## 4 Experimental Verification and Discussion
### Measurements of the WPT System's PTE  
Though the power received by the receiving antenna can be routed to the RF power meter via a coaxial cable similar to that used to deliver the generator's produced power to the TX antenna, its presence in the interior of the metallic enclosure would likely have an adverse effect on the field distribution and therefore would negatively affect the measurement results. To mitigate this issue, an RF-DC converter was designed and fabricated. The converter was equipped with an SMA connector, enabling it to be connected to the Rx antenna via a male-male transition. The converter's PCE must be retrieved to extract pure PTE of the cavity-based WPT system free from the effect of the experimental setup elements, such as the coaxial cables utilized in the measurement and the RF-DC converter. To that end, a separate set of measurements was made, where an RF sine wave was directly fed through the input of the RF-DC converter. No cables were used during the measurements to reduce the measurement uncertainty associated with various reflections and imperfections of connecting elements. Throughout the experimental study, two measurement scenarios were adopted. In the first scenario, the RF signal generator frequency was kept fixed at 865.5 MHz, while the output power of the generator was successively set to the following levels: \(-10\), \(-3\), \(0\), \(3\), \(5\), \(6\), \(7\), \(8\), \(9\), and \(10\) dBm. Since for the PTE extraction, one needs the PCE values for the input power levels that, in general, differ from those used in the measurements, a cubing spline interpolation was employed; it was built-in MATLAB function _interp10_, with the fourth and fifth arguments assigned '_pchip_' and '_extrap_', respectively. All the measurement results presented in this contribution were made with the RF-DC converter, thereby obviating the need for a coaxial cable. However, the RF-DC converter's output voltage was measured by a multimeter connected in parallel with a \(1\) k\(\Omega\) load resistor. It should be noted that the load resistance was chosen based on a series of measurements involving the RF-DC converter only. The values of the converter's matching  
Figure 13: Experimental setup involving a carbon steel closet used as a resonant cavity in the WPT system under study: antenna-based WPT system inside the metal closet (**a**), experimental setup involving the TX and RX antennas, the signal generator operating at 865.5 MHz, and the power meter used to measure the received power (**b**). network were chosen so that the input impedance was as close to 50 \(\Omega\) as possible to ensure adequate impedance matching. The 1 k\(\Omega\) load resistance was found to ensure the highest RF to DC power conversion efficiency (PCE). The measured PCE as a function of the input RF power level at the operating frequency is shown in Figure 14a. It is seen that the PCE increases monotonically with the input power level reaching about 70% at 12 dBm. The sensitivity of the RF-DC converter to frequency variations is also of interest for the present study, as it may affect the overall performance of the WPT system when adjusting the operating frequency to improve its PTE slightly. Figure 14b presents the PCE plotted against the frequency from 800 MHz to 960 MHz, measured at different input power levels listed above. The converter's PCE shows slight variation with the frequency regardless of the input power level, as apparent in Figure 14b. A few series of received power measurements were made for different positions of the receiving antenna to verify the simulated results. The main objective was not only to find the maximum achievable PTE but also to determine how sensitive the PTE is to small variations in the position of the receiving antenna. The sensitivity is crucial, as the receiving antenna is intended to be incorporated into a smart suit. Hence, it is not easy to ensure precise antenna placement in practice due to the softness of the suit material.