## 5 Discussion  
Starting from the preliminary design of the RF sources presented in [21], this work describes the full end-to-end design of a wirelessly powered IIoT node designed to guarantee both insensitivity to materials it is placed on, and robustness concerning the ability to store energy. This has been reached with the employment of a conformal illuminator consisting of three circularly-polarized radiating elements, unlike the sensor node power receiving  
Figure 10: Front and side views of the scenario under test. The RF illuminator and the six positions of the sensors are highlighted both for setup 1 (**a**) and setup 2 (**b**). antenna (linearly polarized), with the aim of facing possible changes of the EM waves polarization that can easily happen in metal-rich environments. Furthermore, the choice of a patch antenna at the node side has been done knowing that its bottom layer has to be placed in contact with metallic parts; in this way, its ground plane is virtually enlarged by the metallic components of the engine itself, thus increasing the radiation efficiency. During the measurement campaign conducted in the engine compartment of the car (see Figure 10), a maximum node-illuminator distance of 112 cm has been considered. It is worth mentioning that sensor nodes are, in this case, placed in a quasi-total metallic environment, which is affected by interfering reflections and non-line-of-sight (NLOS) propagation conditions. Moreover, all the sensors are attached to metallic parts and, in most cases, are also placed close to other metallic elements with the aim of coupling them with the key components of the engine. During these measurements, the bonnet was closed, and the instrumentation was placed outside the car, for the best mimic of realistic operations. Figure 10(a),b reports a summary of each sensor node's charging times for the two setups. In particular, the three noteworthy charging times (end of cold-start, first power-on, and full charge) have been monitored, reporting the node-illuminator distance computed along a straight line, together with the minimum sleep time between two subsequent turn-ons of the node (minimum sleep interval). With regard to the related works, EH techniques for industrial machineries have been investigated in [25] exploiting light, RF, thermal, and kinetic energy. However, at the moment, EM waves coming from RF signals are rarely used for predictive maintenance of mechanical systems. In particular, for what concerns far-field WPT applications [26], useful RF-to-DC PCE are achieved for RF-input power higher than 0 dBm, whereas, in this work, the main goal is to deal with lower power received at the node antenna, given the budget of  
Figure 11: Charging times of the six different nodes for (**a**) setup 1 and (**b**) setup 2.  
the radio link at 2.45 GHz and the illuminator-node distances reported in Figure 11. Hence, the targeted low-level of input power in this work is \(-\)10 dBm, for which an RF-to-DC PCE of 25% is reached, whereas with \(-\)5 dBm a PCE of 34% is achieved: this means that, even with these low input power levels, DC power (25 uW) and voltage (466 mV) at the output of the rectifier (and, equivalently, at the input of the PMU), are sufficient to complete the cold-start phase of the TI bq25570 (taking place from 15 uW and 330 mV). Finally, circuits for powering wireless sensor networks have been designed making use of commercially available components at the transmitter side in the 915 MHz RFID ultra-high frequency (UHF) band [27], showing a comparable PCE at \(-\)10 dBm, but exploiting a receiving tag which integrates a matching network with an area in excess of 16 cm\({}^{2}\) and a monopole antenna of approximately the same area, which is too large for the specifications and the requirements of our target application. A higher PCE of 45% at \(-\)10 dBm is achieved in [28], however operating at 900 MHz, with much larger antenna size of 80 mm \(\times\) 30 mm \(\times\) 1.5 mm.