## 3 Results
### Material Measurements  
Materials samples with different powder areal weights have been prepared for the evaluation of the free-space technique (see Table 4).  
Given the electrical nature of the materials considered with relatively low conductivity contrast between them, a reliability study has been performed. Tests in the roll-to-roll pilot lines have been performed. Measurements are done at room temperature around 18 \({}^{\circ}\)C with humidity rate around 52%. The antenna is positioned in the orientation of the carbon fibers for all tests. Each measurement is done two times by moving the sample. Between two samples characterization, the empty fixture is measured as a reference to check the environment variations (denoted FREE SPACE). The measured amplitudes and phase-shifts of the complex reflection coefficient \(S_{\text{11m}}\) are given in Figure 7. After the first round of measurements, each sample is measured one more time.  
From these data, unique microwave signatures are noticed for the different powder areal weights considered. For each sample under test, a vector calibration is performed at the material reference plane. Note that the procedure does not require any other measurements as the reference plane can be chosen arbitrary. From the calibrated complex reflection coefficients \(r_{01}\), the relative complex permittivity \(\varepsilon_{r}^{*}\) of the material can be extracted using the inversion of the following relationship. From the imaginary part \(\varepsilon_{r}^{\prime\prime}\) of the relative complex permittivity \(\varepsilon_{r}^{*}=\varepsilon_{r}^{\prime}-je_{r}^{\prime\prime}\) and assuming that the dielectric losses are only  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample Identification**} & \multicolumn{6}{c}{**Measured Thickness \({}^{*}\)[mm]**} & \multirow{2}{*}{**Main Thickness**} & \multirow{2}{*}{**Weight (g)**} & \multirow{2}{*}{**Power Area**} \\  & & & **(\({}^{*}\) at Different Locations: One Measurement per Sample’s Side)** & & & & & **Weight (g/m\({}^{2}\))** \\ \hline Hexcel’s & T1.1 & 0.37 & 0.34 & 0.35 & 0.34 & 0.35 & 37.66 & 0 \\ M21/34\%/UD194/T800s/300 AT & T1.2 & 0.35 & 0.38 & 0.38 & 0.35 & 0.37 & 37.90 & 0 \\ \hline FXplyTM-EL & T2.1 & 0.48 & 0.41 & 0.44 & 0.45 & 0.45 & 40.04 & 25.11 \\  & T2.2 & 0.40 & 0.41 & 0.41 & 0.42 & 0.41 & 40.12 & 26.00 \\ \hline FXplyTM-EL\_low & T3.1 & 0.48 & 0.50 & 0.42 & 0.45 & 0.46 & 39.09 & 14.56 \\  & T3.2 & 0.32 & 0.36 & 0.39 & 0.42 & 0.37 & 39.49 & 19.00 \\ \hline FXplyTM-EL\_high & T4.1 & 0.49 & 0.48 & 0.47 & 0.43 & 0.47 & 40.69 & 32.33 \\  & T4.2 & 0.46 & 0.47 & 0.41 & 0.41 & 0.44 & 40.99 & 35.67 \\ \hline Prepreg Paper only & - & 0.1 & 0.1 & 0.09 & 0.09 & 0.10 & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 4: Thin film prepre parameters.  
Figure 7: Measured amplitude and phase-shift of the complex reflection coefficient as a function of the powder areal weight (+ First measurement round, \(\square\) Second measurement round)—f = 10 GHz.  
attributed to the RF conductivity \(\sigma_{RF}\) (case of metals) at 10 GHz, we can extract the value of \(\sigma_{RF}\) by  
\[\sigma_{RF}=\omega\varepsilon_{0}e_{r}^{\
u}. \tag{4}\]  
where \(\omega\) is the microwave excitation pulsation and \(\varepsilon_{0}\approx\frac{1}{36\pi}\times 10^{-9}\) F/m denotes the vacuum permittivity. The extracted complex permittivity and related RF conductivities are summarized in the following table.