Content uploaded by Son Xuat Ta
Author content
All content in this area was uploaded by Son Xuat Ta on Apr 08, 2014
Content may be subject to copyright.
1464 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013
Circularly Polarized Crossed Dipole on an HIS
for 2.4/5.2/5.8-GHz WLAN Applications
Son Xuat Ta, Ikmo Park, and Richard W. Ziolkowski, Fellow, IEEE
Abstract—A triband circularly polarized (CP) crossed-dipole
antenna is introduced for 2.4/5.2/5.8-GHz wireless local area net-
work (WLAN) applications. It employs a single feed and only two
crossed trident-shaped dipoles as the primary radiating elements.
To achieve a compact radiator size, two techniques are utilized,
namely, insertion of a meander-line segment in the middle branch
of the tridents and termination of all trident arms with arrow-
head-shaped tips. The crossed trident-shaped dipoles are backed
by a high impedance surface (HIS) to achieve a broadband char-
acteristic and unidirectional radiation pattern at three bands. The
measured impedance bandwidths, based on the 10-dB reflection
coefficient values, are 2.21–2.62 GHz (410 MHz), 5.02–5.44 GHz
(420 MHz), and 5.62–5.96 GHz (340 MHz), and the measured
3-dB axial-ratio bandwidths are 2.34–2.58 GHz (240 MHz),
5.14–5.38 GHz (240 MHz), and 5.72–5.88 GHz (160 MHz). The
proposed antenna exhibits right-hand circular-polarized radiation
with high gain.
Index Terms—Circular polarization, crossed-dipole, high
impedance surface reflector, wireless local area network.
I. INTRODUCTION
OWING to the proliferation of wireless communica-
tion systems nowadays, people rely more and more
on their information search applications using mobile, hand-
held, and portable terminals. Thus, wireless local area net-
works (WLANs) have become the popular choice for Internet
access. WLAN uses a lower frequency band of 2.4–2.485 GHz
for the IEEE 802.11b/g standard and two upper frequency
bands of 5.15–5.35 and 5.725–5.875 GHz for the IEEE 802.11a
standard. The antennas for some applications in the WLAN
bands, such as Wi-Fi access points [1], gap fillers [2], and
RFID readers [3], require a unidirectional pattern to provide
high security and efficiency of the propagation channels.
Additionally, to mitigate the multipath problem due to the
reflections from building walls and ground surfaces, circularly
polarized (CP) antennas have been widely used in these WLAN
applications [4]. A single antenna for all of these WLAN ap-
plications would require stable triband CP operation covering
entirely the 2.4/5.2/5.8-GHz bands with broad impedance and
3-dB axial-ratio (AR) bandwidths, as well as similar radiation
Manuscript received August 02, 2013; revised September 02, 2013 and Oc-
tober 01, 2013; accepted October 24, 2013. Date of publication November 07,
2013; date of current version November 14, 2013.
S. X. Ta and I. Park are with the Department of Electrical and Com-
puter Engineering, Ajou University, Suwon 443-749, Korea (e-mail:
tasonxuat@ajou.ac.kr; ipark@ajou.ac.kr).
R. W. Ziolkowski is with the Department of Electrical and Computer
Engineering, University of Arizona, Tucson, AZ 85721 USA (e-mail:
ziolkowski@ece.arizona.edu).
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LAWP.2013.2288787
pattern characteristics. Several CP antenna types for WLAN
applications have been reported, including a dielectric res-
onator antenna [4], a microstrip patch antenna [5], and a slot
antenna [6]. However, these antennas were presented simply
for single- or dual-band operations.
On the other hand, in the past few years, various antennas
have been intensively engineered using electromagnetic meta-
materials to improve their performance characteristics. Linearly
polarized antennas have been incorporated with metamaterial
substrates to achieve impedance bandwidth enhancements and
unidirectional radiation patterns along with profile miniaturiza-
tion [7]. Furthermore, metamaterial structures have been widely
applied in CP antennas [8], [9]. However, the above-mentioned
CP antennas are not suitable for multiband communications
with large frequency ratios because of the nature of their
radiators and the bandwidths of the metamaterial structures.
This letter introduces a triband antenna for 2.4/5.2/5.8-GHz
WLAN applications. Two compact-sized trident-shaped dipole
elements are employed as the primary radiating elements for the
indicated three bands. They are fed by a vacant-quarter printed
ring that acts as a 90 phase delay to generate the desired CP
radiation [10]. The crossed dipoles are characterized first in free
space (without a reflector) and then in the presence of a high
impedance surface (HIS) reflector. The HIS reflector-backed
trident-shaped elements achieve broad impedance bandwidth,
good AR performance, and unidirectional radiation patterns
over all of the operating bands. Compared to the crossed dipoles
on the metallic cavity-backed reflector [10], [11], the presented
antenna yields a lower profile andanimprovement
in the 3-dB AR bandwidth at the lower band. The resulting
antenna system is characterized first with the ANSYS-Ansoft
High Frequency Structure Simulator (HFSS); its simulated
performance is then verified by measurements.
II. ANTENNA GEOMETRY AND DESIGN
Fig. 1 shows the geometry of the proposed antenna. It is com-
posed of two printed dipole elements, a coaxial line, and an HIS
reflector. The HIS reflector is constructed as a compact two-di-
mensional array of square patches printed periodically on a con-
ductor-backed substrate [7]. The RT/Duroid 6010 board mate-
rial was selected for the HIS substrate. It has a relative permit-
tivity of 10.2, a loss tangent of 0.0023, and a thickness of .
The size of the square patch in a unit cell is and the
overall size of the unit cell is . The printed dipole elements
are suspended at a height of above the HIS reflector. The ra-
diatingelementsareprintedonbothsidesofa -sized
sheet of RT/Duroid 5880 substrate, which has a relative permit-
tivity of 2.2, a loss tangent of 0.0009, and a thickness of .
Each trident-shaped arm of each dipole element is divided into
1536-1225 © 2013 IEEE
TA et al.: CP CROSSED DIPOLE ON HIS FOR 2.4/5.2/5.8-GHz WLAN APPLICATIONS 1465
Fig. 1. Geometry of the crossed trident-shaped dipole elements: (a) top view,
(b) side view with coaxial feed, and (c) a trident arm with its vacant-quarter
printed ring.
three branches with different lengths, which were specifically
designed to operate in the 2.4/5.2/5.8-GHz WLAN bands. The
center branch of each trident arm is designed to operate over
the 2.4-GHz band. It contains a compact meander line and has
an end that is shaped like an arrowhead to reduce its size [10].
The meander line was placed at a distance from the center
with trace width , gap size , and length . The two other
branches also are barbed at their ends; their sizes are adjusted
to operate separately in the 5.2- and 5.8-GHz bands. To gen-
erate the desired CP radiation, two of these trident-shaped el-
ements are crossed via a 90 phase delay line that consists of
a vacant-quarter printed ring, whose radius and width are
and , respectively. One pair is located on the top side of the
RT/Duroid 5880 sheet, the other on its bottom side. The com-
bined pairs are arranged to form two dipole radiators. The va-
cant-quarter printed ring has a length of approximately at
the lower band and at the relatively close upper bands
(being the guided wavelength at the center frequency). In
this manner, the requisite 90 phase difference is obtained for
each of the three different frequency bands.
A simple model based on simulating the scattering parame-
ters of a single-port air-filled waveguide with two PEC amd two
PMC walls was used for the HIS simulation [12]. The HIS thick-
ness was mm ( at 2.45 GHz). The size of
a unit cell was mm (at 2.45 GHz),
and each metal patch in the unit cell had mm. This
design yielded a frequency of 2.475 GHz for the 0 reflection
phase and frequencies in the range of 2.30–2.60 GHz for the 90
Fig. 2. Simulated (a) reflection coefficient, (b) AR, and (c) broadside gain of
the crossed trident-shaped dipole elements radiating in different configurations.
For the metallic and HIS reflector cases, the spacing from the bottom of the
radiating elements to the top of the reflectors was mm.
to reflection phase values, which completely covered the
2.4-GHz WLAN band.
The crossed trident-shaped dipole elements were first
optimized in free space for triband operation covering the
2.4/5.2/5.8-GHz WLAN bands with good CP radiation charac-
teristics. Referring to Fig. 1, the parameters were as follows:
mm, mm, mm, mm,
mm, mm, mm, mm,
mm, mm, mm, mm,
mm, mm, mm,
mm, mm, mm, and mm.
As shown in Fig. 2(a) and (b), the antenna in free space yielded
impedance matching bandwidths of 2.33–3.00, 5.07–5.51, and
5.68–6.00 GHz for the 10-dB reflection coefficient and 3-dB
AR bandwidths of 2.44–2.52, 5.20–5.31, and 5.75–5.85 GHz
with CP center frequencies of 2.47, 5.3, and 5.83 GHz, respec-
tively. The CP center frequency is defined here as the frequency
at which the AR has its minimum value.
1466 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013
In order to generate the desired unidirectional radiation
pattern, the crossed trident-shaped dipole elements were placed
first above a mm metallic reflecting surface. The
presence of the metallic surface significantly affected the
impedance matching and the CP radiation performance of
the antenna. From HFSS simulations for different air gaps
between the reflector and the radiator, it was found that
mm ( in the 5-GHz WLAN bands) exhibited
the best results in terms of the impedance and CP radiation
bandwidths, as well as the minimum AR for the two upper
bands (see Fig. 2(a) and (b)). Additionally, the combination
with mm yielded an dB in the lower op-
erating band. On the other hand, mm ( in
the 2.4-GHz WLAN band) yielded the widest impedance and
3-dB AR bandwidths in that lower band. Unfortunately, the
unidirectional patterns in both upper bands were degraded un-
acceptably. These results indicated that the optimized antenna
characteristics in terms of impedance matching and CP radia-
tion were achieved at each band when the air gap between the
metallic surface and the radiating elements was approximately
a quarter-wavelength at each center frequency. Therefore, it
was not possible to determine an appropriate value of for all
of the operating bands, particularly since the frequency ratio
between lower and upper ones is greater than two.
To circumvent this difficulty, the HIS structure was employed
as the reflector for the crossed trident-shaped dipole radiating el-
ements. For such a design, the HIS is required to exhibit a high
surface impedance over the 2.4-GHz band, but must operate as a
finite-sized metallic reflector over the 5.2- and 5.8-GHz bands.
The HIS and reflector parameters were judiciously selected to
achieve these characteristics. It was determined that suspending
the radiating elements above the HIS at mm offered
the best performance. Moreover, a 6 6 version of the HIS
structure with dimensions of mm was chosen
for the final design based on compromises between the overall
size and stable antenna performance. Because of the presence
of this HIS, the optimized design parameters of the crossed tri-
dent-shaped dipole elements had to be modified slightly from
its free-space design. The final design parameter values were as
follows: mm, mm, mm,
mm, mm, mm, mm,
mm, mm, mm, mm,
mm, mm, mm, mm,
mm, mm, mm,
mm, mm, mm, mm,
and mm.
Fig. 2 also provides comparisons of the performance of
the crossed trident-shaped dipole elements over the metallic
surface, over the HIS, and in free space (without any reflector).
Fig. 2(a) shows that the reflection coefficient changed only
slightly in the two upper bands for all cases, whereas the
metallic reflector case yielded narrower impedance band-
widths, particularly over the lower band, than the other two
cases. Fig. 2(b) shows that the AR performance remained
almost the same in all cases at the upper bands. On the other
hand, at the lower band, the CP radiation degraded significantly
in the metallic reflector case. In fact, it yielded a zero 3-dB AR
bandwidth. The HIS reflector exhibited a significant improve-
mentinthe3-dBARb
andwidth even when compared to the
Fig. 3. (a) Top view of the fabricated antenna. Comparisons of the simulation
and measurement results: (b) reflection coefficients and (c) AR values.
free-space case. It yielded a 2.36–2.60-GHz range for the 3-dB
AR. As shown in Fig. 2(c), without any reflector, the crossed
trident-shaped dipole elements radiate a quasi-bidirectional
electromagnetic wave and have a gain of only approximately
2 dBi in all of the bands. Fig. 2(c) also shows that the gain
is improved significantly with the presence of the metallic or
HIS reflectors. These two cases showed a gain higher than
6.0 and 7.5 dBi in the 2.4- and 5.2/5.8-GHz WLAN bands,
respectively. In addition, the proposed antenna has wider
beamwidths and consequently yielded slightly lower broadside
gain as compared to the metallic reflector case at the lower
band. These results indicate that the HIS reflector enhanced
the performance of the crossed trident-shaped dipole radiating
elements in terms of the resulting broadband impedance and
CP radiation characteristics and unidirectional gain patterns.
III. MEASUREMENTS RESULTS
The antenna system consisting of the crossed trident-shaped
dipole elements combined with the HIS reflector [Fig. 3(a)] was
constructed and tested. The printed dipoles on both sides of
the RT/Duroid 5880 sheet and on the top surface of the HIS
TA et al.: CP CROSSED DIPOLE ON HIS FOR 2.4/5.2/5.8-GHz WLAN APPLICATIONS 1467
Fig. 4. Comparisons of the simulated and measured gain patterns at (a) 2.45,
(b) 5.25, and (c) 5.80 GHz.
reflector RT/Duroid 6010 sheet were constructed via a stan-
dard wet etching technology. The comparison of the simulated
and measured reflection coefficients of the proposed antenna is
shown in Fig. 3(b). The measured impedance bandwidths for
the 10-dB reflection coefficient were 2.21–2.62, 5.02–5.44,
and 5.62–5.96 GHz, which agreed quite closely with the simu-
lated bandwidths of 2.28–2.69, 5.01–5.44, and 5.64–5.98 GHz,
respectively. Similar good agreement between the simulated
and measured ARs of the proposed antenna is demonstrated in
Fig. 3(c). The measured 3-dB AR bandwidths were 2.34–2.58,
5.14–5.38, and 5.72–5.88 GHz, whereas the simulated 3-dB AR
bandwidths were 2.36–2.60, 5.19–5.31, and 5.74–5.85 GHz.
The measurements yielded 2.49, 5.24, and 5.82 GHz for the
CP center frequencies in the three operating bands with ARs
of 1.84, 1.75, and 1.62 dB, respectively.
The gain patterns of the crossed trident-shaped dipole ele-
ments combined with the HIS reflector are shown in Fig. 4
at 2.45, 5.25, and 5.80 GHz. The radiated fields had a right-
hand circular polarization (RHCP) and a quite symmetrical pro-
file in both the -and -planes. At 2.45 GHz, the measure-
ments yielded a gain of 6.87 dBic and half-power beamwidths
(HPBWs)of83 and 77 in the -and -planes, respectively.
At 5.25 GHz, the measurements yielded a gain of 6.6 dBic and
HPBWs of 81 in both the -and -planes. At 5.80 GHz,
the measurements yielded a gain of 6.8 dBic, and HPBWs of
68 and 65 in the -and -planes, respectively. The dif-
ference between the measurement and the simulation (back-ra-
diation LHCP) is attributed to the effects of the plastic rack
and tape that were used to anchor the antenna during pattern
measurement. Additionally, the measured radiation efficiencies
were 90.2%, 85.7%, and 87.4% in comparison to the simulated
values of 95.2%, 90.3%, and 88.1% at 2.45, 5.25, and 5.8 GHz,
respectively.
IV. CONCLUSION
A triband CP antenna was introduced for 2.4/5.2/5.8-GHz
WLAN applications. Two trident-shaped dipole elements
were employed as the radiating elements. Meander lines and
arrowhead- and barbed-shaped tips were employed to achieve
a compact radiator size. To generate the CP radiation, the
dipoles were crossed via a vacant-quarter printed ring that
allows broadband characteristics. An HIS was designed as
the reflector of the proposed radiating dipole elements for
broad impedance and AR bandwidth performance and stable
unidirectional gain patterns in all of the operating bands. The
proposed HIS-reflector-based triband dipole antenna resulted
in measured 10-dB impedance bandwidths of 2.21–2.62 GHz
(410 MHz), 5.02–5.44 GHz (420 MHz), and 5.62–5.96 GHz
(340 MHz), as well as 3-dB AR bandwidths of 2.34–2.58 GHz
(240 MHz), 5.14–5.38 GHz (240 MHz), and 5.72–5.88 GHz
(160 MHz). Additionally, the measured radiation performance
characteristics of the antenna system included unidirectional
RHCP gain patterns, high radiation efficiencies, and a stable
operation over all three operating bands.
REFERENCES
[1] C.R.Medeiros,E.B.Lima,J.R.Costa,andC.A.Fernandes,“Wide-
band slot antenna for WLAN access point,” IEEE Antennas Wireless
Propag. Lett., vol. 9, pp. 79–82, 2010.
[2] B.Kim,J.Ryu,H.Choo,H.Lee,andI.Park,“DualISM-bandgap-
filler microstrip antenna with two Y-shaped sots for satellite internet
service,” Microw. Opt. Technol. Lett., vol. 52, no. 8, pp. 1825–1827,
Aug. 2010.
[3] A. T. Mobashsher, M. T. Islam, and N. Misran, “A novel high-gain
dual-band antenna for RFID reader applications,” IEEE Antennas Wire-
less Propag. Lett., vol. 9, pp. 653–656, 2010.
[4] Y.M.Pan,K.W.Leung,andK.Lu, “Omnidirectional linearly and
circularly polarized rectangular dielectric resonance antennas,” IEEE
Trans. Antennas Propag., vol. 60, no. 2, pp. 751–759, Feb. 2012.
[5] G. Yang, M. Ali, and R. Dougal, “A wideband circularly polarized mi-
crostrip patch antenna for 5–6-GHz wireless LAN applications,” Mi-
crow. Opt. Technol. Lett., vol. 45, no. 4, pp. 279–285, May 2005.
[6] Y. Sung, “Dual-band circularly polarized pentagonal slot antenna,”
IEEE Antennas Wireless Propag. Lett., vol. 10, pp. 259–261, 2011.
[7] H. Mosallaei and K. Sarabandi, “Antenna miniaturization and band-
width enhancement using a reactive impedance substrate,” IEEE Trans.
Antennas Propag., vol. 52, no. 9, pp. 2403–2414, Sep. 2004.
[8] Y. Dong, H. Toyao, and T. Itoh, “Compact circularly-polarized patch
antenna loaded with metamaterial structures,” IEEE Trans. Antennas
Propag., vol. 59, no. 11, pp. 4329–4333, Nov. 2011.
[9] K. Agarwal, Nasimuddin, and A. Alphones, “RIS based compact cir-
cularly polarized microstrip antennas,” IEEE Trans. Antennas Propag.,
vol. 61, no. 2, pp. 547–554, Feb. 2013.
[10] S.X.Ta,I.Park,andR.W.Ziolkowski,“Dual-bandwide-beamcrossed
asymmetric dipole antenna for GPS application,” Electron. Lett., vol.
48, no. 25, pp. 1580–1581, Dec. 2012.
[11] S. X. Ta, H. Choo, I. Park, and R. W. Ziolkowski, “Multi-band, wide-
beam, circularly polarized, cross, asymmetrically barbed dipole an-
tennas for GPS applications,” IEEE Trans. Antennas Propag., vol. 61,
no. 11, pp. 5771–5775, Nov. 2013.
[12] A.E.I.Lamminen,A.R.Vimpari,andJ.Saily,“UC-EBGonLTCCfor
60-GHz frequency band antenna applications,” IEEE Trans. Antennas
Propag., vol. 57, no. 10, pp. 2904–2912, Oct. 2009.
