## 3 Multiband Ambient RF Energy Harvester Using the Proposed Antenna
### Rectifiers Design and Full Testing System  
First, we designed four single-band rectifiers. Because of its high conversion efficiency at low power densities, the SMS7630 Schottky diode was used in the voltage doubler rectifier circuits. This diode is suitable for an input power level of around of \(-10\) to 0 dBm. Figure 11 depicts the prototype and configuration of these rectifier circuits. The operating frequency was controlled by \(l_{1}\), \(w_{1}\), \(R_{L}\), and \(R\), and they were optimized in simulation. Four resistive loads of 800 \(\Omega\), 870 \(\Omega\), 1200 \(\Omega\), and 3300 \(\Omega\) were used for the 1.8 GHz, 2.1 GHz, 2.45 GHz, and 2.6 GHz rectifiers, respectively. The capacitor was chosen as 470 pF. The DC filters were large radial stubs that reject the harmonics in order to improve conversion efficiency. We proposed a quad-band rectenna working at 1.8 GHz, 2.1 GHz, 2.45 GHz, and 2.6 GHz, four common frequency of bands available in the environment, formed by the proposed wideband circularly polarized antenna, the quadplexer, and four single rectifiers. The quadplexer helps separate the four frequencies and transfer them to corresponding rectifiers. The rectifiers are single-banded, so they are easy to design and also likely to achieve good performance. The proposed quad-band rectenna is shown in Figure 12.  
Figure 10: (**a**) The proposed quadplexer (dimension in mm); (**b**) simulated S-parameters of the quadplexer. We investigated the advantages of employing the \(2\times 2\) circular polarized array for RF energy harvesting in self-power devices. As shown in Figure 13a, the highest simulated AC-DC efficiency of the 1.8 GHz rectifier is 65% at \(-1\) dBm input power, for the 2.1 GHz it is 64%, the 2.45 GHz is 61%, and the 2.6 GHz is 61%, respectively. The maximum measured AC-DC efficiency was 60%, 55%, 55%, and 54% at \(-1\) dBm, 0 dBm, \(-1\) dBm, and 0.5 dBm input power, respectively. The highest reported RF-DC output voltage was 1568 mV at 2 dBm, occurring with the 2.6 GHz rectifier. After connecting the rectifiers to the quadplexer, forming a quad-band rectifier, the AC -DC efficiency slightly decreased to 49%, 43.5%, 41%, and 38% for the 1.8, 2.1, 2.45, and 2.6 GHz bands at \(-0.5\) dBm input power, as shown in Figure 13b. Figure 11: Schematic and fabricated rectifiers. Figure 12: Quad-band rectenna system using the wideband and CP antenna with a maximum output voltage at 2.6 GHz. In the full system testing, the proposed wideband and CP antenna were connected to the quadplexer and four rectifiers to form the multiband rectenna prototype via SMA connectors, as shown in Figure 12. A signal generator excited monotone incident waves via a reference antenna. The RF-DC efficiency was smaller than the AC-DC due to the loss on the connectors and quadplexer. The maximum RF-DC efficiency was 27%, 26%, 25.5%, and 27.5% at \(-\)6 dBm, respectively, as in Figure 13c. The maximum measured voltage was 484 mmV at 2.6 GHz, at nan input power level of \(-\)6 dBm. This result shows its potential for a self-powered wireless sensor when connecting with a power management circuit. The quad-band rectenna was also tested in the ambient environment, 70 m from the base station. The rectenna collected and harvested the 3G and 4G waves transmitted from the base station, along with the Wi-Fi signal in the surrounding houses into DC voltage. The maximum recorded voltage was 194.5 mV, corresponding to 47 \(\upmu\)W power, as shown in Figure 14. When the rectenna is put closer to the base station, the amount of power gathered should be substantially higher. It should be noted that for practical application, the locations of the power sources are unknown. For locations very close to power sources, such as base stations, the harvested power can be high enough to support a wireless sensor,  
Figure 13: (**a**) Simulated and measured AC–DC efficiency of the rectifiers; (**b**) measured AC–DC efficiency of the quadplexer and rectifiers; (**c**) measured RF–DC efficiency.