# A Power Processing Circuit for Indoor Wi-Fi Energy Harvesting for Ultra-Low Power Wireless Sensors
## 7 Actual Experimentation of the RFEHS  
In this section, the performance of the proposed power processing circuit experimented in a real environment is highlighted. In the experiment, the input terminal of the proposed power processing circuit is connected to a 50-Ohm antenna, where the antenna is designed to operate at 2.45 GHz. The energy harvester is a printed co-planar waveguide antenna. The antenna is attached to the input of the circuit through a Sub-Miniature Version A (SMA) connector. The SMA connector impedance is 50 Ohm. The actual figure of the energy harvester is depicted in Figure 15. The detailed explanation of the antenna and its performance has been reported in [14]. The impedance losses when the power processing circuit is attached to the harvesting antenna is another system characteristic which needs to be highlighted. The effect on impedance losses can be seen by measuring the return loss with and without the power processing circuit attached to the antenna. Figure 16 illustrates the measured return loss, \(S_{1I}\) for both conditions. As can be seen from the figure, the impedance losses are small, and the value is above the equator of the Smith chart, reflecting the harvesting antenna and the proposed power processing circuit inductive characteristics. A PMU is attached to the output terminal of the proposed power processing circuit as shown in Figure 17. On the PMU, a supercapacitor is connected to the energy storage component. The arrangement of the harvesting antenna, the proposed power processing circuit and a PMU, made a complete RF energy harvesting system. During the measurement period, the radio frequency  
Figure 16: Measured impedance losses of the harvesting antenna without and after attachment of the power processing circuit at 2.45 GHz. Figure 15: A 50 \(\Omega\), 2.45 GHz coplanar waveguide antenna. signal strength inside the selected indoor area is monitored using Wi-Fi analyser software that is installed in a notebook and using a spectrum analyser connected to a similar antenna of the energy harvester for the proposed system. It has been found that the signal strength measured using both measurement devices are in good agreement. The measured signal strength result is illustrated in Figure 18. It has been found that during the test period, the transmitted signal is low. However, even the incident power is low, there are multiple numbers of incident power received at the energy harvester, continuously, and the amount is sufficient for the operation of the proposed harvesting system. During the measurement periods, it has been found that four 2.4 GHz Wi-Fi access points are covering the selected area. As can be seen in Figure 18, the maximum peak of the signal strength is \(-55\) dBm, and the lowest is between \(-85\) dBm. The available power that could be transferred from the antenna to the matching circuit can be calculated by a function of \(P_{INC}\) = \(P_{measured}\) + 30 dBm. For an example, let us assume the transmitted power (\(P_{TX}\)) for Wi-Fi transmitter is 0 dBm, and the average power received at the antenna is approximate \(-60\) dBm. The losses introduced during the power transmission can be calculated by substituting the transmitted power with the average received power which gives the estimated losses to be 0 dBm(\(-60\) dBm) = 60 dBm. Then, the loss introduced by antennae and connector can be  
Figure 17: The actual figure of the proposed RF energy harvesting system. Figure 18: Measured 2.45 GHz Wi-Fi signal strength inside the indoor area. calculated which is 60 dBm/2 = 30 dBm. Hence, \(P_{INC}\) for the level of the signal is \(-\)30 dBm. In practical, as shown in Figure 18, if the lowest measured signal strength is \(-\)85 dBm, the available power that is transferred to the matching circuit is at \(-\)55 dBm. The voltage across the 0.22 F super capacitor connected to the output of the PMU is measured. The value is recorded using a data logging instrument from National Instrument known as ELVIS II.