Design of an Enhanced RF Energy HarvestingSystem for Wireless Sensors

*Ahmed Al-Khayari*, Hamed Al-Khayari*, Sulaiman Al-Nabhani*, Mohammed M. Bait-Suwailam**, and Zia Nadir***Student/**Faculty in the Department of Electrical and Computer Engineering,

Sultan Qaboos University, P.O.Box 33, Al Khoud,

Muscat, Oman, PC. 123,

Email: msuwailem@squ.edu.om

Abstract—In this paper, a technique is proposed for enhancingenergy harvesting devices. The design is capable of chargingsmall power consumption sensors by capturing ambient radiofrequency (RF) waves. The technique comprises the use ofhigh-profile monopole antenna array with a reflector in orderto maximize the RF energy received by the antenna system.Furthermore, a prototype of the proposed RF energy harvestingsystem was built, and tested in laboratory. The proposed design iscompared with a reference case that uses only a single monopoleantenna element at both transmitter and receiver sides. Theachieved results show better performance in terms of capturedoutput direct current (DC) voltage in comparison to that capturedenergy by the reference prototype with a single antenna element.The designed system is useful in many wireless applications andcan easily be integrated with other high-gain antenna systems tofurther maximize the harvested energy.

I. INTRODUCTION

Nowadays, wireless devices are growing in many applica-tions like mobile phones or sensor networks. Such wirelessdevices provide efficient and practical solutions to consumer,industrial, and military needs [1]. This growth in wirelessapplications produces a large usage of batteries that givesalternative solutions for empowering most wireless devices.However, such batteries pose constraints due to packaging size,operational time, and their deposition cause environmentalissues. Furthermore, many applications, like wireless sensornodes are located in difficult or inaccessible places so theirbattery maintenance becomes a crucial problem. This problemmotivated lots of designers to think of infinite power supplyresources or enhanced power over a period of time. Never-theless, in many instances only a few milliwatts of power areneeded to power wireless sensors [2].

Currently, many technologies have been developed that at-tempt to overcome the limitations imposed on wireless devices.For instance, recent advancements in rechargeable batteries anduse of double-layer capacitors, as well as technologies thatharvest solar, wind, or even kinetic energy to name a few havebeen effective at satisfying a large portion of the establishedmarket need. Another alternative is to get an advantage ofthe electromagnetic radio frequency (RF) waves that existin air. Such waves can propagate through the surroundingspace and hence can be captured by an antenna. This kindof energy harvesting is still a new research direction and is ofgreat interest to engineers and researchers worldwide. Thereexists couple of sources for RF energy harvesting, that are:intentional sources, anticipated ambient sources, and unknown

ambient sources [2]. The intentional sources are in fact ded-icated power transmitters that provide controlled amount ofpower. Unlike intentional sources, anticipated ambient sourcesact as uncontrolled sources of power, like TVs, radio, Wi-Fiand mobile base station transmitters. Unknown ambient RFsources can still provide minimal amount of power, however,with no control and no knowledge of the source of suchtransmitted energy.

Fig. 1. Block diagram of an energy harvesting system.

The main objective of the RF energy harvesting system isto convert the RF power from the space into usable electricalenergy source. In order to achieve that, a system with severalstages is required as shown in Fig. 1. The RF signals aregenerated from a transmitting antenna (i.e., RF source) andthey are received by a receiving antenna transferring theminto AC current as an input to the harvesting circuit. In orderto maximize the captured power, both the transmitting andreceiving antennas should have same polarization. In this work,we propose the use of linear polarization for ease of impedancematching. Second step is to transfer the power received bythe antenna to the circuit with minimal losses. Therefore amatching circuit is placed at the input stage of the harvestingcircuitry to equalize the impedance between the antenna andthe next component of the circuit. Furthermore, a voltagebooster circuit is added to the device as an amplifier for thevoltage input to increase the output voltage of the system.Since most of consumer devices require DC supply to chargeand the fact that the AC voltage cannot be stored, a rectifiercircuit takes its function to convert the AC voltage to DCvoltage.

Significant amount of research work have been conductedin this exciting field ranging from antenna design to high-efficiency rectifying devices [3]-[7]. In this work, an enhancedRF energy harvesting circuit is designed and tuned to work

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at the unlicensed ISM band at 868 MHz. The aim of thiswork is to provide a cost effective and efficient antenna designto maximize the amount of power that is received by theantenna system. The enhancement is achieved here throughincorporating a monopole antenna array designed at that par-ticular frequency with a reflector to maximize the amount ofcaptured energy by the antenna system. Several experimentswere carried out in laboratory and results are presented in thispaper. For comparison purposes, single antenna elements atboth transmitter side and receiver end of the harvesting circuitwere adopted.

II. PROPOSED DESIGN

Based on the block diagram of the harvesting systemdescried in Fig. 1, a prototype for an RF energy harvestingsystem was designed, built in laboratory, and tuned to operateat the ISM band of 868 MHz. An anticipated RF source wasused at transmitter side to provide a sufficient RF energyin order to quantify the performance and efficiency of theproposed harvesting circuit. An image of the built circuit isshown in Fig. 2.

Along the edge of the printed circuit board, an SMAconnector was soldered to connect the antenna system withthe remaining parts of the circuit. Then three stages of voltagemultiplier are used with Schottky BAT41 diode and 560 pFceramic capacitors. The three stages can easily be viewed fromFig. 2, since each two diodes and two capacitors form onestage. Upon boosting the output voltage, a load capacitor witha value equal to 2.2 mF is used to store the output power.The ground plate is important to shield the circuit from anyelectromagnetic interference and to provide better performancefor the monopole antenna array.

Fig. 2. An image of the proposed circuit for enhancement of RF energyharvesting for wireless sensors.

III. RESULTS

In the experimental setup, an RF source, Agilent n9310a,was used to supply RF signal from a monopole antenna at thetransmitting side. A frequency of 868 MHz was used so that nointerference can result from licensed frequency bands used by

other agents. On the receiving side, another monopole antennahad been used in order to capture the radiated signals. Foroptimum receiving signals, the sending and receiving antennasshould be identical in shape, size and with same polarization.Additionally, it is important to place the receiving antenna atthe far field region to make sure that the receiving signals areonly the radiated waves and there are no reactive components.The receiving antenna is connected to the harvesting circuit.Fig. 3 depicts the measurement scenario as conducted in thelaboratory. A Spectrum Analyzer was used at the receivingside in order to measure the receiving signals.

Fig. 3. Measurement setup with distance controlled using measuring ruler.

In this experiment, the RF generator transmitting signalswith 868 MHz frequency and power amplitude equal to 20dBm. The expected receiving signals should be less than thetransmitted signals due to some losses. One of these losses isthe path loss. This loss can be calculated using Friis equation

Pr = PtGtGr(λ

4πd)2, (1)

where Pr is the received power at the receiver side, Pt isthe transmitted power, λ is the wavelength of the transmittingsignal, Gt and Gr are transmitting/receiving antenna gains, andd is the separation distance between the transmitting/receivingantennas. It is clear that the received power is inverselyproportional to the distance between the transmitting and thereceiving antennas. In fact, several measurements had beencarried out in the lab to validate this relationship for theproposed harvesting circuit.

In the first measurement setup, one antenna was used as atransmitter and another was used as a receiver. The capturedenergy was measured directly after the receiving antenna usingthe spectrum analyzer. Then, measured received power datawere converted to RMS voltages using Ohm’s Law Powerequation

Pr =(Vin)

2

Z0, (2)

Vrms =√

Z0Pr, (3)

where Pr is the received power, Z0 is equal to 50 Ω, and Vin isthe input voltage. At the end, the output voltage was measuredat the end of the harvesting circuit. The input RMS voltageand the output DC voltage were recorded and the results areas shown in Fig. 4 below.

In order to enhance the harvested energy, we propose touse a conducting reflector along with a monopole antenna

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Fig. 4. Input AC voltage Vin and output DC voltage Vout of the RFharvesting circuit as a function of separation distance.

array. Fig. 5 shows the circuit prototype after incorporat-ing the proposed enhancement technique. In this work, onlytwo monopole antennas are adopted. Adding another antennashould increase the amount of captured energy. In additionto that, the proposed use of the parabolic reflector shouldqualitatively focus more the ambient RF signals into theantenna system. The reflector used here is a simple aluminumfoil placed on the back side of a parabolic-shaped thick paper.In brief, this will increase the directivity of the monopoleantenna system in one direction instead of radiating in alldirections. Fig. 6 shows the radiation pattern of a singleantenna before and after adding the reflector. It is clear that thereflector will direct the pattern to be more directive. Different

Fig. 5. An image of the designed antenna array along with the conductingreflector for further enhancement of the received RF energy.

measurements were taken in order to determine the effects ofthe enhancement methods. This can be done by measuringthe output DC voltage for different cases. The results weretaken for one antenna without any enhancement, one antennawith the reflectors, two antennas only and two antennas withthe reflectors. Fig. 7 summarizes the results between the threecases.

As can be seen from Fig. 7, the recorded DC outputvoltage at a separation distance of 25 cm between trans-

Fig. 6. Simulated radiation pattern of a single antenna element without andwith a reflector.

Fig. 7. Measured output voltage for the three cases as a function of separationdistance between transmitter and receiving antenna system without a loadcapacitor.

mitting/receiving (Tx/Rx) single antenna elements is approxi-mately equal to 570 mV. This result without any enhancementbut when reflectors were incorporated to both Tx/Rx antennas,the output voltage was increased to reach 1.06 V. Moreover,adding another antenna will increase the output voltage more.The use of two monopole antennas has increased the outputvoltage to 630 mV (without reflectors) and to 1.27 V withthe use of reflectors. It is clear from results above that theenhancement has been achieved from this proposed design.

Other measurements were taken. In this case the capacitiveload was added. The output voltage without the loadingcapacitor is instantaneous. However, adding a capacitor atthe output stage of the harvesting circuit makes the readingmore stable. Hence, it makes the readings more reliable andaccurate, however, at cost of time as it does take long time tostabilize the measured output voltage due to charging delay foreach single separation distance. Nevertheless, capacitors withminimal charging delay will overcome this issue.

Fig. 8 depicts the measured results obtained when using aload capacitor. Data were taken for several separation distancesbetween transmitting antenna and receiving antenna system.Comparison is made with a reference circuit when adoptingsingle antenna elements. As can be seen, capacitors requirelong amount of time to reach stabilized output DC voltagewhen comparing these results with those obtained in Fig. 7

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without using the load capacitor. However, results obtainedwhen using the load capacitor confirm the proposed conceptthat is adding the reflectors and adopting a high-gain antennaarray increase the received input (AC) and output (DC) voltageof the harvesting circuit.

Fig. 8. Measured output voltage for the antenna array and single antenna atboth Tx/Rx sides as a function of separation distance between transmitter andreceiving antenna system when using a load capacitor.

IV. CONCLUSION

In this paper, an enhanced RF energy harvester circuit forpowering low consumption electrical devices were designedand tested. The designed system consists of monopole antennaarray, a 3-stage voltage booster, rectifying circuit and a unitstorage device. To enhance the captured energy, an in-housemade parabolic conducting reflector was used.

The experimental results show that the power harvested byone antenna decreases when separation distance between trans-mitting antenna and receiving circuitry increases. Furthermore,the use of high-gain antenna array as well as RF reflector atthe input of the harvesting system has resulted in dramaticincrease in the harvested power. Currently, more work is beingcarried out with the use of high-gain planar antennas in orderto maximize the captured energy by the harvesting circuit.

V. ACKNOWLEDGMENT

The authors would like to thank SQU for providing thefinancial resources to accomplish this research work.

REFERENCES

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[2] Harry Ostaffe, RF Energy Harvesting Enables Wireless Sen-sor Networks @ONLINE, (http://www.sensorsmag.com/sensors-mag/rf-energy-harvesting-enables-wireless-sensor-networks-6175), 2009.

[3] Z. W. Sim, R. Shuttleworth, M. J. Alexander, and B. D. Grieve,”Compact patch antenna design for outdoor RF energy harvesting inwireless sensor networks,” Progress In Electromagnetics Research, Vol.105, 2010, pp. 273-294.

[4] P. Nintanavongsa, U. Muncuk, D. Lewis,and K. Chowdhury, ”DesignOptimization and Implementation for RF Energy Harvesting Circuits,”IEEE Journal On Emerging and Selected Topics in Circuits andSystems, VOL. 2, NO. 1, March 2012, pp. 24-33.

[5] N. Md. Din, C. K. Chakrabarty, A. Bin Ismail, K. K. A. Devi, andW.-Y. Chen, ”Design of RF energy harvesting system for energizinglow power devices,” Progress In Electromagnetics Research, Vol. 132,2012, pp. 49-69.

[6] Masotti, D.,Costanzo, A., and Adami, S., ”Design and realization of awearable multi-frequency RF energy harvesting system,” In Proceed-ings of the 5th European Conference on Antennas and Propagation(EUCAP), 11-15 April,2011, pp. 517 - 520.

[7] Maryam Al-lawati, Manar Al-Busaidi, and Zia Nadir, ”RF energyharvesting system design for wireless sensors,” In the 9th InternationalMulti-Conference on Systems, Signals and Devices, 2012, pp. 1-4.

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