3.7 Rectenna results and discussion

The entire system (antenna, matching circuit and rectifier) is tested over different input power levels with different resistive load values at two frequencies

Figure 23. Photo of the measurement setup.

Rectenna Systems for RF Energy Harvesting and Wireless Power Transfer DOI: http://dx.doi.org/10.5772/intechopen.89674

Figure 24. Simulated and measured conversion efficiency in addition to the DC output voltage versus input power (a) at f1 (b) at f2.

(f1 and f2); Figure 24(a) and (b) show the comparison between the measured and simulated results of RF-to-DC conversion efficiency and the DC output voltage versus the input power at f1 and f2, respectively. The maximum measured conversion efficiency is 63% with input power range of 14 dBm (from 3.5 to 10.5 dBm) at f1, while the measured efficiency at f2 is 69% with input power from 4.5 to 11 dBm (15.5 dBm). There is a slight shift between the simulation and measurement results, where the maximum simulated RF-DC conversion efficiency are 66 and 73% at the same two frequencies, respectively. Due to the limitations in the experiments, the received input power is limited only up to 11 dBm.

## 4. Conclusions

ANSYS high-frequency structure simulator (HFSS). The enhanced-gain antenna described in [45] is used as a receiving antenna in the proposed rectenna to increase the rectifier sensitivity. Hence, increasing rectenna capability to harvest from low input power levels. The receiving antenna and the rectifier are integrated on the same substrate, fabricated and measured in the measurement setup shown in Figure 22. An Agilent Technologies E8257D Analog signal generator is used to send a microwave signal which is connected to a horn antenna with 9 dBi gain at the two frequencies. On the other hand, the rectenna under test (RUT) is connected with a voltmeter to measure the DC output voltage. To take the antenna radiation characteristics into account, the antenna effective area is considered. Hence, the RF-DC conversion efficiency of the proposed rectenna (η) is calculated as the following:

<sup>η</sup> <sup>¼</sup> <sup>V</sup><sup>2</sup>

power and RL is the resistive load. Pin is defined in Eq. (19)

Aeff are calculated using Eqs. (20) and (21), respectively.

3.7 Rectenna results and discussion

Recent Wireless Power Transfer Technologies

Figure 23.

170

Photo of the measurement setup.

DC Pin � RL

where VDC is the measured DC output voltage, Pin is the received RF input

where PD is the RF power density and Aeff is the antenna effective area. PD and

PD <sup>¼</sup> PtGt

λ2

Aeff ¼ Gr

The entire system (antenna, matching circuit and rectifier) is tested over different input power levels with different resistive load values at two frequencies

40 cm. Figure 23 shows the photo of the rectenna measurement setup.

Pt is the transmitting power, Gt is the horn antenna gain and r is the distance between the transmitter and the rectenna all are known. Therefore, the RF-to-DC conversion efficiency can be measured. For far-field measurements, r is chosen of

� 100 (18)

Pin ¼ PD � Aeff (19)

<sup>4</sup>πr<sup>2</sup> (20)

<sup>4</sup><sup>π</sup> (21)

This chapter presents a study of rectenna systems for RF energy harvesting and wireless power transfer. A survey about employing rectennas in WPT, low input received power rectennas, single and multi-band rectennas, wide input received power rectennas are introduced. Finally, dual-band rectenna using voltage doubler rectifier and four-section matching network is discussed. The first part of the rectenna design is the dual-band disc antenna with enhanced gain in order to collect a highest amount of RF energy. It radiates at 1.95 and 2.45 GHz. The measured results showed the gain of 8.3 and 7.8 dBi at 1.95 and 2.45 GHz, respectively. The disc antenna is integrated with a dual-band rectifier with four-section matching network to introduce a dual-frequency rectenna with higher conversion efficiency over wide band of the input power for multiband RF energy harvesting. The rectenna gives maximum RF-DC measured conversion efficiency of 63% and 69% at 1.95 GHz and 2.5 GHz, respectively. it also operates over a wide range of the input power; it covers the range of 14 and 15.5 dBm at f1 and f2, respectively for a conversion efficiency higher than 50% with load resistance (RL) = 1K. The rectenna is simulated, fabricated and measured. The simulated and measured results show good agreement.

Recent Wireless Power Transfer Technologies

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