.54 GHz/13.51-17.72 GHz and 2.02-2.62 GHz /5.08-6.27 GHz/11.97-18 GHz covering the 2.4/5 GHz WLAN/Ku-band applications, respectively [18]. Besides the impedance characteristics, the 2D radiation patterns in \(xz\)- and \(yz\)-planes of the proposed antenna were also measured and plotted in Figures 11-13. Evidently, this proposed antenna furnished a bidirectional pattern with a different main beam shape for each band. At the operating frequency of 2.45 GHz, the main beam propagated in forward- and backward-directions, while at the operating frequency of 5.5 GHz, the main beam elevated up about 25\({}^{\circ}\). At the operating frequency of 15.5 GHz, this antenna provided a bidirectional pattern with a beam peak directed at about 180\({}^{\circ}\) in both numerical and experimental results. The application of an antenna was in an outdoor WLAN system, where the pattern in the \(yz\)-plane elevated up from a 0\({}^{\circ}\) to receive the signal from the WLAN transmission tower as well as around a high angle for the application of Ku band. The added reflector could be helpful to increase the antenna gain at the expense of size enlargement. Nonetheless, the arrangement of a number of antenna elements in an array was an alternative means to enhance the antenna gain. Furthermore, an appropriate design for a metal ground structure of vehicular surface could also achieve the additive reflection to improve the radiation pattern and antenna gain. Overall, this antenna provided linear polarization with less than \(-15\) dB cross-polarization for all intended frequency ranges. It provided simulated/measured gains of 2.34/2.63 dBi, 4.21/4.6 dBi and 4.35/4.14 dBi, at 2.45 GHz, 5.5 GHz and 15.5 GHz, respectively. The simulated radiation efficiency values were at least 73%, 82% and 73% for the lower-, middle- and upper-band, respectively.