## 7 Experimental Validation of the Adaptive RF WPT Harvester  
Operating in the ISM band at 915 MHz, the designed adaptive rectifier (without the active load modulation technique) was optimized to operate for maximum power conversion efficiency over a wide range of RF input power levels. Simulated in Keysight's Advanced Design System simulator [22], it uses a 31-mm-thick Rogers 5880 substrate and two transistor design kits (NE3210S1 and NE68133) from the Renesas Electronic Active Device Library. Also, the diode SPICE models (for HSMS2850 and HSMS2860) were extracted from Avago Technologies datasheets [20]. The S-parameters of the passive components (capacitors, inductors, and resistors) were taken from the datasheets provided in [23].  
The fabricated adaptive rectifier circuit (Figure 11) attained a co-simulated maximum power conversion (PCE) of 66% at 15 dBm input power as depicted in Figure 12. Furthermore, it exhibits 40% of PCE over a wide dynamic input range of incident RF power levels from \(-6\) to 25 dBm. Also, the co-simulated results are in good agreement with the measured data. However, due to equipment and test bench limitations, only measurements up to 15 dBm were performed. The rectifier achieved 4 V with an input power of 15 dBm.  
Figure 11: Adaptive rectifier circuit: (**a**) co-simulation layout view (without the active load modulation block); (**b**) the rectifier’s printed circuit board.  
Figure 12: Simulated and measured parameters at 915 MHz for the designed adaptive rectifier with \(R_{L}=1\) k\(\Omega\): (**a**) Efficiency at 915 MHz; (**b**) output direct current (DC) voltage at 915 MHz.  
Figure 13 shows the adaptive rectifier with the active load modulation block as well as a comparison of the expected PCE while using active load modulation versus fixed load values. As expected, by including the active load, the circuit presents a higher co-simulated efficiency, over 40%, and within a wider range of RF input power levels (\(-7\) dBm to 32 dBm) compared to fixed loads. The active load resistors were adjusted to attain maximum efficiency over the desired wide range of input power levels (\(R_{1}=1\) k\(\Omega\) and \(R_{2}=R_{3}=0.3\) k\(\Omega\)). Therefore, the load varies from 0.3 k\(\Omega\) to 1.3 k\(\Omega\). A low pass filter, L, and C\({}_{2}\) were added to the rectifier circuit. Since the real part is almost constant in the Smith chart, the series capacitor (C\({}_{1}\)) is enough to match the rectifier circuit to the antenna input (50 \(\Omega\)). The adaptive active load modulation achieved a DC output voltage of around 3.25 V at the input power of 15 dBm. The reconfigurable rectifier consumes an area of \(30\times 17\) mm\({}^{2}\).  
To conclude, as reported in Table 2, the proposed rectifier performance was demonstrated through a successful comparison with existing published designs. The proposed designs demonstrated an extensive range of input power levels to address the issue of conventional rectifiers by simultaneously utilizing the adaptive reconfigurable configuration along with the active load modulation approach.  
Figure 13: Adaptive rectifier circuit with an active load configuration: (**a**) Co-simulation layout view; (**b**) Efficiency with active load and fixed passive load values; and (**c**) output DC voltage at 915 MHz.