This document summarizes a research paper on the design of a low-power rectenna for wireless power transfer. It discusses the analytical modeling and optimization of individual rectenna elements, including the microstrip patch antenna, Schottky diode model, and output filter. Simulation and experimental results show that directly matching the rectifier impedance to the antenna improves efficiency over traditional designs using a coupling capacitor. Optimizing the output filter also reduces harmonic power dissipation, further improving efficiency. The rectenna efficiency is found to increase with higher input power levels as discussed.

1. 978-1-4244-4547-9/09/$26.00 ©2009 IEEE TENCON 2009
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,
2
1
)
2
1
(
2
−+
= r
rf
c
W
ε
(1)
l
f
c
L
rr
Δ−= 2
2 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
1
School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798
3
Position and Wireless Technology Centre, Nanyang Technological University
2
Singapore Institute of Manufacturing Technology, Singapore
1

2. where c = velocity of light in free space (3x108
m/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( ) 1
qV
KT
I 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
dV
d biϕ
η
−
=
(4)
where dφbi / dV is the variation in barrier height with
applied voltage.
4
3
32
3
][
32
−
−−−=
q
KT
V
Nq
dV
d
fcbi
s
db
ϕϕ
επ
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
)(
q
KT
Vdc
η
= )(ln( 0
KT
qV
I RF
η
(6)
The DC output is given as the function of RF
voltage.
2

3. Figure 3: Circuit Model of Packaged Schott
C. Filter: The harmonics generat
results in the unnecessary power dissipati
effectively reduced by introducing the filte
output. The purpose of the output filter
spurious harmonics generated due to the
diode. The filters suppress the harmonics
desired output to the load. The effect of the
been discussed by the results of Har
simulation in the later part of the paper.
III. DESIGN OF RECTENNA SY
The Rectenna is designed to reson
We found the structure has a good retu
designed frequency. The measured and sim
the structure is shown in Fig 4a and Fig
simulated and the test measurement results h
agreement. The return loss (S11) is found
The antenna is matched directly to the S
sweep of frequency is performed for the Sc
the impedance is measured at the operating
impedance is found to be 124.8-j109.49 as s
5a .
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 c
implemented with the Schottky
connections 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
3

4. 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.
4

5. 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
P
P
η (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.
REFERENCES
[1] W.C. Brown, “The history of power transmission by radio waves,”
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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