## 4 Ambient RF Energy Harvesting Prototype Performance in Real Conditions  
Numerical and experimental data discussed in the previous sections show that the most efficient RF rectifier for \(P_{in}=-30\) dBm is the simple diode-capacitor circuit when 1 k\(\Omega\leq R_{L}\leq 10\) k\(\Omega\). The implementation of an ambient RF energy harvester is conditioned by practical issues related with the impedance-matching network that should be considered. Theoretically, a lumped C-L impedance network can be used to match the diode-capacitor rectifier with the standard 50 \(\Omega\) antenna. However, tolerances and parasitic capacitances and inductances of the lumped elements complicate the fabrication of the prototype. All these effects can be compensated by tuning the constituent elements of the impedance matching network.  
Figure 6 shows the picture of the implemented prototype of an RF rectifier. The matching network has been initially designed by using the ADS Smith chart, taking into account the measured input impedance of the fabricated diode-capacitor rectifier. The final values of the impedance matching network have been tuned to optimize the adaptation. Figure 7 shows the measurements of a fabricated RF diode-capacitor rectifier prototype finally matched at 2.47 GHz for \(-30\) dBm input power. The impedance-matching network is a C-L network with C = 0.1 pF and L = 6 nH, as shown Figure 7a's inset. Figure 7b shows the measured input impedance of the matched rectifier (\(Z_{rec}\)) at the operating frequency.  
Figure 6: Implemented prototype of an ambient RF energy harvesting system composed by the input antenna port, the C-L impedance network, a thru connector, the diode-capacitor rectifier, and a connector to the load resistor. Fabrications have been performed using the following: SMS-7630 [33] diode, capacitors from the HIQ-CBR Series, Case Size 0402 Radio Frequency and Microwave Ceramic Capacitor Kit from Kennet, and the inductors from the High Frequency Inductor Designer Kit EIA 0402 from Johanson Technology.  
Figure 8 overlaps the measured \(S_{11}\) parameter of the fabricated and matched RF rectifier prototype with the power max-hold measurement of the ambient RF spectrum in our laboratory. As observed, the matching frequency coincides with one of the active channels of the WIFI band. The efficiencies shown in Figure 5 indicate that only 10% of \(-\)30 dBm (1 uW) input power would be delivered to a 10 k\(\Omega\) load, producing a maximum of 32 mV voltage and supplying 33.2 uA to the load. However, it must be considered that the ambient RF signals are constantly oscillating depending on the wireless traffic. As a result of these fluctuations, the real effective output power delivered to the load will be lower than that indicated by Figure 5 efficiencies.  
Figure 8: Max-hold ambient RF power spectrum measurement, between 2.35 GHz and 2.55 GHz (in black and left-scale): \(S_{11}\) of the RF rectifier with 6.8 mF and 10 k\(\Omega\) load in the output (in red and right scale).  
Figure 7: Measured and simulated \(S_{11}\) parameter (**a**) and impedance in the Smith Chart (**b**) of a fabricated RF-rectifier prototype matched at 2.45 GHz for -30 dBm input power. Fabrication has been done using: SMS-7630 [33], \(C_{Load}=C_{RF}+C_{sto}\) being \(C_{RF}=2.2\) pF, \(C_{sto}=6.8\) mF: \(R_{L}=10\) k\(\Omega\) and \(C_{m}=0.1\) pF using a CBR04C108B1 GAC RF-capacitor and \(L_{m}=6\) nH L-07C12 NJ/65 RF-autoinduction for the matching circuit. Simulated \(S_{11}\) only have sense below the series resonant frequency of the RF inductor used (3.5 GHz).  
As observed in Figure 9, the output voltage oscillates between 16 mV and 36 mV corresponding to 25.6 nW and 129.6 nW. In the low voltage region, the harvesting system can supply 2.5 uA, which should be enough to power ultra-low commercial power sensors. However, for a proper power function, several systems should be combined to achieve the minimum voltage requirements.