## 4 System Measurement Campaigns
### Rectifying and WPT Characterization  
Validation measurements regarding charging times and powers at stake have been performed, adopting a static line-of-sight (LOS) channel on-air, in a typical laboratory environment with the presence of various RF sources, i.e., Bluetooth and WiFi, just to mention two of the signals present and working in the same frequency band of the presented system. These measurements have been carried out for various orientations of the node with respect to the maximum link directions to confirm that the circular polarization of the illuminators transmitting antennas secures the insensitivity of received power to possible orientations of the node antenna.  
In Figure 9, the measurement results obtained during these tests are reported, for increasing illuminator-node distance from 30 up to 100 cm, which is from an almost near-field location up to the far-field condition. The charging times for the PMU cold-start, and for charging the storage capacitance C\({}_{\text{STORE}}\) up to 2 V (first power-on) and up to 3.3 V (complete charge) are shown.  
Figure 8: Node’s charging efficiency, versus input power, for different halved open circuit voltages and configurations: (**a**) exit cold-start, (**b**) first power-on, (**c**) fully charged.  
Moreover, it has been noticed that the maximum allowed distance from the illuminator to power the node is less than 120 cm, confirming also that the bq25570 cold-start takes place at 600 mV, at least.  
Secondly, another set of measurements has been conducted to characterize the MPPT operations of the bq25570 and verify the accuracy of the nonlinear optimization carried out to maximize the efficiency of the rectifier, in optimum loading conditions, for a reference RF input power (chosen equal to \(-10\) dBm).  
The open voltage at the rectifier's output has been first measured; then, the rectifier DC output has been connected to a potentiometer in order to halve the open voltage (V\({}_{\text{OPEN}}\)), which is how the MPPT of the power management unit (PMU) operates. The measured load values registered for V\({}_{\text{OPEN}}\)/2 are reported in Table 2. It is worth noticing that, for an illuminator-node distance of 80 cm, the open voltage is 920 mV, which approximately corresponds to an input power of \(-10\) dBm, which matches with a rectifier load (R\({}_{\text{LOAD}}\)) of 9.40 k\(\Omega\), close to the optimum one predicted by the ADS optimizer (Keysight Technologies, Santa Rosa, CA, USA) (6.44 k\(\Omega\)) and measured (8.85 k\(\Omega\)) for an input power of \(-10\) dBm.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Illuminator-Node Distance** & **VOPEN** & **RLOAD**\(\oplus\)**VOPEN/2** \\ \hline
30 cm & 3.24 V & 7.58 k\(\Omega\) \\
40 cm & 2.12 V & 10.43 k\(\Omega\) \\
60 cm & 1.58 V & 12.38 k\(\Omega\) \\
80 cm & 0.92 V & 9.40 k\(\Omega\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Measurements of the values of the load emulating the bq25570 MPPT operations.  
Figure 9: Representation of required times for the nodes to exit the cold-start and charge the storage capacitance at 2 and 3.3 V. Different distances and orientations with respect to the illuminator antennas have been considered: frontal node with (**a**) 0\({}^{\circ}\), (**b**) 30\({}^{\circ}\), and (**c**) 45\({}^{\circ}\) inclination with respect to the vertical axis.