882 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 56, NO.

3, MARCH 2008

Compact Circularly Polarized Rectenna

With Unbalanced Circular Slots
Tzong-Chee Yo, Chien-Ming Lee, Chen-Ming Hsu, and Ching-Hsing Luo, Member, IEEE

Abstract—A novel rectifying antenna (rectenna) using a com- be generated. The unwanted harmonics cause problems of har-
pact circularly polarized (CP) patch antenna with RF-to-DC power monics re-radiation and efficiency reduction of rectenna; then
conversion part at 2.45 GHz is introduced, in which the unbal- the LPF is required to suppress harmonics to improve system
anced slots structure is adopted for size reduction and 2nd har-
monic rejection. To contribute a rectenna for RF power conversion, performance and prevent harmonics interference. For size re-
the back side of the CP antenna is the doubler rectifier circuit with duction and cost down, the antenna with harmonic rejection
3rd order harmonic rejection radial stub for efficiency optimiza- property was proposed to eliminate LPF [11]. In addition, for
tion and harmonic power re-radiation elimination. The adopted satellite power transmission as well as portable application, CP
CP antenna built on low cost FR-4 substrate has measured band- has more fade resistant than linear polarization (LP) and low
width of 137 MHz (10 dB return loss) as well as 30 MHz CP band-
width (3 dB axial ratio). By up to 3rd order harmonic rejection, the
polarization loss between transmitting and receiving antennas
RF-to-DC conversion efficiency would reach 53% and 75% with without regarding its rotating angle. The combination of har-
1 K resistor load under ANSI/IEEE uncontrolled and controlled monic rejection and CP property would bring the advantages of
RF human exposure limit respectively. low polarization loss, conversion efficiency enhancement and
Index Terms—Circular polarization, patch antenna, rectifiers, simpler design.
rectifying antenna (rectenna), wireless power transmission (WPT). In this paper, a compact circularly polarized rectenna with
harmonic rejection property is proposed. The unbalanced
slots on the circular patch antenna achieve CP operation, size
I. INTRODUCTION reduction and the second harmonic rejection property so that
only third harmonic rejection stub is necessary omitting the re-
quirement of LPF and bringing the benefit of low insertion loss
R ECTIFYING antenna (rectenna) which can convert RF en-
ergy to DC power plays an important role in free space
wireless power transmission (WPT). Over the last century, the
at fundamental frequency associated with LPF in conventional
rectennas. Furthermore, the CP operation is excited by single
development of rectenna for space solar power transmission feed avoiding complex branch coupler feeding. The doubler
(SSPT) [1] as well as WPT [2] had great achievement with spe- rectifier with small third harmonic rejection stub placed on the
cific functions; and the applications e.g., actuator [3] or wireless back side of rectenna provides high output voltage as well as
sensors [4] were also applied with WPT techniques. It shows great RF-to-DC conversion efficiency.
that the WPT techniques using rectennas have great potential to
deliver energy to load through free space without using physical II. ANTENNA DESIGN
transmission line.
The typical rectenna in the prior literatures [1]–[4] basically The geometry along with its parameters of the CP patch
consists of four elements: antenna, low pass filter (LPF), diodes, rectenna with two unbalanced circular slots is shown in Fig. 1.
and DC pass capacitor. The initial development of rectenna fo- The slotted CP antenna has the advantages of compact size,
cuses on its directivity and efficiency for great power reception circularly polarized characteristic, and second harmonic rejec-
and conversion, hence, large array [5] was usually adopted for tion property and the design procedure is discussed in next two
microwave power reception. Afterward, many functions were paragraphs. When integrating circuits with antenna, the trans-
added to enhance the performance of the rectenna array, such mission line feed would cause complicated layout schematic
as arbitrary polarization [6], dual-polarization [7], CP [8], dual for antenna array application. Besides, the nearby circuit would
band [9] and wide band [10] operation. Besides, for the antenna introduce coupling effects and affect antenna characteristics.
integrated with nonlinear circuits, such as diodes and FETs, it is Placing circuit on the back side of the antenna through coaxial
well known that harmonics of the fundamental frequency would feed can bring the benefits of low transmission line loss and less
occupied rectenna area to decrease the mentioned problems.
Manuscript received May 2, 2007; revised September 10, 2007. This work
The optimal antenna element parameters were obtained by
was supported by the Ministry of Education (Ex: HH94-A13) and Excellence using the Ansoft simulation software: High Frequency Struc-
University project of National Cheng-Kung University (NCKU), Taiwan, ture Simulator (HFSS) [12]. The circular patch antenna and rec-
R.O.C.
T.-C. Yo, C.-M. Lee, and C.-H. Luo are with the Department of Electrical
tifying circuit are designed at the center frequency of 2.45 GHz
Engineering, National Cheng-Kung University, Tainan, Taiwan, R.O.C. (e-mail: on low cost FR-4 substrate. The two layer structures are of the
robin@ee.ncku.edu.tw). same material with a thickness mm and mm,
C.-M. Hsu was with the Department of Electrical Engineering, National a dielectric constant and the conductor thickness of
Cheng-Kung University, Tainan, Taiwan, R.O.C. He is now with Lite-Med,
Inc., Taiwan, R.O.C. 36 m (equivalent of 1-oz copper). Both the antenna feed and
Digital Object Identifier 10.1109/TAP.2008.916956 rectifier input port have characteristic impedance of 50 for
0018-926X/$25.00 © 2008 IEEE
YO et al.: COMPACT CP RECTENNA WITH UNBALANCED CIRCULAR SLOTS 883

Fig. 1. Antenna and rectifying circuit configuration and the photographs of

proposed rectenna. Geometry parameters for the CP antenna are: W = 60; L =
60; R = 15:5; F = 6:5; d = 5:2; r = 5:2; d = 8:3; r = 2:3; h =
1:6; h = 0:8 (Dimension: mm).

good impedance matching to reduce signal reflection between

these components.
The unbalance slots are placed on the diameter line, which
has 45 counterclockwise rotation of the vertical diameter of cir-
cular patch, with different size and position relative to the center
of the circular disk. Initially, the radius of circular patch is
15.5 mm which has resonant frequency of 2.6 GHz without em-
bedding slots. To achieve the operation frequency range from
2.4–2.6 GHz, the slots are loaded at mm with
the radii mm initially and two resonant modes
with linear polarization are excited. The effects of slot size
and position as representative of slots’ size and position
are plotted in Fig. 2(a) and (b) in which the size and position
dominate the frequency of low and high resonant mode respec-
tively. The larger slots size decreases the frequency of low res-
onant mode and the longer distance of slots from patch center Fig. 2. Computed return loss versus frequency for the parameter of (a) r : 3.5,
increases the frequency of high resonant mode. By the analysis, 4, 4.5 mm and (b) d : 5.8, 7, 8.2 mm.
the parameters of the slots were designed accordingly to excite
orthogonal modes with quadrature out of phase as CP operation.
Besides, the unbalanced slots also introduce the shift of second
harmonic of fundamental band and will be discussed in the next
section.
The antenna shown in Fig. 1 provides left-handed CP.
Right-handed CP can be obtained by placing the slots mirror
to x-axis. Fig. 3 shows good agreement between the measured
and simulated return loss. The measured 10 dB return loss at
fundamental frequency of 2.45 GHz extended from 2397 to
2534 MHz ( MHz) shown in Fig. 3. It has measured
minimum axial ratio (AR) of 1.5 dB with 3 dB AR bandwidth
of 30 MHz as well as radiation pattern at 2455 MHz and plotted
in Fig. 4 and Fig. 5 separately. The CP antenna measured by
LP horn antenna in anechoic chamber has 2dBi LP gain at its
broadside corresponding to 3.36 dBic CP
gain. For perfect CP operation (i.e., dB), the slots’ size
and position can be tuned further for larger CP gain. Fig. 3. Simulation and measurement result of return loss from 2 to 3 GHz.
In addition to CP operation, the slots also increase the cur-
rent path of radiation patch which equivalent to smaller antenna
size at the same frequency. By introducing slots in the LP cir- is reduced from the calculated result of 16.5 mm from [13] to
cular microstrip disk antenna at 2.45 GHz, the antenna radius 15.5 mm of the proposed one, yielding 12% size reduction.
884 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 56, NO. 3, MARCH 2008

Fig. 4. Computed and measured axial ratio (AR). Fig. 6. Measured (with and without 3rd harmonic rejection stub) return loss of
the proposed CP antenna.

Fig. 7. Schematic and photograph of doubler rectifier with 3rd harmonic rejec-
tion radial stub.

is optimized by Ansoft Designer shown in dashed square of

Fig. 7 while its center rejection frequency is at 7.35 GHz (3rd
harmonic of 2.45 GHz) and has wide rejection band covering
3rd harmonic frequency range (7.2–7.45 GHz) of fundamental
band (2.4–2.483 GHz). After that, the input return loss of the
proposed antenna integrated with 3rd harmonic rejection stub
is verified in Fig. 6 (solid line). The measured second and third
harmonic return losses of the proposed CP antenna with radial
Fig. 5. Measured radiation pattern (at 2455 MHz) of proposed CP antenna stub are 0.92 dB and 0.58 dB, respectively. It is clearly shown
(a) x-z plane (b) y-z plane. that the 3rd order harmonic return loss is largely reduced from
dB to dB at 7.35 GHz in Fig. 6 (dash line to solid
line).
III. RECTIFIER DESIGN The proposed CP antenna with the integrated radial stub
A typical rectenna usually consists of a power receiving an- has 50 input impedance for easy measurement. Impedance
tenna, a low-pass filter or band-pass filter to suppress harmonic matching network for the rectifier also has been added for
signals, diodes for RF-to-DC conversion, a DC pass filter ca- better matching. The schematic and photograph of the adopted
pacitor, and a resistive load. The diode plays the key role in rectifier, which is voltage doubler for at least twice output
determining the RF-to-DC conversion efficiency. HSMS-282c DC voltage and half input impedance of the single diode
Schottky detector diode pair is adopted here for rectenna de- rectifier, are shown in Fig. 7. The double output voltage is
sign. The equivalent circuit of a single diode consists of created by storing charge at the series capacitor during
V are derived the negative phase of RF signal through the shunt diode while
from its datasheet [14]. Since the proposed antenna just pro- charge in is accumulated with the input potential during
vides second harmonic rejection property shown in Fig. 6 (dash the positive signal phase by turning on the series diode. The
line), a 3rd harmonic rejection radial stub is added between an- impedance matching network has one transmission line-Line1
tenna and rectifying circuit for efficiency improvement. The de- with mm and mm as well as 4.7 pF series
sign and layout of the 3rd harmonic rejection radial stub [15] connected capacitor-C between the 3rd harmonic rejection
YO et al.: COMPACT CP RECTENNA WITH UNBALANCED CIRCULAR SLOTS 885

Fig. 8. Measured output voltage and conversion efficiency of the proposed

rectenna as a function of load resistance at 20 dBm. Fig. 9. Measured output voltage and efficiency of the proposed rectenna as a
function of incident power density with 1 K load resistance.

stub and doubler rectifier for 50 characteristic impedance.

By the 2nd harmonics rejection CP antenna and 3rd harmonic power density is presented as the ratio of incident power on
rejection stub, the harmonics generated by the non-linear antenna and the equivalent antenna area [17]. From Fig. 8, load
diodes can be largely suppressed for efficiency improvement resistance chosen for the rectenna test as a function of power
and eliminating interference to other communication system. It density is 1 K . The measured output voltage and conversion
should be noted that high output dc voltage generated by the efficiency of the rectenna are plotted in Fig. 9. In the measured
doubler is able to decrease the diode’s junction capacitance to power density range, the optimum RF-to-DC conversion effi-
further improve the RF-to-DC conversion efficiency [16]. The ciency is 78% at 16.5 mW/cm power density as well as 15.3 V
unwanted RF signals leaking to load are also suppressed by the output DC potential with 1 K load resistance. Since the WPT
dc pass filter-C which is parallel connected with the load. technique may be applicable on wireless sensors near or on
human body, the corresponding ANSI/IEEE [18] RF human
IV. RESULT exposure limits for uncontrolled and controlled environment at
2.45 GHz are 1.63 mW/cm and 8.16 mW/cm , yielding good
In order to minimize the junction capacitance of diodes for ef-
RF-to-DC conversion efficiency, 53% and 75%, respectively.
ficiency improvement, the output voltage corresponding to the
From the measured result in Fig. 8 and Fig. 9, the rectenna
specified input power (20 dBm) is maximized. Therefore, the
using CP antenna with 2nd harmonic rejection property and 3rd
output load resistor is chosen to maximize the RF-to-DC con-
harmonic rejection radial stub has great performance in output
version efficiency of the adopted Schottky barrier diodes. Due
DC voltage and RF-to-DC conversion efficiency.
to the trade off between output DC voltage and RF-to-DC con-
version efficiency, the 1K resistor load is selected as the load
value for conversion efficiency optimization. Here, the output V. CONCLUSION
DC voltage across the resistor load was measured by voltmeter. A compact circular polarized rectenna with two unbalanced
By load resistance and output voltage, the corresponding output circular slots and 2nd order harmonic rejection property is pro-
DC power is obtained. The RF-to-DC conversion efficiency posed. The 2nd order harmonic rejection property originated
is defined as from the two unbalanced circular slots simplifies the design
of rectify circuit which only adds 3rd order harmonic rejec-
% % % (1) tion radial stub for efficiency improvement. The circular polar-
ized rectenna operated at 2.45 GHz with measured bandwidth
Where is the output DC power, is the power re- of 137 MHz and minimum 1.5 dB AR. The used doubler rec-
ceived by the rectifier, is output DC voltage and is the tifier provides optimum output DC voltage of 15.8 V as well
load resistance. In specific input power (20 dBm), the output as 78% RF-DC conversion efficiency at 16.5 mW/cm incident
DC voltage as a function of resistance is depicted in Fig. 8 power density and 1 K load resistance. Corresponding to the
showing that the larger the resistance, the higher the output DC RF exposure limit of ANSI/IEEE standard, the rectenna can also
voltage. By (1), the RF-to-DC conversion efficiency represented provide good efficiency of 53% and 75% in uncontrolled and
as a function of resistance is also plotted in Fig. 8, which indi- controlled environment respectively. In this study, the proposed
cates the optimum conversion efficiency 70.6% just located at rectenna can not only provide CP characteristic for power re-
1K . ceiving with low polarization loss but also have large output
For further investigation of rectenna characteristic corre- DC voltage that achieves great RF-to-DC conversion efficiency.
sponding to the transmitting and receiving antennas distance The circular slots structure and double layer configuration make
variation, the output DC voltage and RF-to-DC conversion the rectenna much small (12% of size reduction) and compact
efficiency is measured as the function of power density. The (2.4 mm in thickness).
886 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 56, NO. 3, MARCH 2008

ACKNOWLEDGMENT [18] IEEE Standards Coordinating Committee 28, Standard for Safety
Levels With Respect to Human Exposure to Radio Frequency Electro-
The authors gratefully acknowledge basic support from the magnetic Fields 3 kHz to 300 GHz. New York: IEEE, 1999.
Center for Micro/Nano Science and Technology and the Center
Tzong-Chee Yo received the B.S. and M.S. degrees
of Bioscience and Biotechnology, National Cheng-Kung Uni- in electrical engineering from National Cheng Kung
versity, Taiwan, R.O.C. University, Tainan, Taiwan, R.O.C., in 2001 and
2003, respectively, where he is currently working
toward the Ph.D. degree.
REFERENCES His research interests include RF circuits and
devices, microstrip antenna, rectenna, and wireless
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Antennas and Propag, vol. 43, no. 11, pp. 1331–1333, Nov. 1995. Hsinchu, Taiwan, R.O.C., the M.S. degree in elec-
[14] HSMS-282x Surface Mount RF Schottky Barrier Diodes, Data sheet, trical engineering from National Taiwan University,
Avago Technology, 2007. in 1982, the M.S. degree in biomedical engineering
[15] R. Sorrentino and L. Roselli, “A new simple and accurate formula for from The Johns Hopkins University, Baltimore,
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IEEE Trans. Microw. Theory Tech., vol. 40, no. 12, pp. 2359–2366, trical Engineering, National Cheng Kung University,
Dec. 1992. Tainan, Taiwan, R.O.C., since 1996 and was honored as a Distinguished
[17] Y. J. Ren and K. Chang, “New 5.8 GHz circularly polarized retrodi- Professor in 2005. His research interests include biomedical instrumenta-
rective rectenna arrays for wireless power transmission,” IEEE Trans. tion-on-a-chip, assistive tool implementation, cell modeling, signal processing,
Microw. Theory Tech., vol. 54, no. 7, pp. 2970–2976, Jul. 2006. home automata, RFIC, gene chip, and quality engineering.