RFID antenna for reading tags

1. IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 5, MAY 2009 1275
A Universal UHF RFID Reader Antenna
Zhi Ning Chen, Fellow, IEEE, Xianming Qing, Member, IEEE, and Hang Leong Chung
Abstract—A broadband circularly polarized patch antenna is
proposed for universal ultra-high-frequency (UHF) RF identifica-
tion (RFID) applications. The antenna is composed of two corner-
truncated patches and a suspended microstrip line with open-cir-
cuited termination. The main patch is fed by four probes which
are sequentially connected to the suspended microstrip feed line.
The measurement shows that the antenna achieves a return loss of
15 dB, gain of 8.3 dBic, axial ratio (AR) of 3 dB, and 3-dB AR
beamwidth of 75 over the UHF band of 818–964 MHz or 16.4%.
Therefore, the proposed antenna is universal for UHF RFID appli-
cations worldwide at the UHF band of 840–960 MHz. In addition,
a parametric study is conducted to facilitate the design and opti-
mization processes for engineers.
Index Terms—Axial ratio (AR), broadband antenna, circularly
polarized (CP), RF identification (RFID), sequential feed, ultra
high frequency (UHF).
I. INTRODUCTION
RF IDENTIFICATION (RFID), which was developed
around World War II, is a technology that provides
wireless identification and tracking capability. In recent years,
RFID technology has been rapidly developed and applied to
many service industries, distribution logistics, manufacturing
companies, and goods flow systems [1], [2].
In an ultra-high-frequency (UHF) RFID system, the reader
emits signals through reader antennas. When an RFID tag com-
prising an antenna and an application-specific integrated circuit
(ASIC) is located in the reading zone of the reader antenna,
the tag is activated and interrogated for its content information
by the reader. The querying signal from the reader must have
enough power to activate the tag ASIC to perform data pro-
cessing, and transmit back a modulated string over a required
reading distance. Since the RFID tags are always arbitrarily ori-
ented in practical usage and the tag antennas are normally lin-
early polarized, circularly polarized (CP) reader antennas have
been used in UHF RFID systems for ensuring the reliability of
communications between readers and tags [3], [4].
Globally, each country has its own frequency alloca-
tion for UHF RFID applications, e.g., 840.5–844.5 and
920.5–924.5 MHz in China, 866–869 MHz in Europe,
902–928-MHz band in North and South of America, 866–869
and 920–925 MHz in Singapore, and 952–955 MHz in Japan,
and so on, so that the UHF RFID frequency ranges from 840.5
to 955 MHz (a fractional bandwidth of 12.75%) [5]. Therefore,
Manuscript received May 24, 2008; revised November 18, 2008. First pub-
lished March 27, 2009; current version published May 06, 2009.
The authors are with the Institute for Infocomm Research, Singapore 138632
(e-mail: chenzn@i2r.a-star.edu.sg; qingxm@i2r.a-star.edu.sg; changleo@dso.
org.sg).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TMTT.2009.2017290
a universal reader antenna with desired performance across the
entire UHF RFID band would be beneficial for RFID system
configuration and implementation, as well as cost reduction.
In this paper, we propose a sequentially fed stacked CP patch
antenna for UHF RFID applications. The antenna comprises two
suspended truncated patches and a suspended microstrip line.
The main patch is sequentially fed by four probes which are con-
nected to the microstrip line. A parasitic patch is positioned right
above the main patch for enhancing the bandwidth. The corners
of the patches are truncated to enhance the axial ratio (AR) per-
formance. The proposed antenna is designed to cover the UHF
RFID band of 840–960 MHz with acceptable performance in
terms of gain, AR, and impedance matching. Meanwhile, the
antenna configuration is simple and easy for fabrication.
The remainder of this paper is organized as follows. Section II
describes the geometry of the proposed antenna. The measured
results, analysis, and discussion are presented in Section III.
Section IV demonstrates the results of parametric study. The
validation of the proposed antenna in RFID system applications
is exhibited in Section V. Finally, a conclusion is drawn in Sec-
tion VI.
II. ANTENNA CONFIGURATION
CP antennas can be realized when two orthogonal modes
of equal amplitude are excited with a 90 phase difference
[6]. In general, the feeding structures of CP antennas can be
categorized into single and hybrid feeds. A single feed of
a CP antenna has the advantages of simple structure, easy
manufacture, and small size in arrays. However, the single-fed
single-patch CP antenna in its simple form has inherently
narrow AR and impedance bandwidths of 1%–2% [7]. To
improve the bandwidth, a variety of CP antennas have been
studied, wherein the bandwidth of AR, impedance matching,
and gain have been enhanced, e.g., by modifying the radiator
shape, designing feeding structures, and optimizing antenna
or array configurations [8]–[17]. Usually, a CP antenna with
the hybrid feed features a wide AR bandwidth, but suffers a
complicated structure, expansive manufacture, and increased
antenna size.
Fig. 1 shows the configuration of the proposed antenna. The
antenna comprises four layers of conductor, which include two
suspended radiating patches, a suspended microstrip feed line,
and a finite-size ground plane. Air substrate is used in this con-
figuration to achieve higher gain, broader bandwidth, and lower
cost. The microstrip feed line of a width of 24 mm is suspended
above the ground plane (250 mm 250 mm) at a height of
(5 mm). One end of the feed line is connected to an RF input,
while the other one is open circuited, which simplifies the an-
tenna structure. The main radiating patch of 156 mm 156 mm
and with a truncation of 24.5 mm at two diagonal corners
is placed above the feed line at spacing of mm. The
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2. 1276 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 5, MAY 2009
Fig. 1. Configuration of the proposed antenna. (a) Exploded view. (b) Side
view.
main patch is fed by four probes which are connected to the mi-
crostrip line. The probes are of diameter of mm, and
positioned along the microstrip feed line with adequate distance
to create the 90 phase lag between the probes and brings into
sequential rotation of current on the radiation patch for CP ra-
diation. To enhance the bandwidth, a truncated parasitic patch
with a dimension of 139 mm 139 mm and the truncation
of 17 mm is placed right above the main patch with the spacing
of mm. The truncated patches produce additional de-
generating modes necessary for widening the AR bandwidth.
With aid of simulation by Zeland IE3D, which is based on
the method of moments (MoM), the antenna is optimized and
then prototyped [18]. The prototype and detailed dimensions are
shown in Fig. 2. The truncated patches, feed line, and ground
plane are all made of copper and fixed using plastic spacers.
Four metallic screws are used as the probes to connect the mi-
crostrip feed line and the main patch. A coaxial cable is directly
connected to the microstrip feed line to simplify the assembly
of the antenna, where the coaxial cable is split into two wires
(screen and core) and the wires are soldered to the suspended
feed line and the ground plane separately.
III. RESULTS AND DISCUSSION
The antenna was measured in an anechoic chamber using the
Orbit MiDAS far-field measurement system and Agilent 8510C
vector network analyzer.
Fig. 3(a) shows the simulated and measured return loss of
the antenna. The measured return loss is less than 15 dB over
the frequency range of 760–963 MHz (25.6%). Fig. 3(b) ex-
hibits the simulated and measured AR at boresight. The mea-
sured 3-dB AR bandwidth of 818–964 MHz or 16.4% is ob-
tained. The simulated and measured boresight gain is illustrated
Fig. 2. Antenna prototype and detailed dimensions (h = 5 mm,
h = 20 mm, h = 10 mm, and L = 250 mm). (a) Photograph of
the antenna prototype. (b) Main patch. (c) Parasitic patch. (d) Microstrip feed
line.
in Fig. 3(c). The antenna exhibits the measured gain of more
than 8.3 dBic over the band of 815–970 MHz with a peak gain
of 9.3 dBic at 900 MHz. The measured and simulated return
loss, AR, and gain show good agreement.
Figs. 4 and 5 show the measured radiation patterns at 840,
910, and 955 MHz in the – and – planes, respectively.
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3. CHEN et al.: UNIVERSAL UHF RFID READER ANTENNA 1277
Fig. 3. Simulated and measured results of the proposed antenna. (a) Return
loss. (b) AR. (c) Gain.
In both planes, symmetrical patterns and wide-angle AR char-
acteristics have been observed. The beamwidth of 3-dB AR
is more than 75 , which is desirable for wide-coverage RFID
applications. The wider 3-dB AR beamwidth is accredited to
the sequential feed arrangement. The advantage stems from the
symmetry of the feeding structure, which cancelled out the un-
wanted cross polarization radiation. The 3-dB AR beamwidth
of the antenna prototype at selected frequencies are tabulated in
Table I.
In addition, the front-to-back ratio of the antenna is better than
15 dB in both the – and – planes at all measured frequen-
cies, although a finite-size ground plane is used.
IV. PARAMETRIC STUDIES
Parametric studies are conducted to provide more detailed in-
formation about the antenna design and optimization. The para-
metric study is carried out by simulation because good agree-
ment between the simulation and measurement has been ob-
served. The parameters under study include the truncation of
Fig. 4. Measured radiation patterns in the x–z plane at: (a) 840, (b) 910, and
(c) 955 MHz.
the patches, the height of the parasitic patch, the size of feeding
probes, the extension of the open-circuited microstrip line end,
and the size of the ground plane. Since the effects of some pa-
rameters, such as the size and height of the main patch and the
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4. 1278 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 5, MAY 2009
Fig. 5. Measured radiation patterns in the y–z plane at: (a) 840, (b) 910, and
(c) 955 MHz.
size of the parasitic patch, have been well known, the study of
these parameters is excluded in this paper. To better understand
the influence of the parameters on the performance of the an-
tenna, only one parameter at a time will be varied, while others
are kept unchanged unless especially indicated.
TABLE I
3-dB AXIAL RATIO BEAMWIDTH OF THE PROPOSED ANTENNA
Fig. 6. Effect of the truncation of the main patch 1L on the antenna perfor-
mance. (a) Return loss. (b) AR.
A. Truncation of the Main Patch
Fig. 6 shows the effect of on the return loss and AR of
the antenna. It is found that the truncation of the main patch
shows a significant effect on the AR of the antenna.
The nontruncated patch mm exhibits the widest
impedance bandwidth, but the narrowest AR bandwidth. The
increasing of improves the AR bandwidth and achieves
better impedance matching. However, over truncating (such as
mm) of the patch will degrade all the bandwidths.
The gain of the antenna is hardly affected by so that the
results are not exhibited. In practical design, the truncation can
be optimized for specific design requirement.
B. Truncation of the Parasitic Patch
Similar to , has a greater effect on the AR and
impedance bandwidths, while the gain of the antenna is hardly
affected. As illustrated in Fig. 7, when the parasitic patch be-
comes a square mm , the antenna features dramatic
AR bandwidth reduction. The impedance and AR bandwidths
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5. CHEN et al.: UNIVERSAL UHF RFID READER ANTENNA 1279
Fig. 7. Effect of the truncation of the parasitic patch 1L on the qantenna
performance. (a) Return loss. (b) AR.
change modestly if are kept within 10–20 mm and decline
when the patch is over truncated.
C. Height of the Parasitic Patch
Fig. 8 exhibits the effect of varying height of the para-
sitic patch on the performance of the antenna. It is observed that
the operating band is shifted down as the height increases. Fur-
thermore, the effect is more severe at higher frequencies. When
the parasitic patch is placed close to the main patch (such as
mm), a slight effect on the performance of the antenna
is observed. Increasing makes the antenna size larger, and
thus, shifts down the operating band.
D. Diameter of Feeding Probes
The study shows that the diameter of the feeding probe
has a slight effect on impedance matching, AR, and gain. How-
ever, the very thin probe causes poor impedance matching and
AR, as shown in Fig. 9. The long and thin feeding probes in-
troduce a large inductance to degrade the impedance matching.
Furthermore, the large inductance also disturbs the phase char-
acteristic at the feeding point, and thus, degrades the AR per-
formance. The feeding probes with a diameter of 2–3 mm are
recommended in practical design.
E. Extension of the Open-Circuited Strip
The open-circuited feed line configuration simplifies the an-
tenna implementation and reduces the fabrication cost. How-
ever, the open-circuited termination will cause reflection on the
feed line, and thus, affect the magnitudes and phase difference
of the feeding currents at the four probes. The effect of the exten-
sion of the open-circuited strip is illustrated in Fig. 10. A severe
Fig. 8. Effect of the height of the parasitic patch h on the antenna perfor-
mance. (a) Return loss. (b) AR. (c) Gain.
effect on the AR has been observed. Optimal AR is achieved
when the last probe is positioned at the edge of the strip line. In-
creasing greatly degrades the AR. When reaches 25 mm,
the AR is larger than 3 dB over the entire frequency band.
F. Size of the Ground Plane
The effect of the size of ground plane on the performance of
the antenna is exhibited in Fig. 11. As expected, the antenna
with the larger ground plane has superior performance over the
smaller ones. When the ground plane is smaller than 200 mm
200 mm, the performance of the antenna degrades in terms
of impedance, gain, and AR, especially at the lower frequen-
cies. For instance, the AR bandwidth is reduced to less than
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6. 1280 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 5, MAY 2009
Fig. 9. Effect of the probe diameter d on the antenna performance. (a) Return
loss. (b) AR.
Fig. 10. Effect of the extension of the open-circuited strip d on the antenna
performance. (a) Return loss. (b) AR.
5%. Increasing the ground plane size properly, for example,
up to 250 mm 250 mm, achieves better performance. Fur-
ther increasing the ground plane size only enhances the gain.
Fig. 11. Effect of the size of the ground plane L on the performance of the
antenna. (a) Return loss. (b) AR. (c) Gain.
The change of the ground plane size offers a simple way to im-
prove the antenna performance, but at the price of increasing
the overall antenna volume. Unfortunately, practical antenna de-
signs are always subject to certain size constraints.
V. RFID VALIDATION: READING-RANGE MEASUREMENT
To validate the superior features of the proposed antenna
in RFID reader applications, the reading-range measurement
was carried out using the proposed antenna incorporated into a
UHF RFID reader to detect a UHF RFID tag. The Omron 750
series reader and an in-house developed UHF tag were used;
the Omron 750 series reader can operate at different frequency
bands of 865.6–867.6, 902.75–927.75, and 952–954 MHz with
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7. CHEN et al.: UNIVERSAL UHF RFID READER ANTENNA 1281
TABLE II
READING RANGE OF THE ANTENNA (EIRP OF THE READER: 4 W)
4-W effective isotropic radiated power (EIRP). The reading
range indicates the maximum distance of the tag from the reader
antenna, where the tag can be detected properly by the reader.
The measurement was conducted in a full anechoic chamber at
boresight and 30 offset from the boresight of the antenna for
all the frequency bands. The results are tabulated in Table II,
the maximum reading range of 7.1–7.5 m has been achieved at
boresight and 6.1–6.5 m is achieved at the directions of 30
offset from the boresight. The reading range is comparable with
that achieved by reader with single band antennas.
VI. CONCLUSIONS
In this paper, a broadband sequentially fed CP stacked patch
antenna has been presented for universal UHF RFID appli-
cations. By using a simple feeding structure and combining
several band broadening techniques, the optimized antenna
has achieved the desired performance over the UHF band of
818–964 MHz or 16.4% with the gain of more than 8.3 dBic,
AR of less than 3 dB, return loss of less than 15 dB, and
3-dB AR beamwidth of larger than 75 . Therefore, this uni-
versal design can be applied to all the UHF RFID applications
worldwide. The reading-range measurement has validated that
the proposed antenna can be incorporated into the multiband
RFID readers or/and readers operating at different RFID bands
to achieve desired reading ranges. This feature will benefit
RFID system configuration and implementation, as well as cost
reduction.
Furthermore, the parametric studies have addressed the ef-
fects of the truncations of the patches, height of the parasitic
patch, size of the feeding probes, extension of the open-circuited
feed line, and size of the ground plane on the performance of the
antenna. The information derived from the study will be helpful
for antenna engineers to design and optimize the antennas for
UHF RFID applications.
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Zhi Ning Chen (M’99–SM’05–F’08) received the
B.Eng., M.Eng., and Ph.D. degrees in electrical
engineering from the Institute of Communications
Engineering (ICE), Nanjing, China, and the DoE
degree from the University of Tsukuba, Tsukuba,
Japan.
In 1988, he joined ICE as a Teaching an Assis-
tant, a Lecturer, and then an Associate Professor. He
subsequently joined Southeast University, Nanjing,
China, as a Postdoctoral Fellow and then an Asso-
ciate Professor. In 1995, he continued his research
with the City University of Hong Kong, China. From 1997 to 1999, he was
with the University of Tsukuba, Tsukuba, Japan, as a Research Fellow awarded
by Japan Society for the Promotion of Science (JSPS). In 2001 and 2004, he
visited the University of Tsukuba, again under Invitation Fellowship Program
(senior level) of the JSPS. In 2004, he conducted his research with the Thomas J.
Watson Research Center, International Business Machines Corporation (IBM),
Yorktown Heights, NY, as an Academic Visitor (Antenna Designer). In 1999,
he joined the Institute for Infocomm Research (I R) [formerly known as the
Centre for Wireless Communications (CWC) and Institute for Communications
Research (ICR)] as a Member of Technical Staff (MTS), and then the Principal
MTS. He is currently Principal Scientist and Department Head for RF and Op-
tical. He is concurrently an Adjunct Associate Professor with the National Uni-
versity of Singapore (NUS) and Nanyang Technologies University (NTU), Sin-
gapore, and an Adjunct/Guest Professor with Zhejiang University, Nanjing Uni-
versity, Shanghai Jiao Tong University, and Southeast University. Since 1990,
he has authored or coauthored over 220 technical papers published in interna-
tional journals and presented at international conferences. He holds two patents
with seven patent applications filed. He authored Broadband Planar Antennas
(Wiley, 2006), coedited UWB Communications (Wiley, 2006), and edited An-
tennas for Portable Devices (Wiley, 2007). He is the Editor for the “Field of
Microwaves, Antennas and Propagation” for International Journal on Wireless
and Optical Communications. He is an Associate Editor for Research Letters in
Communications and Journal of Electromagnetic Waves and Applications. He
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8. 1282 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 5, MAY 2009
also reviews papers for many prestigious journals and conferences. His main
research interests include applied electromagnetics, antenna theory, and design.
In particular, his research and development focuses on small and broadband an-
tennas and arrays for wireless systems, such as multiinput multioutput (MIMO)
systems and UWB systems, bio-implanted systems, and RF imaging systems.
Dr. Chen founded the IEEE International Workshop on Antenna Technology
(IEEE iWAT) and as general chair, organized the first IEEE iWAT: Small An-
tennas and Novel Metamaterials, 2005, Singapore. He chairs the iWAT Steering
Committee. He has been invited to deliver keynote addresses and talks at several
international events and serves many international conferences as key organizers
He currently serves New Technology Directions of the IEEE Antenna and Prop-
agation Society (IEEE AP-S) (2005–2010) as a member.
Xianming Qing (M’90) received the B.Eng. degree
from the University of Electronic Science and Tech-
nology of China (UESTC), Chengdu, China, in 1985.
From 1985 to 1996, he was with UESTC, where
he taught and performed research, became a Lecturer
in 1990, and then an Associate Professor in 1995.
In 1997, he joined the Physics Department, National
University of Singapore (NUS), Singapore, as a Re-
search Scientist, where he focused on development of
high-temperature superconductor (HTS) microwave
devices. Since 1998, he has been with the Institute
for Infocomm Research (formerly known as CWC and ICR), Singapore. He
is currently a Research Scientist with the RF and Optical Department. He has
authored or coauthored over 60 papers in international journals and confer-
ences. He has authored two book chapters. His current research interest includes
RFID reader/tag antennas, ultra-wideband (UWB) antennas, antenna measure-
ment technology, and antenna co-design.
Mr. Qing has been a member of the IEEE Antennas and Propagation Society
(IEEE AP-S) since 1990. He received seven Awards of Advancement of Sci-
ence and Technology in China. He was also the recipient of the IES Prestigious
Engineering Achievement Award 2006, Singapore.
Hang Leong Chung was born in Singapore, in 1979. He received the B.E.
degree in electrical engineering from the University of Queensland, Brisbane,
Qld., Australia, in 2005.
From 2005 to 2007, he was a Research Engineer with the Institute for Inf-
comm Research (I R), A*Star, Singapore. He is currently with DSO National
Laboratories, Singapore.
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