# Influence of Dielectric Plate Parameters on the Reflection Coefficient of a Planar Aperture Antenna
## 4 Measurement Results  
For long-term research to be meaningful, simulation, i.e., the simulated values of the reflection coefficient S\({}_{11}\), must be validated via laboratory measurements. What can be measured with certainty is S\({}_{11}\) for a given thickness d of the dielectric plate on the antenna. Laboratory measurements of S\({}_{11}\) at the waveguide-free space boundary and the waveguide-dielectric boundary were performed for d = 0 mm, d = 4.78 mm, d = 7.08 mm, d = 9.26 mm, and d = 11.08 mm. Simulation calculation precedes the measurements for these dielectric plate thickness values by using paper as a dielectric. Comparison results of the simulated and measured values of S\({}_{11}\) are shown in Figure 12a-e. Table 6 shows the comparative values of the reflection coefficients (simulated and measured) for different values of dielectric thickness.  
It can be seen that there are some differences between the simulated and the measured curves. The measured curves show a small shift towards lower frequencies. This can be explained by the fact that when the sheets of paper are stacked on top of each other to achieve the specified thickness of the dielectric plate, there is obviously an unwanted amount of air between the two sheets. This can be explained by the fact that simulation models do not take into account a number of factors that are present in the laboratory models on which the measurements were carried out. The measured results are affected by the calibration of the measuring device, the conditions under which the measurement is performed (a laboratory with an EM echo), the performance (errors) of the measurement model, and the lack of knowledge of the exact structural-geometric and electromagnetic parameters of the measured model.  
Figure 11: Radiation pattern of an aperture antenna with a dielectric plate: (**a**) \(E\) plane; (**b**) \(H\) plane.  
Figure 12: _Cont._  
To conclude this section, the measurement of S\({}_{11}\) at the boundary between a waveguide-dielectric and a dielectric-free space was carried out by using an N9914A FieldFox Handheld RF spectrum analyzer, the operating range of which extends up to 6.5 GHz. The laboratory in which the reflection coefficient measurement was performed is not a chamber without an electromagnetic echo (Figure 13).  
\begin{table}
\begin{tabular}{c c c c c} \hline
**d, mm** & \(\mathbf{f_{sim}}\)**GHz** & \(\mathbf{S_{11 sim}}\)**, dB** & \(\mathbf{f_{meas}}\)**, GHz** & \(\mathbf{S_{11 meas}}\)**, dB** \\ \hline
0 & 1.6944 & \(-\)12.57 & 1.7147 & \(-\)13.15 \\
4.78 & 1.7753 & \(-\)15.23 & 1.7550 & \(-\)16.81 \\
7.08 & 1.7820 & \(-\)19.99 & 1.7550 & \(-\)24.41 \\
9.26 & 1.7820 & \(-\)27.73 & 1.7600 & \(-\)25.49 \\
11.08 & 1.7820 & \(-\)45.17 & 1.7550 & \(-\)43.93 \\ \hline \end{tabular}
\end{table}
Table 6: Comparative values of the reflection coefficient S\({}_{11}\) for an antenna with and without dielectric plate at the antenna aperture.  
Figure 12: Simulated and measured reflection coefficient S\({}_{11}\) for an aperture antenna: (**a**) without dielectric plate (d = 0 mm) and with dielectric plate thickness: (**b**) d = 4.78 mm, (**c**) d = 7.08 mm, (**d**) d = 9.26 mm, and (**e**) d = 11.08 mm.