# Influence of Dielectric Plate Parameters on the Reflection Coefficient of a Planar Aperture Antenna
## 5 Discussion
A very significant contribution to the properties of the antenna is made by the change in the permittivity of the dielectric plate because it is basically the measure of the energy stored in the dielectric. In the simulations, the permittivity \(\varepsilon_{r}\) ranges from 1 to 10, and the best result of S\({}_{11}\) is obtained for \(\varepsilon_{r}\) = 2.5. This \(\varepsilon_{r}\) value corresponds to the relative permittivity of the paper used as a dielectric material in the measurements, but such permittivity has some other materials suitable to be used for the antenna radome. The total permittivity also consists of an imaginary component, the size of which provides information on how dissipative the dielectric material is in relation to the applied electric field. This component, in proportion to the aforementioned real part of the dielectric constant, forms loss tangent \(\tan\delta\). The loss tangent \(\tan\delta\) ranges from 0.01 to 0.098, and the simulations showed the minimal value for \(\tan\delta\) = 0.09. Electrical conductivity of the dielectric plate \(\sigma\) takes values from 0 to 10 S/m in the simulations, and, as expected, it showed the minimal value for \(\sigma\) = 0 S/m. At the very end of the study of antenna behavior with and without a dielectric plate on the aperture, laboratory measurements of the reflection coefficient were obtained by using an N9914A FieldFox Handheld RF spectrum analyzer. The chamber in which laboratory measurements were obtained is a chamber with an electromagnetic echo. Because the laboratory is not isolated from electromagnetic interference and the dielectric plate can never be arranged as in the simulations (in practice, homogeneity of the dielectric plate is not perfect: there is an undesirable amount of air between the layers of paper), there is some variation in the values obtained for the reflection coefficients S\({}_{11}\). This statement is also evident in Figure 12a-e, in which the graphs of the measured values show a frequency shift to the left, i.e., to lower frequency values. Another circumstance favors the difference between the simulated and the measured values; in practice, it is very difficult to adjust dielectric parameters to the value used by the simulation. The only parameter value that can be measured very accurately is the dielectric thickness d.  
The focus of this article is not on the effects of edge scattering of the grounded plane of the aperture antenna. However, because the effects of edge scattering are visible on the side and back lobes, it is evident from this article that the effect of the finite edge must be included in the calculation in order to obtain more accurate results of the radiation patterns. The measurement was possible for a larger number of dielectric thicknesses, and it was found that for the given aperture antenna, a dielectric plate thickness of 11.08 mm gives the minimum value of the reflection coefficient S\({}_{11}\) of \(-\)43.93 dB at the frequency of 1.7550 GHz. The minimum reflection coefficient obtained by additional simulation for d = 11.08 mm is \(-\)45.17 dB at the frequency of 1.7820 GHz. Apart from d, which is variable, other parameters are set within the limits given in [17; 20]. Despite the differences mentioned between the measured and simulated values, it can be stated that the curves of the reflection coefficients match very well. Furthermore, the assumption was confirmed that by adjusting the value of the geometric and electromagnetic parameters of the dielectric plate placed on the antenna, the reflection coefficient can be significantly improved (minimized), which ultimately results in good impedance matching of the antenna opening. In the context of this investigation (simulations and measurements), it was assumed that a dielectric is a linear, homogeneous, and isotropic material.