Compact Circularly Polarized Multiband Antennas for RFID Applications

Abstract

This paper presents multiband circularly polarized (CP) antennas for
radio frequency identification (RFID). A coax-fed and a microstrip-line-fed antennas having optimized cross-slots in their patches are first
designed for dual-band CP operation. The microstrip-line-fed design is then modified, by incorporating a U-shaped slot in its partial ground
plane, to achieve additional operation band with a CP characteristic. Simulation and measured results of the presented designs are reported.
The measured results are in accordance with the computed ones. The compact size and CP property make these designs suitable for RFID
applications.

Full text

Research Article
Compact Circularly Polarized Multiband Antennas for
RFID Applications
H. M. El Misilmani, M. Al-Husseini, K. Y. Kabalan, and A. El-Hajj
ECE Department, American University of Beirut, P.O. Box 11-0236, Beirut 1107 2020, Lebanon
Correspondence should be addressed to H. M. El Misilmani; hilal.elmisilmani@ieee.org
Received  January ; Accepted  March ; Published  April 
Academic Editor: Christoph F. Mecklenbr¨
auker
Copyright ©  H. M. El Misilmani et al. is is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
is paper presents multiband circularly polarized (CP) antennas for radio frequency identication (RFID). A coax-fed and a
microstrip-line-fed antennas having optimized cross-slots in their patches are rst designed for dual-band CP operation. e
microstrip-line-fed design is then modied, by incorporating a U-shaped slot in its partial ground plane, to achieve additional
operation band with a CP characteristic. Simulation and measured results of the presented designs are reported. e measured
results are in accordance with the computed ones. e compact size and CP property make these designs suitable for RFID
applications.
1. Introduction
Radio frequency identication (RFID) is dominating the new
technologies through its advantage of using wireless non-
contact radio communication,using radio frequency electro-
magnetic elds, to transfer data and information radio waves
from a tag attached to an object, for the purposes of automatic
identication and tracking []. RFID system consists of a
reader (or interrogator), a tag (or transponder), and antennas.
Designing antennas for RFID readers necessitates wideband,
recongurable band, or multiband performance with circular
polarization. e wideband, recongurable band, and the
multiband are used to make the system operable in dierent
applications and standards where circular polarization is
meant to overcome problems such as absorption, phasing
issues, multiphases, inclement weather, and line-of-sight path
[,]. UWB antennas have the advantage of covering a
very wide frequency range, but they are prone to noise
from unwanted frequencies, which could degrade the original
message []. On the other hand, recongurable antennas
aredesignedtobeabletocontroltheresonanceofthe
antenna and limit the disadvantage of UWB antennas. ese
recongurable antennas are complex as they require the
use of switching elements and their biasing lines or other
complicated reconguration mechanisms [].
e resonant frequencies of the proposed antenna lie in
the microwave frequency range. Karmakar et al. presented
in [] the development of a low-cost active RFID tag using
an annular ring-slot antenna working at . GHz. Cao et
al. presented a Minkowski fractal reader antenna composed
of three layers, designed for handheld RFID reader systems
working at . GHz in []. In [], Pitukwerakul et al.
presented a channel of wooden and metal book shelves mod-
eled for RFID library management system using microstrip
antennas as both transmitter and receiver antennas working
at . GHz. Popplewell et al. presented in []acompletely
integrated . GHz BFSK RFID transceiver suitable for RFID
and medical sensor applications. In [], iruvalar Selvan
and Raghavan presented a coplanar-waveguide-fed spiral
monopole antenna for RFID applications at . GHz. A small,
low-cost hairpin antenna-in-package concept for RFID appli-
cations working around  GHz is presented by Papatheologou
et al. in [].
Multiband circularly polarized (CP) antennas can be
thought of as an intermediate solution combining simplicity
and multifrequency operation. e advantage of the multi-
bandcircularlypolarizedantennasistobeabletointegrate
several frequency bands on one single antenna, making it
useful for several frequency ranges. ese multiband anten-
nas could contain frequency ranges from several wireless
Hindawi Publishing Corporation
International Journal of Antennas and Propagation
Volume 2014, Article ID 783602, 10 pages
http://dx.doi.org/10.1155/2014/783602

 International Journal of Antennas and Propagation
applications, and hence the antenna could be used for several
applications []. Another important feature in designing
these antennas, achievable with the use of microstrip anten-
nas, is to make them compact, lightweight, and low-cost
antennas [].
Due to the previously discussed advantages of multiband
antennas, several designs with dual-band operation and
circularpolarizationoverthetwobandshavebeenpre-
sented. Achieving dual-band CP operation is a challenging
task, especially when the ratio of the center frequencies
ofthetwobandsislarge,thatiswhenthetwobands
arespacedapartinthespectrum.ChenandYung[]
proposed a unidirectional dual-band circularly polarized
antenna working in .–. and .–. GHz bands, by
loading a pair of L-shaped stubs outside a truncated patch.
Heikkinen and Kivikoski []presentedadual-bandCP
microstrip-fed shorted ring-slot rectenna working at .
and . GHz where two shorted annular ring-slot antennas
are combined in order to achieve a dual-CP operation. A
dual-band circularly polarized microstrip antenna for China’s
Compass Navigation Satellite System is proposed by Bo et
al. [], with two pairs of T-shaped slots embedded close
to the edge of the circular patch for achieving dual-band
circularly polarized operation in two frequency bands –
 MHz and – MHz. Liao et al. []presented
a compact single-feed dual-band circularly polarized patch
antenna with small frequency ratio consisting of an unequal
cross-slot embedded in a circular patch and a narrow annu-
lar ring operating at . and .GHz. A dual-frequency
circularly polarized microstrip antenna is also proposed by
Wang []. e suggested antenna, operating at . GHz
and.GHz,hasastructureoftwomicrostripboardlayers,
employing two circular patches which have dierent radius
in order to achieve the dual-band resonance []. Nayeri et
al. [] presented a single-feed dual-band circularly polarized
microstrip antenna with a dual-stacked-patch conguration.
e dual-CP operation is achieved by using asymmetrical U-
slots on the patches and the antenna is operating at . and
. GHz. Qian and Tang [] presented a dual-band circularly
polarized multilayer microstrip antenna for . and . GHz.
Intheproposedantenna,astacked-patchcongurationis
used along with truncated corners and inserted slits of the
hexagonal patches in order to achieve the dual-CP operation.
For RFID applications, a dual-band circularly polarized
RFID antenna utilizing the feeding network with Wilkinson
power divider is proposed by Shin et al. []. e design con-
sists of two square radiating patches, an isolator for dual-band
performance, a bottom ground plane, and microstrip feeding
networks. e antenna is operating in the – MHz and
.–. GHz bands. Jung and Lee []presentedadual-
band circularly polarized aperture coupled microstrip RFID
antenna using a metamaterial (MTM) branch-line coupler.
e proposed antenna operates at  MHz and . GHz.
e radiating patch antenna is fed by the two output lines of
the designed dual-band MTM branch-line coupler through
two slots, and a cross-slot is implemented at the center of the
radiating patch to enhance isolation. On the ground plane
the slots are positioned in a T-shape in order to increase the
isolation.
R1
W
2
W
1
L1L2
Arm 2
Arm 1
Feed gap
W
s
Ls
F : Conguration and parametrized dimensions of the coax-
fed dual-band CP antenna.
Since the design goal is heading towards compact light-
weight antennas, and due to the advantages of multiband
antennas and circular polarization type previously discussed,
thegoalofthispaperistocombinethemultibandwithcir-
cular polarization and study the design of several multiband
circularly polarized antennas for RFIDs. Accordingly, this
paper presents novel multiband circularly polarized antennas
for radio frequency identication (RFID). e paper starts
with a procedural overview on obtaining a dual-band charac-
teristic along with dual-band circular polarization and move
to present a multiband circularly polarized antenna. e
antennas were simulated, fabricated, and tested. Similarity
between the measured and the simulated results is shown in
all cases.
2. Design, Simulations, and Results
2.1. Coax-Fed Dual-Band Circularly Polarized Antenna. e
rst antenna is based on a coax-fed circular patch with radius
𝑅1printedonanFRsubstratewithdimensionsof50 ×
50mm2, a thickness of .mm, and a dielectric constant 𝜖𝑟
of .. A cross-slot with two arms of lengths 𝐿1and 𝐿2
and widths 𝑊1and 𝑊2is incorporated on the patch. e
conguration of this design is shown in Figure .
e circular polarization operation is generated when
two degenerate orthogonal linearly polarized modes, of
equal amplitude and 90∘phase dierence, are independently
excited. e cross-slot adopted here is one method that can
lead to this operation, as a result of the reection of the signals
from the cross-arms with a 90∘phase shi, which results in
CP radiation. However, to guarantee the CP operation, the
armlengthsofthecross-slothavetobeoptimized,while
preserving the sought resonance frequencies. Studies have
shown that, to obtain CP, the lengths of the two arms should

International Journal of Antennas and Propagation 
Reection coecient (dB)
R1=13.5 R1=16.5
R1=14.5 R1=17.5
R1=15.5 R1= 18.5
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Frequency (GHz)
0
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−25
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F : Simulations for dierent values of circular patch radius.
notbeequal,orelsetheantennawilloperatewithalinear
polarization.
Comparing the objective of this paper and the antenna
introduced in [], the diculty in our case is to be able
to achieve resonance in the . GHz and a higher frequency
in the .–. GHz band (large frequency ratio), which will
require some heavy work on the parameters of the antenna,
where each change in one parameter will aect not only the
resonance frequency but the circular polarization as well. e
antenna presented in []hasequalarmwidthsandveryclose
arm lengths in order to achieve a small frequency ratio of ..
2.2. Parametric Study
2.2.1. Choosing the Appropriate Radius 𝑅1.e rst step in
ourdesignistochoosetheappropriateradiustostartwith.
Starting from a length of  mm for each arm of the cross-
slot with a width of  mm for each and a feed gap of mm,
varying the radius of the patch 𝑅1starting from . mm to
., the best radius to start with is that of . mm, giving
two resonance frequencies in two bands that could be used
for RFID, at . GHz and in the .–.GHz band, as shown
in Figure .
2.2.2. Choosing the Appropriate Feed Gap. Inthesecondstep,
the feed gap or the gap between the feeder and the center of
the circular patch is investigated. e values for the length
and width of the two arms are kept xed, that is,  mm and
mmrespectively.evalue of𝑅1is set to . mm, which is
obtained from the optimization in Section ... Simulating
the design while varying the feed gap value from . mm to
 mm, the resonance frequency is seen to be stable with little
variations. As a result, the feed gap value of choice is one that
resultsinsimilarlylowSvaluesforthetwofrequencybands.
is value is found to be  mm.
2.2.3. Studying the Eect of Arms Lengths. Aer choosing the
valueoftheradiusandfeedgap,itisnowfeasibletostudy
the eect of arms lengths. As stated above, the cross is one of
the methods that could be used to get circular polarization.
However the cross by itself cannot give circular polarization
unless some specications are taken into consideration. One
of these specications is the arm length. In order to have
circular polarization the lengths of the two arms of the cross
should not be equal. A simulation with equal arms lengths of
 mm resulted in axial ratio of >. dB over all the frequency
range and thus no circular polarization operation is achieved.
In addition, each length is responsible for the frequency
where a circular polarization is achieved, as will be seen in
the following sections. e CP will result from the dierent
lengths of the two perpendicular arms reecting the current
from the coaxial feed in -degree phase shi achieving
circular polarization operation.
2.3.HowtoGetDual-BandandCircularPolarization. e
goal now is to achieve two resonance frequencies and circular
polarization at . GHz. Following are the methods and steps
which will be used:
(i) vary the length of arm  (𝐿1),
(ii) vary the length of arm  (𝐿2),
(iii) vary the width of arm  (𝑊1),
(iv) vary the width of arm  (𝑊2).
2.3.1. Vary the Length of Arm 1 (𝐿1). Var y i n g t h e l e ng t h
of arm  (𝐿1) and keeping the other parameters constant,
the best value to get circular polarization at approximately
. GHz with resonance at . GHz and in the .–.GHz
band was noticed to be  mm. In order to achieve an
additional circular polarization operation in one frequency of
the .–. GHz band and enhance the circular polarization
operation achieved at . GHz, the above mentioned steps are
processed while keeping 𝐿1=15mm. e diculty in this
situation is that every parameter (𝐿1,𝐿2,𝑊1,and𝑊2)will
have major eect on the resonance and circular polarization
operation.
2.3.2. Vary the Length of Arm 2 (𝐿2). Keeping the widths of
the two arms equal to  mm and varying the length of arm
(𝐿2)whilekeepingtherstone(𝐿1) equal to  mm, the
best result is achieved at 𝐿2equal to . mm. e antenna
has three resonance frequencies, at ., .–., and . GHz
(RFID frequencies), for which two of them have CP, . and
. GHz.
2.3.3. Vary the Width of Arm 2 (𝑊2). In order to further
enhancetheCPresults,thetwowidthsofarms𝑊1and 𝑊2are
changed. Starting with varying the width of arm (𝑊2) while

 International Journal of Antennas and Propagation
Case 1
Case 2
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Frequency (GHz)
0
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Reection coecient (dB)
(a) Reection coecient S
Axial ratio (dB)
10
8
6
4
2
7654321
0
Frequency (GHz)
Case 1
Case 2
(b) Axial ratio
F : Coax-fed dual-band CP antenna S and axial ratio results.
T : e parameters of the three coax-fed dual-band antenna
cases.
Parameters Case  Case 
Size (mm) Size (mm)
Arm  length(𝐿1). .
Arm  length (𝐿2) 
Arm  width (𝑊1)  .
Arm  width (𝑊2) . .
keeping the width of arm  equal to  mm, the best results are
achieved with the parameters presented in Tab l e  (Case ),
for which the value of the lengths of arm  and arm  (𝐿1
and 𝐿2)hadtobechangedaswelltoaccommodateforthe
change in the width of arm  (𝑊2). e antenna has three
resonance frequencies, at ., .–., and .– GHz (RFID
frequencies), for which two of them have CP, at . and .–
. GHz.
2.3.4. Vary the Width of Arm 1 (𝑊1). Aer varying the
length of arm  (𝐿1), then the length of the second arm
(𝐿2),andthenthewidthofarm(𝑊2) and aer achieving
two designs working on dual-frequency bands with circular
polarization, in this step the width of arm  (𝑊1)isvaried.It
isimportanttonoteherethatchangingthevalueofthewidth
of the second arm will aect the total results, and thus some
enhancement in the other parameters should be done in order
to achieve the dual-band and circular polarization operation.
Focusing on the widths parameters, it is seen that a dual-band
characteristic with circular polarization could be achieved
by the use of . mm and . mm values for arm  and 
widths, respectively. Tab l e  (Case ) lists the parameters of
this second dual-band CP antenna design having dual-band
resonance with circular polarization at . and . GHz.
T : Dual-band antenna parameters.
Parameter Size (mm)
𝑅1.
𝑊𝑓
𝐿𝑓.
Arm  length (𝐿1).
Arm  length (𝐿2) .
Arm  width (𝑊1)
Arm  width (𝑊2)
e corresponding reection coecient S and axial
ratio results for the two nal dual-band antennas are shown in
Figures (a) and (b). Case  has resonance frequencies and
CP in the . and the .– GHz bands (both can be used
for RFIDs). Case  reveals that a small change in the cross-
slot widths leads to a variation in the circular polarization
operation in the higher band, where the new CP is achieved
at . GHz instead of . GHz.
2.4. Microstrip-Line-Fed Dual-Band Circularly Polarized
Antenna. e second antenna is based on a microstrip-line
feed and also has a cross-slot incorporated in a circular patch.
An initial design of this antenna is shown in Figure (a).
Simulating the new design using the parameters listed in
Table  resulted in poor S and axial ratio results compared
to those of the coax-fed dual-band antenna designed in
Section .. To obtain dual-band CP operation with this
design, the cross-slot dimensions have to be optimized but
this requires bulky work, or, as we did, the feed-patch
connection could be further enhanced. Using the feed line
in Figure (a) alargeportionofEMwavesisconnedinthe
end part of the feed, and this degrades the S and the axial
ratio results. To accommodate for this issue and allow larger

International Journal of Antennas and Propagation 
R1
W
2
W
1
L1L2
Arm 2
Arm 1
W
s
W
f
Ls
Lf
(a) Initial microstrip feed line
R1
W
2
W
1
L1L2
Arm 2
Arm 1
W
s
W
f
Ls
Lf
(b) Enhanced microstrip feed line
F : New dual-band design with microstrip feed line.
Reection coecient (dB)
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−30
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Frequency (GHz)
Dual-band case
Triple-band case
(a) Reection coecient
10
8
6
4
2
0
Axial ratio (dB)
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Frequency (GHz)
Dual-band case
Triple-band case
(b) Axial ratio
F : Dual- versus multiband microstrip-fed antennas S and axial ratio simulated results.
portion of EM to reach the cross, curved edges were added
to the end of feed line as shown in Figure (b).Inthisway,
more EM waves are now reaching the cross to be able to get
good resonance and circular polarization results. is smooth
transition between the microstrip feed and the patch helps to
improve the S parameter, and with the two arms of the slot
having dierent lengths, CP operation in two bands can be
obtained.
Simulating the enhanced antenna with the parameters
indicated in Tab l e  , two resonance frequencies have been
achieved, one in .–. GHz band and another in .–
. GHz both with circular polarization, as shown in
Figures (a) and (b).
2.5. Microstrip-Line-Fed Multiband Circularly Polarized
Antenna. e rst two antennas were dual-band CP. is
third design aims at having CP in a third band. is can
be done through defecting the ground plane by etching a
U-shaped slot, as the one used by the author in []. is slot
inthegroundisusedtoradiateinanadditionalfrequency
according to the its dimensions. Care should be taken to
properly design this slot to have resonance and CP at the
added resonant frequency. In this case, the slot dimensions
have been optimized to have resonance at . and . GHz.
Investigating the eect of the slot on the operation of the
antenna, the optimal performance was seen with the design
and parameters shown in Figure (a).efabricatedantenna

 International Journal of Antennas and Propagation
15.5
14.90
1.00
11.75
1.00
6.80
3.806.50
2.57
1.00
3.00
50.00
50.00
(a) Antenna parameters (b) Fabricated: top view (c) Fabricated: bottom view
F : Multiband antenna, (a) antenna parameters, and fabricated antenna: (b) top view and (c) bottom view.
Reection coecient (dB)
Simulated
Measured
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(a) Reection coecient
Simulated
Measured
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(b) Axial ratio
F:MultibandCPantennaSandaxialratiosimulatedversusmeasuredresults.
isshowninFigures(b) and (c).ecompareddual-and
multiband S and axial ratio simulated results are shown
in Figure . Clearly, an additional band has been added to
theoperatingbandsoftheantennaintherangeof.to
. GHz. Also, the measured and simulated S and axial
ratio results of the multiband antenna are shown in Figures
(a) and (b). e gain patterns, simulated and measured,
at each resonant frequency, are indicated in Figure .As
canbeseen,theantennahasdirectionalpatterns,with
gain of ., ., ., and . dB at ., ., .,
and . GHz, respectively. e antenna is RHCP, with the
current distribution at dierent resonant frequencies being
also shown in Figures and . e antenna eciency ranges
between .% and .%.
As can be seen, a good analogy is revealed between
thesimulatedandmeasuredresults,andanewmultiband
circularly polarized antenna is achieved, operating at .,
., ., and . GHz, making it suitable for multiband
circularly polarized RFID applications operating at these
frequencies. e . and . GHz resonant frequencies with
circular polarization have been accomplished through the
use of dierent arms lengths and widths of a cross-slot
embedded on a circular patch fed by a microstrip feed line.
e higher resonant frequencies, . and . GHz with

International Journal of Antennas and Propagation 
0−30
−60
−90
−120
−150
180
150
120
90
60
30
4−5.5−15
(a) . GHz, 𝜙=0
∘
0
−30
−60
−90
−120
−150
180
150
120
90
60
30
4−5.5−15
(b) . GHz, 𝜙=90
∘
0−30
−60
−90
−120
−150
180
150
120
90
60
30
4−5.5−15
(c) . GHz, 𝜙=0
∘
−15
0−30
−60
−90
−120
−150
180
150
120
90
60
30
4−5.5
(d) . GHz, 𝜙=90
∘
0−30
−60
−90
−120
−150
180
150
120
90
60
30
4−5.5−15
(e) . GHz, 𝜙=0
∘
0−30
−60
−90
−120
−150
180
150
120
90
60
30
4−5.5−15
(f) . GHz, 𝜙=90
∘
0−30
−60
−90
−120
−150
180
150
120
90
60
30
4
−5.5−15
Simulated results
Measured results
(g) . GHz, 𝜙=0
∘
0−30
−60
−90
−120
−150
180
150
120
90
60
30
4−5.5−15
Simulated results
Measured results
(h) . GHz, 𝜙=90
∘
F : Gain patterns at dierent resonant frequencies in both E- and H-planes.

 International Journal of Antennas and Propagation
(a) . GHz, phase = ∘(b) . GHz, phase = ∘
(c) . GHz, phase = ∘(d) . GHz, phase = ∘
(e) . GHz, phase = ∘(f ) . GHz, phase = ∘
(g) . GHz, phase = ∘(h) . GHz, phas e = ∘
F : Current distribution at . and . GHz resonant frequencies for dierent phases.
circular polarization, have been achieved through etching the
ground by U-slot shape.
3. Conclusion
In this paper, compact dual- and multiband circularly polar-
ized antennas for radio frequency identication (RFID)
were presented. A coax-fed dual-band CP antenna was rst
designed with the use of cross-slots etched in a circular
patch. By changing the coax feed into a microstrip-line feed,
a dual-band CP operation was obtained by smoothing and
optimizing the connection between the microstrip line and
the patch. e microstrip-line-fed design was then modied
to get a third operation band with a CP characteristic,
by incorporating a U-shaped slot in its partial ground
plane.

International Journal of Antennas and Propagation 
(a) . GHz, phase = ∘(b) . GHz, phase = ∘
(c) . GHz, phase = ∘(d) . GHz, phase = ∘
(e) . GHz, phase = ∘(f ) . GHz, phase = ∘
(g) . GHz, phase = ∘(h) . GHz, phase = ∘
F : Current distribution at . and . GHz resonant frequencies for dierent phases.
Conflict of Interests
e authors declare that there is no conict of interests
regarding the publication of this paper.
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