5924 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO.

12, DECEMBER 2012

Dual-Band Antenna With Compact Radiator for

2.4/5.2/5.8 GHz WLAN Applications
Xiao Lei Sun, Li Liu, S. W. Cheung, Senior Member, IEEE, and T. I. Yuk, Member, IEEE

Abstract—This paper presents a dual-band planar antenna with achieved was with the use of a FR4 substrate (with a relative
a compact radiator for 2.4/5.2/5.8-GHz wireless local area network permittivity ) and [3], which is still
(WLAN) applications. The antenna consists of an -shaped and larger than our proposed antenna of only .
-shaped radiating elements to generate two resonant modes for
dual-band operation. The -element fed directly by a 50- mi- There are also other designs of antennas for WLAN applica-
crostrip line is designed to generate a frequency band at around tions. For example, in [10], a split-ring was used as a monopole
5.5 GHz to cover the two higher bands of the WLAN system (using radiator to generate dual-band operation. The design achieved a
the IEEE 802.11a standard). The -element is coupled-fed through size of when fabricated on a substrate with a large
the -element and designed to generate a frequency band at 2.44 . An interesting design for WLAN application was
GHz to cover the lower band of the WLAN system (using the 802.11
b/g standards). As a result, the - and -elements together are reported in [11], where dual-band operation was achieved by
very compact with a total area of only . Parametric combining two loop antennas together. However, the element
study on the key dimensions is investigated using computer sim- was quite large because both loops required operating in the
ulation. For verification of simulation results, the antenna is fab- one-wavelength resonant modes. In [12], a slot was cut on the
ricated on a substrate and measured. The ground plane to generate the higher band for the WLAN system
effects of the feeding cable used in the measurement system and the
housing and liquid crystal display of wireless devices on the return and the ground plane itself resonated in the lower band. Thus,
loss, radiation pattern, gain and efficiency are also investigated by the antenna size, including the large ground plane, was quite
computer simulation and measurement. large about . In [13], a pair of symmetrical hor-
Index Terms—Cable effects, coupled-fed, dual band, -shaped izontal strips embedded in a slot on the ground plane was used
radiating element, -shaped radiating element, wireless local area to excite a dual-band resonance. The slot occupied a large area
network (WLAN). of about .
The size of the antenna for the WLAN system is mainly de-
termined by the lower band at 2.4 GHz and there are tech-
I. INTRODUCTION niques to reduce the size of the radiating element. Among many
techniques, inverted- structure is an effective one [14]–[17].

C URRENTLY, the wireless local area network (WLAN)

is one of the most popular networks for accessing
the internet. The WLAN uses a lower frequency band,
However, for multiband operation, the antenna needs to have
more radiating elements, other than the inverted- element.
Thus the design challenge is to incorporate the inverted- el-
2.4–2.484 GHz, for the 802.11b/g standards, and two higher ement with other radiating elements, yet maintaining the com-
frequency bands, 5.15–5.35 GHz and 5.725–5.825 GHz, for the pact size. To tackle this challenge, in [18], the antenna em-
802.11a standard. As the demand for smaller sizes of wireless ployed a direct-fed inverted- element for the 2.4-GHz band
devices increases, antennas designers are making tremendous and two other long slots on the ground plane to generate the
efforts in attempts to reduce the physical sizes of the antennas, 5.2/5.8-GHz and 3.5-GHz bands. The inverted- element had
yet covering all the three operation bands. A simple method a very compact area of , but the overall size of
used in WLAN antennas to cover the three bands is to use the antenna was much larger due to the slots on the ground
one monopole for the lower band and another monopole or a plane. In [19], a direct-fed planar inverted- antenna (PIFA)
branch structure for the two higher bands [1]–[9]. However, combined with a parasitic element was proposed for WLAN
due to the required length of monopole for resonating in the applications. The PIFA resonated in the fundamental mode at
lower band, this method leads to a relatively large antenna size. 2.4 GHz and the second-order mode at 5.2 GHz. The parasitic
Different techniques have been proposed to reduce the size of element with one end shorted to ground was used to generate the
the monopole responsible for the lower band. For example, in 5.8-GHz band. The radiator of the antenna had a compact size
[1]–[7], the monopoles responsible for the lower band were of . Although the height had
bent to different shapes for size reduction. The smallest size been reduced compared with other PIFA antennas, the antenna
was rather high profile and still occupied a larger volume than
Manuscript received December 22, 2011; revised June 14, 2012; accepted a planar antenna due to the PIFA structure.
July 10, 2012. Date of publication August 02, 2012; date of current version
In this paper, a planar dual-band antenna using a very com-
November 29, 2012.
The authors are with the Department of Electrical and Electronic Engi- pact radiator to cover all the 2.4/5.2/5.8 GHz
neering, The University of Hong Kong, Hong Kong (e-mail: xlsun@eee.hku.hk; WLAN operating bands is proposed. The radiator consists of
liuli@eee.hku.hk; swcheung@eee.hku.hk; tiyuk@eee.hku.hk).
an -shaped and -shaped elements resonating at around 5.5
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org. and 2.44 GHz, respectively. The -element is microstrip fed.
Digital Object Identifier 10.1109/TAP.2012.2211322 However, the -element is placed very close to the -element

0018-926X/$31.00 © 2012 IEEE

SUN et al.: DUAL-BAND ANTENNA WITH COMPACT RADIATOR FOR 2.4/5.2/5.8 GHZ WLAN APPLICATIONS 5925

TABLE I
OPTIMIZED DIMENSIONS OF RADIATOR (mm)

Fig. 2. Simulated S11 with only -element and both - and -elements.

Fig. 3. Simulated current distribution at (a) 2.44 GHz and (b) 5.5 GHz.

ground plane of and an overall dimensions of

. The microstrip-feed line has a width of
Fig. 1. (a) 3-D view of antenna. (b) Layout of radiator (A: feed point, G: via, 1.8 mm to achieve a characteristic impedance of 50 . (Note
prefixes and represent widths and lengths of elements). (c) prototyped that the length of the feed line depends on the space available
antenna.
on the particular wireless device where the antenna is installed.)
The geometry of the radiator is shown in Fig. 1(b) which con-
sists of two radiating elements. These elements look like the
and is coupled-fed through the -element. Since only one feed
letters and rotated by 90 and so are denoted here as an -
point is used for the two separate elements, the overall size is
and -elements, respectively. The prefixes and used to in-
very compact. The antenna is designed and studied using the EM
dicate the dimensions of Fig. 1(b) denote the widths and lengths,
simulation tool CST. For verification, the antenna is fabricated
respectively, in different parts of the elements. The -element
and measured using the antenna measurement system, Satimo
is direct-fed by the feed line at “A” marked on Fig. 1(b). It gen-
Starlab. The feeding cable used to connect the antenna to the
erates a wide frequency band at 5.5 GHz for the higher WLAN
measurement system affects the measurement results [20], re-
bands at 5.2 and 5.8 GHz. The -element, having a modified
sulting in some deviations from the simulation results. To study
inverted- structure, is coupled-fed from the -element via the
these effects, the feeding cable is also modeled and used in simu-
small gap and is shorted to ground using a via with diameter
lation. Then the simulation results agree very well with the mea-
of 0.3 mm marked as “G” on the shorting element of Fig. 1(b).
surement results. Moreover, the effects of housing and liquid
It generates a band at around 2.44 GHz for the lower band of
crystal display (LCD) of wireless devices are also investigated
the WLAN system. Thus the antenna has dual-band operation
in this paper.
to cover all the 2.4/5.2/5.8 GHz WLAN bands. Since only one
feed point is required for the two separate elements which are
II. ANTENNA DESIGN closely packed together, the radiator size is very compact. The
Fig. 1(a) shows the 3-D view of the proposed dual-band an- antenna is designed on a substrate with a relative permittivity
tenna which has a radiator with an area of ,a of 3.5 and a loss tangent of 0.02, and optimized using computer
5926 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 12, DECEMBER 2012

Fig. 5. Simulation model of antenna with EM suppressant tubing cable.

Fig. 6. Simulated and measured S11 of antenna.

generated at a lower frequency of 2.44 GHz and the resonance

at 7 GHz was shifted down to around 5.5 GHz.
The operation of the antenna was further studied using cur-
rent distributions at 2.44 and 5.5 GHz. At 2.44 GHz, the simu-
lated result in Fig. 3(a) shows that the current was mainly on the
-element which contributed to resonance. While at 5.5 GHz,
Fig. 3(b) shows that the current on the -element was quite
large, contributing to resonance.
Results of computer simulation showed that the antenna di-
mensions such as the coupling gap “ ” between the two radi-
ating elements, the in the -element and the and in
the -element were all quite sensitive to the resonant frequen-
cies, so a parametric study was carried out on these dimensions
using computer simulation. The S11 with , 0.5 and
0.8 mm is shown in Fig. 4(a). As the value of “ ” increased,
i.e. decreasing the coupling between the two radiating elements,
both the lower and higher bands moved up, with the higher band
Fig. 4. Simulated S11 with different values of (a) , (b) , (c) , and (d) . moving more significantly. With “ ” increased from 0.2 mm to
0.5 and 0.8 mm, the lower band moved from 2.39 GHz to 2.44
and 2.445 GHz, respectively, while the higher band moved sig-
simulation. The optimized dimensions are listed in Table I. The nificantly more from 5.23 GHz to 5.5 and 5.7 GHz. More im-
antenna is also fabricated as shown in Fig. 1(c) on a substrate portantly, matching for both lower and higher bands improved
with the same electrical parameters used in simulation. as was increased from 0.2 to 0.8 mm. Thus could be used to
produce good matching for the two frequency bands.
The simulated S11 of the antenna with different values of
III. PARAMETER STUDY
are shown in Fig. 4(b). Note that is the op-
To study the resonant frequencies of the two radiating ele- timized value listed in Table I for our design. Fig. 4(b) shows
ments, the antenna with only the -element present and with that had strong effects on the frequency of the higher band,
both elements present was studied using computer simulation. but very little effects on the lower band. With , 3.25,
The results on the reflection coefficient S11 are shown in Fig. 2. and 4.25 mm, the resonant frequencies for the higher band were
It can be seen that, if only the -element was present, the an- 5.78, 5.5, and 5.37 GHz, respectively, but the lower band at
tenna resonated at a single frequency of about 7 GHz. When the 2.44 GHz was not affected much (simply because the -ele-
- element was added to the antenna, an additional mode was ment was responsible for resonance at around 5.5 GHz). These
SUN et al.: DUAL-BAND ANTENNA WITH COMPACT RADIATOR FOR 2.4/5.2/5.8 GHZ WLAN APPLICATIONS 5927

Fig. 7. Simulated and measured radiation patterns in – and – planes at (a) 2.44, (b) 5.2, and (c) 5.8 GHz.

TABLE II
SIMULATED AND MEASURED EFFICIENCIES AND PEAK GAINS

the bandwidth of the higher band, which was about 1.6 GHz
(for ). Fig. 4(d) shows that had some ef-
fects only on the lower band. With , 2, and 3 mm,
the center frequencies for the lower band were 2.5, 2.44, and
2.36 GHz, respectively. For higher band, the resonant frequency
and bandwidth remained about the same. These results indicated
that and could be used to coarsely and finely, respectively,
tune the resonant frequency of the lower band, without affecting
much the higher band.
Based on these results obtained, the design methodology for
Fig. 8. Simulated and measured (a) efficiencies and (b) peak gains of antenna. the proposed dual-band antenna can be described as follows.
Step 1) For the -element, set , with
the guide wavelength at the high resonant frequency,
results indicated that could be used to independently tune and the ratio of similar to the current design.
the resonant frequency for the higher band. Step 2) For the -element, set ,
The results of parametric study on and of the -el- where is the guide wavelength at the low resonant
ement are shown in Fig. 4(c) and (d). Fig. 4(c) shows that frequency, with the ratios of
had quite strong effects on the resonant frequencies for both the similar to the current design.
lower and higher bands. With increased from 0.55 mm to Step 3) Set and other dimensions using
1.55 and 2.55 mm, the lower band moved from 2.67 GHz to 2.44 Table I.
and 2.26 GHz, and the higher band moved from 5.86 GHz to 5.5 Step 4) The settings in steps 1, 2 and 3 will not produce
and 5.45 GHz, respectively. However, did not affect much exactly the high and low resonant frequencies, so
5928 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 12, DECEMBER 2012

Fig. 9. Antenna with LCD model. (a) Front view. (b) Back view. (c) Side view.
Fig. 10. Simulated S11 with different value of “ ”.

use and to fine tune the low and high bands,

respectively.
Step 5) Optimize the matching using and the ratio
(yet keeping unchanged).

IV. SIMULATION AND MEASUREMENT RESULTS

The S11 and radiation pattern of the antenna shown in
Fig. 1(c) have been measured using the antenna measurement
system, Satimo Starlab. When an antenna is placed in the
Starlab for measurement, a coaxial feeding cable is always
needed to connect the antenna to the system. In measuring
small monopoles, at low frequencies where the electrical size Fig. 11. Simulated and measured S11 with .
of the antenna ground plane becomes relatively small compared
to the wavelength, currents will flow back from the antenna
to the feeding cable, giving rise to “secondary radiation” [20] the same figures. Fig. 7(a) shows that, at the lower frequency
and causing inaccuracy in radiated patterns measurement. This of 2.44 GHz, the simulation results without using the cable
also alter the current distribution on the antenna and hence the in general had the largest gains, for the reasons described
S11. To reduce the effects on the measured radiation pattern, previously. With the use of the measuring cable, the simula-
the feeding cable in the Starlab system is covered with EM tion results and measurement results had better agreements.
suppressant tubing to absorb the currents flowing back to the Moreover, the antenna had omnidirectional radiation patterns
cable and radiation from the cable. However, this method in both the – and – planes, which were similar to those
reduces the measured gain and efficiency of the antenna [21]. of invert- -like antenna. At the higher frequencies of 5.2 and
To study the cable effects on the measurement results, the an- 5.8 GHz, Fig. 7(b) and (c) show that both the simulation results
tenna together with the feeding cable as shown in Fig. 5 was with and without using the cable agreed well with the measure-
modeled in CST [21]. The cable had a length of 186.5 mm ment results. This was because at higher frequencies, the cable
and was covered with EM suppressant tubing having thickness effects were less significant due to the increased electrical size
of 1.25 mm, relative permittivity of 5, relative permeability of of the antenna ground plane. The antenna had omnidirectional
5, loss tangent of 0.004 and magnetic loss tangent of 0.3. A radiation patterns in the – plane and a dip at the -direction
metallic block with a size of was used in the – plane, which was similar to those of inverted- -like
to model the SMA connector. (Simulation showed that a cable antenna.
length of longer than 400 mm would produce the similar results, The simulated and measured efficiencies and peak gains of
so a length of 186.5 mm was used to reduce simulation time). the antenna are shown in Fig. 8, with the numerical results at
The simulated and measured S11 are show in Fig. 6. It can 2.44, 5.2, and 5.8 GHz listed in Table II. When the feeding
be seen that, at 5.5 GHz, the simulation result without using the cable was used in simulation, the simulated and measured re-
cable model had a wider bandwidth than that of the measure- sults agreed very well. Here the small discrepancies were mainly
ment result. With the use of the feeding cable, the simulation due to 1) the parameters used to model the EM suppressant
results and measurement results had a much better agreement. tubing were not exact or constant across the frequency band, and
The measured bandwidths for the lower band 2) the dimensions of the SMA connector and the cable length
was from 2.39 to 2.51 GHz and for the higher band was from 5 were not exact. Fig. 8(a) shows that the simulated efficiencies
to 6.1 GHz, with both bands satisfying the requirements of the without using the feeding cable were always highest and the
WLAN standards. discrepancies between the simulated and measured results were
The simulated radiation patterns of the antenna with and larger at lower frequencies, for the reasons of cable effects de-
without using the cable at 2.44, 5.2, and 5.8 GHz in the – scribed previously. Fig. 8(b) and Table II show that the mea-
and – planes are shown in Fig. 7. For comparison, the sured peak gain was smaller than the simulated gain without
corresponding measured radiation patterns are also shown in using the cable model at 2.44 and 5.2 GHz, but larger at 5.8 GHz.
SUN et al.: DUAL-BAND ANTENNA WITH COMPACT RADIATOR FOR 2.4/5.2/5.8 GHZ WLAN APPLICATIONS 5929

Fig. 12. Simulated and measured radiation patterns using LCD in – and – planes at (a) 2.44, (b) 5.2, and (c) 5.8 GHz.

Simulation on the 3-D radiation patterns was carried out to study connector attached to the antenna, the smallest gap size which
the reason for this. Results showed that at 5.8 GHz, the cable could be used for measurement was .
effects changed the direction of the main lobe on the radiation With , the simulated and measured S11 are
pattern and made it more directional with a higher gain. This ef- shown in Fig. 11. To evaluate the cable effects on the S11, the
fect could not be seen in Fig. 7(c) because the main lobe was not simulated S11 using the cable model is also shown in the same
in the – or – planes. Due to the page limit, the 3-D patterns figure. It can be seen that the simulated S11 using the cable
are not shown here. model had a better agreement with the measured S11. The mea-
sured bandwidth for the lower band remained about the same
V. EFFECT OF NEARBY CONDUCTOR AND DEVICE HOUSING from 2.39 to 2.5 GHz. For the higher band, the measured band-
Although the antenna has been designed using simulation and width was from 5 to 6.06 GHz, slightly narrower than that (in
measurement, when the antenna is installed in a wireless device, Fig. 6) without having the LCD. An additional resonance at
the performance will be affected by the nearby conductors and around 6.5 GHz was generated by the LCD.
the housing. Nowadays, wireless devices such as smart phones Using the LCD model, the simulated and measured radiation
often have a large liquid-crystal display (LCD) mounted on a patterns at 2.44 GHz are shown in Fig. 12(a). The antenna
metal plate. Under the LCD, it is a PCB with electronics com- still had quite omnidirectional radiation patterns in the –
ponents mounted on it. The phone antenna is often installed on and – planes. Without using the cable model and at 5.2
the metal plate and covered by housing. For installing a planar and 5.8 GHz, Fig. 12(b) and (c) show the antenna has slightly
monopole antenna, a small area of the metal plate is normally directional radiation patterns at the -direction. However, with
removed, as shown in Fig. 9, to avoid interference with the an- the cable model, the radiation patterns at these two frequencies
tenna. To study such condition for the proposed antenna, a per- in Fig. 12(b) and (c) were less directional in both the – and
fect electric conductor (PEC) with size of – planes. The simulation results using the cable model were
as shown in Fig. 9 was used to model the LCD in CST. There slightly closer to the measured results.
was a gap between the antenna and the LCD. (In practice, the With , Fig. 13 shows the simulated and measured
antenna ground plane could be connected directly to the LCD efficiencies and peak gains. The simulated results with using the
without have this gap, but then our antenna needed to be re- cable model agreed very well with the measured results. At the
designed because of the larger ground. For convenience, we frequencies of 2.44, 5.2, and 5.8 GHz, the measured efficien-
used the same antenna for studies.) Simulation and measure- cies were 66.8%, 83.6%, and 85%, respectively, with the corre-
ment were used to study the effects of the LCD on the S11, ra- sponding measured gains of 1.66, 4.75, and 5.1 dBi.
diation pattern, gain and efficiency. The effects of having housing on the antenna were also
With , 1, 2.5, and 5 mm between the LCD and the studied using the simulation model shown in Fig. 14. The
antenna, the simulated S11 is shown in Fig. 10. It can be seen housing had a thickness of 1 mm and was made of Acrylonitrile
that, with all the tested values for , the antenna could cover Butadiene Styrene with a relative permittivity of 2.3. Since the
the WLAN bands. However, due to the physical size of the SMA antenna was only affected by the material surrounding it, to
5930 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 12, DECEMBER 2012

Fig. 15. Simulated S11 with LCD and housing model.

WLAN system. In doing this, there was not much change in

the higher band.

VI. CONCLUSIONS

A dual-band monopole antenna with a very compact area

of only for 2.4/5.2/5.8-GHz WLAN applica-
tions has been designed and studied using computer simulation
Fig. 13. Simulated and measured (a) efficiencies and (b) peak gains of antenna
with . and measurement. The antenna radiator consists of an -shaped
and -shaped elements having resonances at about 2.44 and
5.5 GHz, respectively. The two frequency bands can be tuned
independently. The S11, radiation pattern, gain and efficiency
all have been studied.
The feeding cable used in the measurement equipment has
been modeled using computer simulation. The simulation re-
sults using the feeding cable agree very well with the measure-
ment results.
The antenna has also been studied using the simulation
models for the LCD and housing. Results have shown that the
antenna is a very promising candidate for practical WLAN
applications.

Fig. 14. Simulation model for antenna on LCD and surrounded by housing.
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[21] L. Liu, Y. F. Weng, S. W. Cheung, T. I. Yuk, and L. J. Foged, “Mod- tories. He has published over 150 technical papers in international journals
eling of cable for measurements of small monopole antennas,” pre- and conferences in these areas. His current research interests include antenna
sented at the Loughborough Antennas Propag. Conf. (LAPC 2011), designs, 2-, 3-, and 4G mobile communications systems, MIMO systems and
Loughborough, U.K., Nov. 14–15, 2011. satellite communications systems, predistortion of high power amplifiers and
e-learning.
Dr. Cheung has served the IEEE in Hong Kong for the past 20 years. In 2009
and 2010, he was the Chairman of the IEEE Hong Kong Joint Chapter on Cir-
cuits and Systems and Communications. Since 2011, he has been the Treasurer
of the IEEE Hong Kong Section and help organizing different international con-
ferences. He also has served as Reviewer for different international journals and
conferences in the areas of antennas and propagation and mobile communica-
tions.

Xiao Lei Sun received the B.S.E.E. degree from the

Huazhong University of Science and Technology,
Wuhan, China, in 2005, and the M.S. degree in T. I. Yuk received the B.S. degree from Iowa
microelectronics and solid-state electronics from the State University, Ames, IA, in 1978, and the M.S.
Institute of Microelectronics, Chinese Academy of and Ph.D. degrees from Arizona State University,
Sciences, Beijing, China, in 2008. He is currently Tuscan, in 1980 and 1986 respectively.
working toward the Ph.D. degree at the department Since 1986, he has been teaching at the University
of electrical and electronic engineering, The Univer- of Hong Kong, Hong Kong , China. His current re-
sity of Hong Kong, Hong Kong, China. search interests include wireless communications and
His research interests include high power amplifier antenna designs.
linearization, RF and microwave circuits, multiband
antennas, and frequency tunable antennas.