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Abstract— Current research trend in the RF wireless power transfer has a shift to new paradigm. Optimization of power transfer for wireless sensor networks has a new facelift due to the evolution of rectifier antenna (Rectenna). Wireless powering of sensors with Rectenna has kindled the interest of research society in the present trend. The research developed in the paper bounds within the wireless powering of miniaturized electronics with low power ratings. Wireless powering of sensors has a future potential to be tapped yet. The modelling and optimization that has been carried out have been backed up with simulations and practical experiments which are dealt later in the paper. The efficiency of Rectenna can have a significant improvement in working onto the each element of the Rectenna circuit as presented in the paper.

I. INTRODUCTION

The design for the transfer of power through RF-signals had a very rich classical research concepts and varied interest in applications as evidence by the number of patents. The classical work dates back from Hertz (1888), who first demonstrated the Electromagnetic (EM) wave’s propagation in free space. Thereafter there have been keen research interest and experimentation tests were carried over by Tesla (1899) [1]. He inculcated the idea of power transfer through RF-signals. A classical breakthrough is the transmission of signals by Marconi (1901) over Atlantic Ocean. This paved the way and turned the attention of Scientist towards the RF-signals. The development of Klystron and Magnetron which are capable of generating high powered microwave signals gave a new face lift for this unfolding technology. The modern history takes it fold when P.E.Glaser (1968) proposed the concept of SPS (Solar Power Station). The power generated in the space can be beamed down to earth using RF-signal. He gave a theoretical prediction of around 10GW of power that can be beamed down by the RF-signal of frequency 5.8GHz [1]. The research in the field of domesticating the technology for industrial applications is the current interest of study. Some of these include harvesting energy from broadband RF-signals. In this paper, we focus on the work on a low power rectenna designed for harvesting the power for sensors [2]. The paper is organised by the studying of individual elements of the Rectenna. The modelling of individual

elements such as patch antenna, diode and low pass filter give us a better insight towards the optimization of parameters for enhanced efficiency of rectenna. The modelling of circuit elements are carried out in EMDS and ADS software’s of Agilent provided us a better approximation towards the measured results. The Harmonic balance (HB) simulation of voltage doubler configuration of Schottky diode is performed. The effect of output low pass filter on the performance of the rectenna is also performed. The design discussed in the paper is suitable for low power rating applications. The practical experiment has been carried out with a model of Rectenna resonating fr at 2.45 GHz.

Figure 1: Schematic diagram for Microwave Power Transmission

II. ANALYTICAL MODELING OF RECTENNA ELEMENTS

A. Antenna: The Micro strip patch Antenna is designed

in accordance to the transmission line model formulation. The model is validated by including the fringe effects and effective dimensions are evolved. The Antenna is edge fed. The main advantages of patch antenna are, easy to design, compact and conformal. The length W and width L are given by,

21

)2

1(2

−+= r

rfcW ε

(1)

lf

cLrr

Δ−= 22 ε

(2)

Design of Low Power Rectenna for Wireless Power Transfer

R.Selvakumaran1,3, W. Liu2, Boon-Hee Soong1, Luo Ming2 and Y.L.Sum1 1School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798

3Position and Wireless Technology Centre, Nanyang Technological University

2Singapore Institute of Manufacturing Technology, Singapore

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where c = velocity of light in free space (3x108m/s), Δl is the line extension caused by fringing effect and εre = effective dielectric constant.

Based on the formulation the width and length are calculated to be 37.26mm and 28.83mm for a dielectric substrate with relative permittivity εr of 4.4 and h = 1.6mm.

B. Diode Modeling: The rectifier circuit connected to

the antenna rectifies the RF power to give DC output. The rectifying circuit consists of one or more diodes connected in accordance to the requirement. Diode rectifies the input available RF power efficiently only at higher input levels. We have modeled the rectifier using the Schottky diode. Schottky diode generally has low built in potential compared to the ordinary P-N junction diode. The diode current equation is given by,

0( ) 1qVKTI V I eη

⎛ ⎞⎜ ⎟= −⎜ ⎟⎝ ⎠

.

(3)

The barrier height of the Schottky diode varies effectively with applied voltage. The metal surface attracts more electrons effectively lowering the barrier and allowing the voltage dependent deviations from the ideal behaviour.

The ideal factor of the diode deviates slightly from unity at forward biases approximately above 0.1V. The ideal factor is given by,

)1(

1

dVd biϕη

−=

(4) where dφbi / dV is the variation in barrier height with applied voltage.

43

32

3

][32

−−−−=

qKTVNq

dVd

fcbis

db ϕϕεπ

ϕ

(5)

Φfc - Potential difference between the Fermi level and bottom of the conduction band.

The equivalent circuit model of the packaged Schottky diode is given as shown in Fig. 3 Rs and Cj are series resistance and junction capacitance. The output DC voltage of Schottky diode by applying a sinusoidal varying input to the diode is given by the relation

)(qKTVdc

η= )(ln( 0 KTqVI RF

η (6)

The DC output is given as the function of RF

voltage.

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Figure 3: Circuit Model of Packaged Schott

C. Filter: The harmonics generat

results in the unnecessary power dissipatieffectively reduced by introducing the filteoutput. The purpose of the output filter spurious harmonics generated due to the diode. The filters suppress the harmonicsdesired output to the load. The effect of thebeen discussed by the results of Harsimulation in the later part of the paper.

III. DESIGN OF RECTENNA SYThe Rectenna is designed to reson

We found the structure has a good retudesigned frequency. The measured and simthe structure is shown in Fig 4a and Fig simulated and the test measurement results hagreement. The return loss (S11) is foundThe antenna is matched directly to the Ssweep of frequency is performed for the Scthe impedance is measured at the operatingimpedance is found to be 124.8-j109.49 as s5a .

Figure 4a : Measured return loss of Rectenn

tky diode

ted by the diode ion. This can be er to the rectifier is to reduce the non-linearity of

s and present the e output filter has rmonic Balance

YSTEM nate at 2.45 GHz. urn loss for the

mulated pattern of 4b. We find the have a very close

d to be -24.8 dB. Schottky diode.A chottky diode and g frequency. The shown in the Fig.

na

Figure 4b : Simulated return loss of

Figure 5a: Impedance vs. Frequency

Figure 5b: Impedance vs. Frequency

The Voltage doubler cimplemented with the Schottkyconnections Fig. 6. The circuit co

f Rectenna.

y for Schottky diode

y for Antenna

circuit configuration is y diode (HSMS-2852) onnection shows that the

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diodes are connected anti parallel to each other. At the frequency of 2.45 GHz, the capacitor present in the output of the rectifier is ideally short circuited. The impedance can be evaluated by considering the anti parallel diodes. The effective impedance presented at the input to the rectifier is given by half the value of the single diode at the measured frequency. The impedance is calculated to be 62.4-j54.75.

A sweep of frequency vs. Impedance is also performed at various feed points along the edge of the patch to obtain optimal impedance for matching the rectifier circuit. We are able to get a feed point along the patch edge at which the impedance is conjugate matched with the input impedance of the rectifier. The feed point is at 0.2 mm distance from the edge of the antenna where the impedance is measured as 62.22+ j54.69 as shown in Fig. 5b. Traditional designs include a capacitive coupling for a match between the antenna and rectifier circuit. We can eliminate the requirement of input filter by directly matching the rectifier with the antenna. This conjugate match between the antenna and rectifier significantly contributes to enhance the efficiency of Rectenna circuit. The Harmonic balance simulation for voltage doubler configuration of Schottky diode is performed. The output power spectrum is analysed for the configuration with and without the output filter. The circuit configuration and power spectrum are as shown in Fig. 7 and Fig. 8.

Figure 6: Voltage doubler configuration of Schottky diode

Figure 7a: HB Simulation without (a) optimized output filter

Figure 7b: HB Simulation with (b) optimized output filter.

The Power spectrum gives a clear insight about the output power available at the rectifier output. Due to the non linearity issues of the diode the harmonics are generated. We can find the DC line and fundamental frequency component seem to be very dominant at the output of the rectifier Fig. 7(a) without output filter Fig. 8(a). The level of first, second & third order harmonics are relatively comparable to the fundamental frequency component. This spurious harmonics level contributes to the power loss. An optimization in the circuit has to be implemented in order to lower the power loss due to harmonics. In Fig. 8(b) the power spectrum for the optimized circuit

Figure 8a Power Spectrum without Output Filter as given in Fig. 7(b) has been shown. The power level of the harmonics has been considerably reduced due to the optimization of the rectifier circuit. A low pass filter at the rectifier output designed based on the fundamental frequency component gives a better optimized result. We can find the DC energy is concentrated in the power spectrum. The power level of first, second & third order harmonics are considerably reduced and they are below -50 dBm. It is not desirable to have fundamental and harmonics component in the output of the rectifier. It contributes to the power loss and decreases the overall efficiency of Rectenna.

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Figure 8b Power Spectrum with output filter. The components of Rectenna are analysed and optimised for better performance. The efficiency of Rectenna is given by the ratio,

%100)( ×=received

DC

PPη (7)

The comparison of Input power available at the rectifier circuit is plotted against the output available DC power as shown in the graph Fig. 9.

Figure 9: Output Power vs. Input Power

We find for the high input power levels the graph seems to be more linear than at low power levels. We can find the conversion efficiency is high at the higher input levels. In general, the conversion efficiency of the diode increases with the increase in the input power levels

IV. CONCLUSIONS The scope of the paper lies in the design of the Rectenna at low power levels. We find the efficiency of the Rectenna can be increased by effectively optimising the design of individual element. The coupling capacitor that has been used in the traditional design can be eliminated by directly matching the rectifier circuit with antenna. The effect of the output filter on the performance has also been studied by the Harmonic balance simulation. The power dissipation due to harmonics is greatly reduced by employing an optimized filter designed for the fundamental frequency at the output of the rectifier circuit. The efficiency of the Rectenna is found to increase at higher input power levels as discussed in the paper. The paper brings out the design and optimisation for low power level operation. The optimization process discussed gives us a promising advantage to enhance the overall efficiency of the Rectenna. In the future, broadband UWB antennas will be considered for recycling the ambient unused RF energy for powering sensors. This may require a lot of efforts to synthesize and design the UWB antenna and power management circuitry.

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IEEE Transactions on Microwave Theory and Techniques, vol. 32, no. 9, pp. 1230–1242, Sept. 1984

[2] J. A. Hagerty, Z. Popovic, “An experimental and theoretical characterization of a broadband arbitrarily polarized Rectenna array,” 2001 IEEE International Microwave Symposium Digest, pp.1855-1858, Phoenix, Arizona, May 2001

[3] J.A. Hagerty, F. Helmbrecht, W. McCalpin, R. Zane, Z.Popovic, “Recycling ambient microwave energy with broadband antenna arrays,” IEEE Trans. Microwave Theory and Techn., pp. 1014-1024, March 2004

[4] J.A.G. Akkermans, M.C. van Beurden, G.J.N. Doodeman, and H.J.Visser, “Analytical Models for Low-Power Rectenna Design,”, Antennas and Wireless Propagation Letters, vol. 4, pp. 187-190, 2005.

[5] J.O. McSpadden and K. Chang, “A dual polarized circular patch rectifying antenna at 2.45 GHz for microwave power conversion and detection,” IEEE MTT-S International Microwave Symposium Digest, pp. 1749–1752, 1994

[6] J.O. McSpadden, F.E. Little, M.B Duke, and A. Ignatiev, “An inspace wireless energy transmission experiment,” IECEC Energy Conversion Engineering Conference Proceedings., vol. 1, pp. 468–473, Aug. 1996

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