## 3 Results
### Tunable Sensor  
As mentioned in Section 2, the reconfigurability of the RFID sensor was obtained by changing the DC voltage of the varactor diode that was fed by a WPT. Regarding the powering part, a wire loop operating at a frequency of 7.2 MHz--chosen to ensure acceptable dimensions of the loop--was used as a transmitting element for the WPT to the rectangular loop, which was used as a receiving element for the WPT on the part of the tag. The 7.2 MHz operating frequency also was chosen based on measuring the voltage needed to supply the capacitance of the varactor diode and the corresponding frequency, as shown in Figure 7. A maximum voltage of 3.8 V was achieved at a frequency of 7.2 MHz for \(P_{\mathrm{g}}=-20\) dBm (generator output) and \(P_{\mathrm{A}}=+31\) dBm (amplifier output). Figure 8 shows the measurement setup of the DC voltage at terminals of the varactor diode. An RF generator (SG 2000) in cascade with a power amplifier (BSA 0101-25/30D), a wire loop, a Voltcraft multimeter (M-4660A), and the sensor tag were used for the measurement. The components used for the measurements are listed in Table 3. The wire loop transmitted a WPT signal to the sensor tag to irradiate it. Then, the rectifier on the back side of the tag converted the energy captured by the rectangular tag loop serving as the receiving element into a DC voltage to bias the varactor diode. Finally, the voltage produced at the terminals of the diode was measured using the Voltcraft multimeter. The  
Figure 6: Photograph of the waveguide chamber measurement setup. Figure 7: Measured rectified DC voltage and the corresponding frequency. reactance of the transmitting loop was compensated by a capacitor value of \(C\) = 10 nF to form a parallel resonant circuit at an operating frequency of 7.2 MHz, which is a free industrial and medical frequency band. The resonant circuit ensured a purely resistive input impedance, as shown by the power amplifier. Figure 9 presents the final prototype of the sensor tag coupled to the rectifying circuit. In this part, the maximum transmitted power (\(P_{\mathrm{A}}\)) that could activate the varactor diode was measured. Based on the datasheet of the varactor, the \(V_{\mathrm{r}}\) should be in the range of 0 to 8 V, providing a capacitance in the range of 1.9 to 30 pF. Following from Table 4, the voltage needed for the diode to operate was achieved by adjusting the input energy of the generator. The maximum power needed here was 41 dBm, corresponding to about 12 W, which would later be rectified to a DC voltage to obtain 7.5 V, which was slightly below the limiting Zener voltage [39]. Figure 8: Setup for measuring the DC voltage of the varactor. \begin{table}
\begin{tabular}{l l} \hline G & Generator: Elsy SG 2000 (100 kHz–1 GHz) \\ A & Power amplifier: BONN Elektronik BSA 0101-25/30D (9 kHz–1 GHz, +46 dB) \\ PM & Power meter: Keysight (Hewlett-Packard) 437B + power sensor \\ WL & Wire loop \\ VM & Multimeter: Voltcraft M-4660A \\ \hline \end{tabular}
\end{table} 
Table 3: The components used in the configuration of the DC voltage measurement. Figure 9: Prototype of the sensor tag coupled to the rectifying circuit. As previously mentioned, the power needed to irradiate the sensor tag was transmitted by the wire loop antenna and received by the rectangular loop on the side of the tag. The rectifier located on the back side of the structure converted the energy collected by the rectangular loop into a DC voltage (\(V_{\text{r}}\)) to bias the varactor diode. By changing the input energy, the DC voltage of the diode changed, which in turn caused a change in frequency, effectively changing the \(P_{\text{A}}\) needed to tune the resonant frequency of the sensor tag. The WPT system controlled the DC bias of the varactor diode. The polarization of the varactor diode in the reverse direction was defined by the controlling circuit; i.e., by the connection of rectifying diodes. The effect of the presence of gas on the stability of the control voltage \(V_{\text{r}}\) on the varactor could be considered negligible.