## 4 Results and Discussion
Therefore, it can be said that, changing the material permittivity and substrate thickness have a small effect on the radiation a pattern properties. Figure 10: The simulated and measured return loss of antenna B with thickness of 1.27 mm. Figure 7.0 The simulated and measured return loss of antenna A with thickness of 0.508 mm. The realized gain over operating frequency range of the four antennas is plotted in the Figure 7.1 As can be seen from the figure, the antenna with lower relative permittivity of 1.96 provided higher gains, while when the substrate thickness is increased to 3.175, the gain also increased. Therefore, it can be said that the realized gain is inversely proportional with relative permittivity and directly proportional with substrate thickness. Figure 7.2 The radiation pattern of the proposed antennas (**a**), E plane of antenna A (**b**), H plane of antenna A (**c**), E plane of antenna B (**d**) and H plane of antenna B.  
Figure 7.3 Realized gain of the proposed antennas versus the frequency in the range of interest. Finally, the polarization of the four antennas was monitored. For this purpose, the axial ration of the antennas was plotted in Figure 7.4 Based on the 3 dB criteria, all antennas provided circular polarization, while an improved circular polarization was obtained from the antennas with relative permittivity of 1.96 and 2.2 with a substrate thickness of 1.275 and 3.175, respectively. As demonstrated in the previous sections, the proposed antennas have a relatively narrow bandwidth. Therefore, broadband bow-tie slot antenna was designed, fabricated and tested. The measured result is compared with the simulation result. The fabricated antenna is measure by the VNA network analyzer. The network analyzer is calibrated by using a calibration tool in order to get more precise results, such as open, short and broadband loads. The simulated and measured return losses are plotted in Figure 7.5  
Figure 7.6 The simulated and measured return losses of the slot antenna (thickness = 1.27 mm). Figure 7.7 Simulated axial ratios of the antennas over the frequency range of interest. From Figure 14, it can be observed that the measured result is relatively matched with the simulation result. The simulation result has a broad bandwidth which is about 33% with \(-27\) dB return loss, while the measured result shows the smaller bandwidth of 14% and the mismatch was higher. The inaccurate result may be referring to the soldering of the SMA connector. Because the antenna gab, which is located between the two feeding lines, is very narrow, as mentioned in Table 5, it is difficult to connect the SMA exactly in the right location. The fabricated antenna with an SMA connector is shown in Figure 7.8  
For the purpose of getting better results and improving the amount of mismatching, the smaller substrate, which is 0.508 mm, was used. The antenna was fabricated with this smaller substrate in which all the other parameters were kept constant as previously illustrated in Figure 7.9 The fabricated antenna was tested with the VNA analyzer again. The simulated and measured results are plotted in Figure 5:0  
From Figure 15, it can be noted that more accurate result was obtained because the measured result is highly correlated with the simulation result. The measured return loss is around \(-25\) dB and the obtained bandwidth is about 1.4 GHz, which is about 22% (obtained bandwidth is 1.3 GHz with 5.65 GHz lower and 6.95 GHz higher frequency limits). Again, due to the availability of the devices, the far-field pattern is not measured. Therefore, only the simulated far-field pattern is presented in Figure 5:1  
Figure 5:2 The radiation pattern of the bow-tie slot antenna (**a**), E-plane and H-plane (**b**). Figure 5:3 The simulated and measured return losses of the slot antenna (thickness = 0.508 mm).