## 2 Methods  
To find out the electrophysical parameters of materials, a technique was chosen that is a combination of the approaches considered in the literature based on measuring the reflected and transmitted signals. Finding the reflected signal is carried out due to the location of the material in the focus of electromagnetic waves. The transmitted signal is determined by placing a metal plate over the material under study. This approach can significantly simplify measurements. The use of a small-sized directional antenna makes it possible to reduce its secondary radiation to the sample. Before the study, it is necessary to measure the background signal to eliminate possible extraneous reflections of the probing signal in the absence of the test material. Further actions of the chosen technique are based on mathematical transformations. One of the important components in finding out electrophysical parameters is taking into account multiple reflections. Thus, it is necessary to take into account the three components \"air-dielectric\", \"air-dielectric-air\", and \"air-dielectric-metal\". To calculate the field after passing through the dielectric layer and reflected from the metal plate, a theoretical calculation of the reflection coefficient was performed. The coefficient of reflection from the <<air-dielectric>> half-spaces interface has the form:  
\[\
u(f)=\frac{z-1}{z+1} \tag{1}\]  
where, \(z(f)=\frac{\mu(f)}{n(f)}\) is the wave impedance, \(n(f)=\sqrt{\mu(f)\cdot\varepsilon(f)}\) is the refractive index of a medium, \(\varepsilon(f)\), and \(\mu(f)\) is the relative electric permittivity and magnetic permeability of the surrounding medium. All parameters have a dependence on the radiation frequency and therefore, are considered as functions of the frequency \((f)\). When considering the case when the reflection from the <<air-dielectric-air>> interface is calculated (Figure 1a), the complex amplitude has the form:  
\[A_{1}(f)=-\frac{(1+\
u(f))a^{2}\cdot\
u(f)}{1-\
u(f)^{2}\cdot a^{2}} \tag{2}\]  
where, \(a=\exp{(iknd)}\). In this case, the field strength reflection coefficient is: \(R(f)=\
u(f)+(1-\
u(f))\cdot A_{1}(f)\). Substituting Formula (2) in this expression, we obtain complex reflection coefficient in terms of field strength:  
\[R(f)=\
u(f)-\frac{(1-\
u(f)^{2})\cdot\
u(f)\cdot\exp{2 iknd}}{1-\
u(f)^{2} \cdot\exp{2 iknd}} \tag{3}\]  
where, \(d\) is the material thickness and \(k\) is the wave number. For the case when the \"air-dielectric-metal\" reflection is calculated (Figure 1b), the complex amplitude has the form:  
\[A_{2}(f)=-\frac{(1+\
u(f))a^{2}}{1-\
u(f)\cdot a^{2}} \tag{4}\]  
In this case, the field strength transmission coefficient is: \(T(f)=\
u(f)+(1-\
u(f))\cdot A_{2}(f)\). Substituting the Equation (4) in this expression, we obtain the complex reflection coefficient for the field strength:  
\[T(f)=\
u(f)-\frac{(1-\
u(f)^{2})\cdot\exp 2 iknd}{1-\
u(f)^{2}\cdot\exp 2 iknd} \tag{5}\]  
To search for the refractive index \(n(f)\), it is necessary to solve the system of equations for \(R(f)\) and \(T(f)\):  
\[\left\{ \begin{aligned} & R(f)=\
u(f)-\frac{(1-\
u(f)^{2})\cdot\
u(f) \cdot\exp 2 iknd}{1-\
u(f)^{2}\cdot\exp 2 iknd}\\ & T(f)=\
u(f)-\frac{(1-\
u(f)^{2})\cdot\exp 2 iknd}{1-\
u(f)^{2} \cdot\exp 2 iknd}\end{aligned} \right. \tag{6}\]  
Solving this system of equations, we find out the value \(\varepsilon(f)\) for the relative dielectric permittivity and \(\mu(f)\) for the relative magnetic permeability. \[\varepsilon(f)=\frac{p(f)+\sqrt{\frac{1}{2R(f)}\cdot p(f)-1}\cdot\exp\left( \frac{i\cdot arg(p(f)^{2})}{2}\right)+T(f)}{1+p(f)+\sqrt{\frac{1}{2R(f)}\cdot p (f)-1}\cdot\exp\left(\frac{i\cdot arg(p(f)^{2})}{2}\right)\cdot T(f)} \tag{7}\]  
\[\mu(f)=\left[\frac{1+p(f)+\sqrt{\frac{1}{2R(f)}\cdot p(f)-1}\cdot\exp\frac{i \cdot arg(p(f)^{2})}{2}}{1-p(f)+\sqrt{\frac{1}{2R(f)}\cdot p(f)-1}\cdot\exp \frac{i\cdot arg(p(f)^{2})}{2}}\right]^{2}\cdot\varepsilon(f) \tag{8}\]  
where, \(p(f)-\frac{T(f)-R(f)T(f)-R(f)-1}{2R(f)}\).