## 2 RF Illuminator Design and Characterization  
The system wirelessly providing RF energy to the sensor nodes has been designed by integrating off-the-shelf components and by a dedicated design of its conformal radiating part. First, a MAX2750 voltage-controlled oscillator (VCO, Maxim Integrated, San Jose, CA, USA) is used, providing an output power of \(-3\) dBm at 2.45 GHz. This frequency has been selected because it results in being the best compromise between the shrinking and the efficiency of the antennas (for both the illuminators and the sensor nodes), and with the aim of exploiting a maximum EIRP of 27 dBm for an 8-MHz bandwidth signal, this being a regulation requirement for short range devices (SRD).  
The VCO can be dynamically tuned in the 2.4-2.5 GHz band to enable frequency diversity between the powering and the communication operations by sweeping the control voltage (V\({}_{\text{TUNE}}\)) between 0.8 and 1.4 V; with the aim of generating the signal at 2.45 GHz (selected frequency devoted to WPT), the correct control value is provided by a voltage regulator element (LT6650, Linear Technology Corporation, Milpitas, CA, USA). This allows a suitable frequency spacing between the WPT signal and the LoRa communication signal (chosen working frequency at 2.401 GHz). However, the illuminator's tuning capability allows its exploitation in the presence of other low-power protocols, such as Bluetooth low energy (BLE), ZigBee [20], or Wi-Fi.  
Hence, the key procedure is to set the channel adopted by the communication protocol far enough from the frequency devoted to WPT, in order to guarantee the coexistence and the simultaneity of these two operations: for these reasons, it is crucial to verify the correspondent frequency spacing in the spectrum between the two center frequencies to reduce the percentage packet error rate (PER) to be lower than 0.1%. From experimental studies, it has been determined that the minimum frequency spacing to be adopted between the LoRa communication channel and the wireless power transmission one is 30 MHz.  
The VCO output signal is amplified by the SE2598L RF amplifier (gain: 26 dB, Skyworks Solutions, Irvine, CA, USA), whose output directly feeds the antenna incoherently with respect to the other two paths. In this way, a 23 dBm transmitted power is available at each antenna input port. Patch antennas have been chosen for the illuminator, due to their robustness with respect to the surrounding environment and have been designed to be circularly polarized (CP) with a 4-dB gain in the maximum radiation direction. Hence, the total EIRP for each illuminator channel is 27 dBm, compliant with the current regulations.  
The chosen substrate for the printed circuit board (PCB) is RO4360G2 (thickness: 0.610 mm, with \(\varepsilon_{\mathrm{r}}=6.4\), tan (\(\delta\)) = 0.0038, Rogers Corporation, Chandler, AZ, USA); it has been selected to obtain reduced microstrip line widths for the connection between the illuminator components and, at the same time, ensure low dielectric losses, with respect to a standard FR-4 substrate (\(\varepsilon_{\mathrm{r}}=4.3\), tan (\(\delta\)) = 0.025).  
For the patch antenna (Figure 2b), a RO4350B (thickness: 1.524 mm, with \(\varepsilon_{\mathrm{r}}=3.48\), tan (\(\delta\)) = 0.0037, Rogers Corporation, Chandler, AZ, USA) substrate has been used, thicker and less dense with respect to the previous one, to emphasize the radiation performance. The three patch antennas are fed through coaxial cables using a right-angle connector.  
Circular polarization has been obtained by means of the corner-trimming technique [22] and chosen to ensure robust WPT performance to the sensors regardless of their orientation with respect to the incident EM waves.  
The photo of the entire board of the conformal illuminator is shown in Figure 2a. This compact designed illuminator's overall dimensions are 13 \(\times\) 6 \(\times\) 5.54 cm\({}^{3}\) and are well suitable to be installed inside internal compartments in industrial machinery and vehicles.