## 5 Full-System Validation  
The full RF energy harvesting system and its circuit diagram are shown in Figure 19. The arms of the Archimedean spiral antenna are welded to the positive and the negative poles of the circuit to maintain a balanced structure. As discussed in Section 4.2, the first \(L\) matching section shown in the circuit diagram of Figures 15 and 16 has been eliminated. The antenna already acts as 188 \(\Omega\) of resistance, so the matching section that transforms 50 \(\Omega\) into 188 \(\Omega\) is no longer needed.  
The operation of the whole system is then tested in the laboratory. For this purpose, the RF harvesting system is placed at a distance of 50 cm from a transmitting source, as depicted in Figure 20. A signal generator injects a 16 dBm tone, centered at 807 MHz, on a wideband transmitting Vivaldi antenna. Then, the spiral collects the incoming power and the circuit attached to the antenna conditions the acquired power in order to supply the temperature sensor. A multimeter located in the center of the figure shows the voltage in the sensor. A rough link budget analysis is carried out to determine the power level that reaches the Archimedean spiral. The gains of the Vivaldi and the spiral antenna are 4.75 dB and 3 dB, respectively. Depolarization losses (3 dB) must be taken into account, since the Vivaldi antenna is linearly polarized and the spiral antenna is circularly polarized. Both antennas are separated by 50 cm, and the transmitting frequency is 807 MHz, so free space path loss is 24.56 dB. If misalignment antenna loss is neglected, the link budget analysis shows that the incoming power  
Figure 18: Measurement setup for the characterization of the minimum powers that turn on the temperature sensor.  
on the spiral antenna is approximately \(-4\) dBm. This amount of power is enough to charge the 1 mF capacitor and to turn on the temperature sensor. In fact, the sensor actually displays on the LCD screen a relative humidity of 26% and a temperature of 25.5 \({}^{\circ}\)C (77.9 \({}^{\circ}\)F) inside the laboratory. As seen in Figure 20, the multimeter reveals that the voltage in the sensor is 1.588 V, which exceeds the required 1.25 V to turn on the sensor.  
Finally, a comparison among different state-of-the-art RF energy harvesting systems is presented in Table 1. As can be noticed, most of the implemented solutions make use of narrowband antennas that work at WiFi/Bluetooth frequency band (2.4 GHz). However, it has already been discussed in the manuscript that they have the disadvantage of not being interoperable among countries with different frequency assignment plans. The rectifying efficiency of our circuit is comparable with the state-of-the-art designs [5; 8; 13; 14; 31], and even higher in some cases. However, unlike all the other references presented in Table 1, our system has been additionally tested with a temperature sensor acting as a load. It has been proved that the sensor is perfectly operational if the antenna is able to collect at least \(-4\) dBm. This is a further step in order to corroborate the fine operation of the RF energy harvesting system and demonstrates its validity for practical applications.  
Figure 19: Full RF harvesting system (**a**) and its circuit diagram (**b**). \(C_{m2}=4.7\) pF, \(L_{m2_{1}}=8.2\) nH, \(L_{m2_{2}}=4.3\) nH, \(C=33\) pF.  
Figure 20: Full-system validation: receiving RF energy harvesting system (on the right) in front of a transmitting Vivaldi antenna (on the left).