240
IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010
Novel CPW-Fed Planar Monopole Antenna
for WiMAX/WLAN Applications
Hsien-Wen Liu, Student Member, IEEE, Chia-Hao Ku, and Chang-Fa Yang, Member, IEEE
Abstract—A coplanar waveguide (CPW)-fed planar monopole
antenna with triple-band operation for WiMAX and WLAN applications is presented. The antenna, which occupies a small size of
25(L) 25(W) 0 8(H) mm3 , is simply composed of a pentagonal radiating patch with two bent slots. By carefully selecting the
positions and lengths of these slots, good dual stopband rejection
characteristic of the antenna can be obtained so that three operating bands covering 2.14–2.85, 3.29–4.08, and 5.02–6.09 GHz can
be achieved. The measured results also demonstrate that the proposed antenna has good omnidirectional radiation patterns with
appreciable gain across the operating bands and is thus suitable to
be integrated within the portable devices for WiMAX/WLAN applications.
Index Terms—Band-notched, monopole antenna, triple-band,
WiMAX, WLAN.
I. INTRODUCTION
R
ECENTLY, some practical requirements such as multiband operation and miniaturized size for current antenna
design in wireless communication systems have attracted high
attention. For short- and long-range applications, many antenna
designs suitable for wireless local area network (WLAN:
2.4–2.483, 5.15–5.35, and 5.725–5.85 GHz) and worldwide
interoperability for microwave access (WiMAX: 2.5–2.69,
3.3–3.8, and 5.25–5.85 GHz) operation have been studied
[1]–[8]. The printed planar wide-slot antennas [1], [2], based
on more than two resonant modes to generate a wider operating
band, were initially proposed to meet the applications for both
WiMAX and WLAN systems. However, in these designs, lower
performance of interference suppression was mainly due to the
absence of bandpass filter. To address this issue, several recent
antennas capable of reducing frequency collision and therefore
enhancing system performance have been developed with open
or symmetric slots for WiMAX and WLAN operation [3]–[8].
As for practical applications, all these designs have complicated
structure that results in more cost for antenna fabrication.
Manuscript received January 27, 2010; revised February 22, 2010; accepted
March 01, 2010. Date of publication March 08, 2010; date of current version
April 12, 2010. This work was supported in part by the National Science Council
of R.O.C. under Grant NSC 97-2221-E-011-024.
H.-W. Liu and C.-F. Yang are with the Department of Electrical Engineering,
National Taiwan University of Science and Technology, Taipei 106, Taiwan
(e-mail: D9407303@mail.ntust.edu.tw; cyang@mail.ntust.edu.tw).
C.-H. Ku is with the Department of Electrical Engineering, Ming Chi University of Technology, Taipei 24301, Taiwan (e-mail: kuchiahao@mail.mcut.edu.
tw).
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.2010.2044860
In this letter, we propose a planar triple-band monopole antenna that is simple and well suited for WiMAX and WLAN
operation. Two thin bent slots etched on the radiating patch are
designed equal to half-guided wavelength at the center-rejected
frequency so that dual band-notched feature may be obtained for
the antenna. Thus, the undesired frequencies can be restricted
in the stopband without affecting the radiation performance of
the antenna. Compared to those designs shown in [3]–[8], the
antenna that has a simple structure is easy to fabricate to be embedded inside the portable devices for achieving WiMAX and
WLAN operation. Note that the band-rejected property of the
antenna is also tunable. Details of the antenna design are then
presented, and a prototype of the proposed antenna has been
constructed and experimentally studied. Tuning effects of major
parameters on the stopband performance are also analyzed and
discussed with the simulations.
II. ANTENNA DESIGN
Fig. 1 exhibits the configuration of the proposed triple-band
monopole antenna. The overall dimensions of the antenna are
mm . This antenna, which is printed on a
only
0.8-mm-thick FR4 substrate with a relative permittivity of 4.4
and a loss tangent of 0.02, is realized using a pentagonal radiating patch with two thin bent slots. The radiating patch is fed
by a 50- coplanar waveguide (CPW) transmission line with
a width of 5 mm and a length of 10 mm. To possess a wider
impedance bandwidth, in this work a tapered structure between
,
the antenna and the CPW’s ground is with an angle of
so good impedance matching across the operating bands for the
antenna can be obtained. Different from the conventional pentagonal design [9], here the radiating patch is developed with
slanted form at both sides to have a proper spacing to mitigate
the effect caused by the circuit and casing. Compared to other
polygonal shapes or a fan shape, the pentagonal radiating patch
used in our design is more suitable to enhance the antenna performance for practical application. To reject the interference out
of bands, two thin bent slots have been printed on the radiating
patch as a half-guided wavelength resonator to generate two
notched bands. Note that the two center-rejected frequencies,
and , for the stopbands may be empirically approximated
by
1536-1225/$26.00 © 2010 IEEE
(1)
(2)
LIU et al.: NOVEL CPW-FED PLANAR MONOPOLE ANTENNA FOR WiMAX/WLAN APPLICATIONS
241
Fig. 2. Simulated and measured return loss of the proposed antenna.
Fig. 1. The proposed planar triple-band antenna. (a) Antenna geometry (L =
40 mm, L = 25 mm, L = 6 mm, L = 3 mm, L = 10 mm, W = 40 mm,
W = 25 mm, W = 16:5 mm, W = 20 mm, W = 16 mm, W = 5 mm,
and W = 0:8 mm). (b) Photograph of the antenna.
where is the speed of light in free space and is the relative
permittivity. According to these formulas, the total lengths for
upper and lower bent slots can be evaluated to be 32 and 22 mm,
which are corresponding to the center-rejected frequencies at 3
and 4.5 GHz, respectively. In our design, these two slots are
etched with a width of 0.2 mm to produce stronger resonance
that guarantees better band-rejected performance. An electromagnetic (EM) solver, Ansoft HFSS ver. 10.0, has been employed to analyze the electrical properties and radiation performance of the antenna. The optimal parameters for the antenna
mm,
mm,
mm,
are designed with
mm,
mm,
mm,
mm,
mm,
mm,
mm,
mm,
mm.
and
III. RESULTS AND DISCUSSIONS
A fabricated prototype for the proposed triple-band antenna
was constructed and implemented. The measured results were
performed by using a vector network analyzer (Agilent PNA
8362B). Fig. 2 describes the simulated and experimental return
loss against the frequency for the antenna, where fairly good
agreements between them have been achieved. Three distinct
operating bands with 10 dB return loss are about 2.14–2.85,
3.29–4.08, and 5.02–6.09 GHz, corresponding to 28%, 21%,
and 10%, respectively. Moreover, two measured stopbands can
Fig. 3. Simulated surface current distributions for the proposed antenna at (a) 3
and (b) 4.5 GHz.
be found about 2.85–3.29 and 4.08–5.02 GHz, which are very
close to the calculations.
To further study the dual stopband rejection property of the
antenna, here we also simulate the surface current distributions
for the center-rejected frequencies at about 3 and 4.5 GHz, respectively. As expected, the strong resonant current shown in
Fig. 3(a) flows along with the upper bent slot to generate the
first stopband. Results in Fig. 3(b) reveal that the second stopband is achieved using the shorter path of the lower bent slot.
Hence, some undesired effects such as frequency collision can
242
IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010
Fig. 4. Simulated return loss against various lengths of bent slots. (a) Upper
bent slot. (b) Lower bent slot.
be avoided well. Referring to these results, the antenna can satisfy not only the WLAN bands of 2.4/5.2/5.8 GHz, but also the
WiMAX bands of 2.6/3.5/5.5 GHz.
Fig. 4(a) and (b) simulate the band-rejected feature against
various lengths for upper and lower bent slots. As shown in
Fig. 4(a), the antenna uses the upper bent slot with different
lengths of about 30–34 mm to achieve a tunable center-rejected
frequency range from 2.65 to 3.35 GHz for the first stopband. In
addition, by selecting various lower bent slots from 20 to 24 mm,
the second stopband is with a changeable range of 3.8–5.4 GHz,
as described in Fig. 4(b). However, it should be noted that a
lower performance in the second stopband with a center-rejected
frequency of 5.4 GHz will occur, while a shorter bent slot is
adopted and near to the feeding line. To address this issue, better
rejection performance can be obtained by shifting the position
up in terms of the lower bent slot. It is also clear that these two
stopbands can be tuned and controlled independently. With the
help of the adjustable filter-like property, the proposed antenna
can be easily integrated within the portable devices.
Fig. 5 exhibits the measured far-field radiation patterns in
plane (E-plane) and
plane (H-plane) for the frequencies
at 2.5, 3.5, and 5.5 GHz, respectively. These results present that
fairly good omnidirectional patterns are achieved in the plane
plane are
over the operating bands, and the patterns in the
close to bidirectional. Fig. 6 shows the measured peak gains
versus the frequency of the antenna. As can be found, significant
gain reduction has been received at two designed stopbands,
Fig. 5. Measured radiation patterns in the xz and yz planes at 2.5, 3.5, and
5.5 GHz.
Fig. 6. Measured gains of the proposed antenna.
and stable gain variation across the three passbands also can be
achieved. For the operating band of 2.4–2.7 GHz, the antenna
gain varies from 1.77 to 2.15 dBi. Results in the medium band
of 3.3–3.8 GHz describe that the gain variation is from 1.72 to
2.47 dBi for the antenna. The measured gain in the highest operating band of 5.15–5.85 GHz is also stable, which varies from
LIU et al.: NOVEL CPW-FED PLANAR MONOPOLE ANTENNA FOR WiMAX/WLAN APPLICATIONS
2.52 to 4.13 dBi. Hence, the proposed compact antenna with
good band-rejected property and stable gain variation is well
suited to be embedded inside the portable devices for application in WLAN/WiMAX systems.
IV. CONCLUSION
A novel CPW-fed planar monopole antenna with triple-band
operation for WiMAX/WLAN applications has been presented.
Compared to conventional designs, the antenna has a simple
fabricated structure and merely uses two bent slots to generate
dual stopband. Simulated and experimental results demonstrate
that the stopbands can be adjusted flexibly and independently,
and large coverage with appreciable gain is also achieved across
the operating bands. Hence, the antenna with good band-rejected characteristic is capable of restricting frequency collision and undesired interference to enhance the radiation performance. For these reasons, the proposed compact antenna is
well suited to be integrated within various portable devices for
WiMAX/WLAN systems.
243
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