PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012
806
Design of a Wide Dual-band Microstrip Antenna for WLANs
Applications
Abdulkareem S. Abdullah and Nabil E. Abdulhussein
Department of Electrical Engineering, College of Engineering
University of Basrah, Basrah, Iraq
Abstract— This paper presents a new design to obtain wide dual-band operation from a singlefeed microstrip antenna loaded with a chip resistor and two cross-slots in ground plane. The
lower band of the proposed antenna has a bandwidth of 12.5% around 2550 MHz, and the upper
band has a bandwidth of 6.3% around 5263 MHz and 6.41% around 5651 MHz. The obtained
bandwidths cover WLANs operations in both the 2.4 GHz (2400–2483.5 MHz) and 5.2 GHz (5150–
5350 MHz) or 5.6 GHz (5470–5825 MHz) bands. A three dimensional finite-difference time-domain
(3-D FDTD) method is employed to analyze the proposed structure and find its performance.
1. INTRODUCTION
Some applications such as satellite links, wireless local networks (WLAN), cellular telephones and
global positioning system (GPS) require wide-band dual-frequency antennas, and microstrip patch
antennas can be used to satisfy these requirements due to their low profile structure, light weight,
ease of integration with microwave integrated circuits (MICs), and capability of producing dual
frequency operations [1, 2].
Various methods have been used to achieve dual frequency operation such as excitation of
orthogonal modes [3], using slots in the patch [4, 5], loading the patch with shorting pins [6, 7], using
stacked patch [8, 9] and using planner antennas with special geometries to create several resonance
paths [10]. Using these methods achieve very narrow bandwidth, 0.5%–1.5% (SWR = 2), so it is
necessary to use additional techniques to improve it.
In this paper, a technique of adding a resistive load between patch and ground plane at suitable
position is used to achieve acceptable return losses at desired frequencies. A two cross slots in
ground plane is created to control the two resonant frequencies and for antenna size reduction.
A full-wave method of a three dimensional finite- difference time-domain (3-D FDTD) method is
employed to analyze the proposed structure and find its performance. A proposed antenna provides
two bands, the lower band covers ISM band (2400–2483.5 MHz) which is required by WLAN IEEE
802.11b,g standard, Bluetooth, and the higher band covers one of U-NII1 (5150–5350 MHz), UNII2 (5470–5725 MHz) and U-NII3/ISM (5725–5825 MHz) which is required by IEEE 802.11a and
HiperLAN2 standards [11].
In Section 2 the details of the design considerations of the proposed antenna are described. The
simulation results are presented in Section 3. Conclusion is presented in Section 4.
2. ANTENNA DESIGN
Figure 1 shows the structure of the proposed antenna. The patch has a width of 13 mm and length
of 10 mm which is approximately equal to the half-wavelength at 6 GHz. The patch is fabricated
on (23 × 20) mm2 FR4 substrate with a thickness of 1.6 mm, a relative permittivity of 4.4 and a
loss tangent of 0.02. A chip resistor is connected between the patch and ground plane at 1 mm
above patches center. A chip resistor presence causes the antenna to operate at two frequencies
with wide bandwidth. A two cross-slots in ground plane are existed to control the two resonant
frequencies and to reduce the antenna size. The vertical arm of the lower cross-slot has a length
of (L) mm and the vertical arm of the upper cross-slot has a length of (L − 2) mm. The horizontal
arms of upper and lower cross-slot have length of (d mm), a width of all arms are 1 mm.
A 50 Ω probe feed is used for feeding the antenna. The optimal feed position is found to be 3 mm
left of the chip resistor along x-axis to ensure acceptable values of return loss at the two bands.
3-D FDTD method is used for the simulation of the complete structure of the antenna including
the resistive load. The FDTD problem space is composed of cells with ∆x = 0.5 mm, ∆y = 0.5 mm
and ∆z = 0.533 mm which are selected to be smaller than (1/20) wavelength of maximum frequency
(6 GHz) in order to ensure the accuracy of the computed results [12, 13]. These cell sizes make
the volume of object to be (40∆x, 46∆y and 3∆z). The boundaries are terminated by 8 cells
conventional perfect match layers (CPML) and 10 cells air gap is left between the object in the
problem space and CPML boundaries.
Progress In Electromagnetics Research Symposium Proceedings, KL, MALAYSIA, March 27–30, 2012 807
(a) Patch side view
(b) Ground plane view
Figure 1: Proposed configuration for dual-band microstrip antenna.
Figure 2: Return losses of the proposed antenna for
different R values at L = 9 mm and d = 8 mm.
Figure 3: Return losses of the antenna for different
value of L at d = 8 mm.
3. RESULTS AND DISCUSSION
3.1. Effect of Chip Resistor R
The effects of chip resistor loading are studied first. In order to obtain minimum return losses at
the desired two frequency bands, the chip resistor is placed at 1 mm above the patches centre, and
its value is changed from 0 Ω (shorting case) to 2 Ω. For the case of using chip resistor of larger
value, the ohmic loss of the antenna quickly increases and the radiation efficiency decreases [14].
Fig. 2 shows the calculated return losses of the proposed antenna at different values of chip resistor
and the corresponding results are given in Table 1. It is noticeable that as the value of the chip
resistor increases from 0 to 2 Ω the bandwidth of the two bands increases. The positions of two
resonant frequencies are very slightly affected by variation of chip resistor value.
The optimal feed position need to be moved further from the chip resistor when the loading
resistance increases. It is seen that at the shorting case the distance between the feed position and
chip resistor is 1 mm, and at the 2 Ω case the distance is 3 mm.
3.2. Effect of Vertical Arms Length L
To find the effect of the vertical arms of the two cross-slots on the two operating frequencies, the
length of the two vertical arms (L) and (L−2) is varied at fixed length of horizontal arms of the two
cross-slots (d = 8 mm) and R = 2 Ω. Fig. 3 shows the calculated return losses for two frequencies
at different values of L.
Figure 4 shows the two resonant frequencies and the frequency ratio (fr2 /fr1 ) against L. The
corresponding calculated dual-frequency performance is listed in Table 2. The results clearly show
that, upon the increasing of the slot length L, the lower frequency fr1 is not affected by the variation
of L and it remains at 2550 MHz. On the other hand, the higher frequency fr2 is decreased and
PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012
808
Table 1: Performance of the antenna for different values of a chip resistor at L = 9 mm and d = 8 mm.
R (Ω)
0
1
1.5
2
fr1
MHz
2571
2554
2550
BW1
MHz, %
142, 5.52
224, 8.77
307, 12.04
fr2
MHz
5197
5231
5232
5263
BW2
MHz, %
127, 2.04
273, 5.22
272, 5.20
332, 6.30
Table 2: Performance of the proposed antenna for different values of L at R = 2 Ω and d = 8 mm.
L
mm
5
7
9
10
fr1
MHz
2550
2550
2550
2550
BW1
MHz, %
307, 12.04
307, 12.04
307, 12.04
307, 12.04
fr2
MHz
5991
5550
5263
5136
BW2
MHz, %
325, 5.45
290, 5.23
332, 6.30
340, 6.62
(a) Patch side view
Figure 4: Calculated resonant frequencies fr1 and
fr2 and frequency ratio (fr2 /fr1 ) against L.
(f2 /f1 )
2.35
2.18
2.06
2.01
(b) Ground plane view
Figure 5: A photograph of the proposed dual-band
microstrip antenna.
is lower than 6000 MHz. Also, from the results, it is found that the optimal feed positions for the
present design are almost about the same, which indicate that the feed position is not affected by
the variation of L.
From Table 2, at L = 9 mm, the lower mode of the proposed design has a 10 dB bandwidth of
307 MHz (2398–2705 MHz), and the upper mode has a bandwidth of 332 MHz (5097–5429 MHz).
The obtained bandwidths cover WLAN operations in two bands, the first band is ISM band (2400–
2483.5 MHz) which is required by WLAN IEEE 802.11b,g and Bluetooth standards, and the second
band is U-NII1 (5150–5350 MHz) band which is required by IEEE 802.11a and HiperLAN2 standards.
The photograph of the proposed design with L = 9 mm, d = 8 mm and R = 2 Ω is shown in
Fig. 5. The measured and simulated return losses are shown in Fig. 6.
Figure 6 shows the simulated bandwidth is 307 MHz at 2550 MHz while the measured bandwidth
is 366 MHz at 2590 MHz.
3.3. Effect of Horizontal Arms Length d
The horizontal arms length d is varied with fixed L (L = 8 mm) and R = 2 Ω. Fig. 7 shows the
calculated return losses for two frequencies at different values of d. The two resonant frequencies
(fr1 and fr2 ) and the frequency ratio (fr2 /fr1 ) against d variation are shown in Fig. 8. The
corresponding antenna performance listed in Table 3.
From the above result, the increasing in slot length d has same effect on the two resonant
frequencies and causes decreasing in both of them, so that the frequency ratio approximately
Progress In Electromagnetics Research Symposium Proceedings, KL, MALAYSIA, March 27–30, 2012 809
Figure 6: Measured and simulated return losses
for L = 9 mm, d = 8 mm and R = 2 Ω.
Figure 7: Calculated return losses for different values
of d at L = 8 mm.
Figure 8: Resonant frequencies fr1 and fr2 and
frequency ratio (fr2 /fr1 ) against d.
Figure 9: Measured and simulated return losses for
L = 8 mm, d = 2 mm and R = 2 Ω.
Table 3: Performance of the proposed antenna for different values of d at R = 2 Ω and L = 8 mm.
d
mm
2
4
8
12
fr1
MHz
2553
2543
2520
2431
BW1
MHz, %
319, 12.5
311, 12.23
294, 11.67
253, 10.41
fr2
MHz
5651
5573
5447
5311
BW2
MHz, %
362, 6.41
351, 6.30
307, 5.64
158, 2.97
(f2 /f1 )
2.21
2.19
2.16
2.18
remains constant compared with the case of varying L. It is clear that, the increasing in d causes
longer paths for two modes.
From Table 3, it is shown that, by set d = 2 mm the lower mode of the proposed design has a
bandwidth of 319 MHz (2394–2713 MHz), and the upper mode has a bandwidth of 362 MHz (5467–
5829 MHz). The obtained bandwidths cover WLAN operations in two bands, the first band is
ISM band (2400–2483.5 MHz) which is required by IEEE 802.11b,g and Bluetooth standards and
the second band is U-NII2 (5470–5725 MHz) and U-NII3/ISM (5725–5825 MHz) bands which is
required by IEEE 802.11a standard.
The proposed design with L = 8 mm, d = 2 mm and R = 2 Ω has been fabricated, and the
measured and simulated return losses of designed antenna are shown in Fig. 9.
It is shown that the simulated bandwidth is 319 MHz at 2553 MHz while the measured bandwidth
is 341 MHz at 2580 MHz.
4. FAR-FIELDS CALCULATIONS
With the FDTD technique, the direct evaluation of the far field calls for an excessively large computational domain, which is not practical. Instead, the far-zone electromagnetic fields are computed
PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012
810
↑y
900
↑y
9010
120
120
60
60
o
xy plane
xy plane
- 10
150
o
θ = 90
-10
θ = 90
0
-20
150
30
30
-30
- 20
↑φ
↑φ
x
0 ÿ→
180
210
330
240
210
330
Dθ
300
dB
x
0 ÿ→
180
240
Dφ
270
Dθ
300
dB
Dφ
270
(a) 2.45 GHz
(b) 5.2 GHz
(c) 5.6 GHz
Figure 10: Radiation patterns in the xy plane cut.
↑z
30
↑z
θ 0 10 θ
→
←
30
θ 0 10 θ
→
←
30
o
φ = 0
0
- 10
60
60
- 10
60
60
- 20
x
90 →
90
120
120
150
dB
o
φ = 0
xz plane
- 20
150
30
0
xz plane
180
x
90 →
90
120
Dθ
120
150
Dφ
dB
(a) 2.45 GHz
Dθ
150
Dφ
180
(b) 5.2 GHz
(c) 5.6 GHz
Figure 11: Radiation patterns in the xz plane cut.
θ
30
←
↑z
0 10 θ
→
↑z
30
30
o
φ = 90
0
- 10
30
yz plane
60
- 10
60
- 20
60
- 20
y
90 →
90
120
120
y
90 →
90
120
120
Dθ
Dθ
150
dB
o
φ = 90
0
yz plane
60
θ 0 10 θ
→
←
150
180
(a) 2.45 GHz
Dφ
150
dB
150
180
Dφ
(b) 5.2 GHz
(c) 5.6 GHz
Figure 12: Radiation patterns in the yz plane cut.
from the near-field FDTD data through a near-field to far-field transformation technique [15].
The directivity patterns are calculated in the xy, xz, and yz plane cuts at 2.45 GHz, 5.2 GHz,
and 5.6 GHz. These patterns are plotted in Fig. 10, Fig. 11, and Fig. 12, respectively.
5. CONCLUSION
A new microstrip patch antenna is proposed in this paper to obtain a wide-band dual-band operation. The proposed antenna is loaded by a chip resistor between patch and ground plane and two
cross-slots in the ground plane are created. The proposed structure is analyzed by the 3D-FDTD
method, and the chip resistor values and dimensions of the two cross-slots are varied to control the
two resonant frequencies and their bandwidths. The calculated results of the proposed antenna
Progress In Electromagnetics Research Symposium Proceedings, KL, MALAYSIA, March 27–30, 2012 811
agree well with experimental results. It is found that antenna can operate at two bands, the first
band covers ISM (2400–2483.5 MHz) band, and the second band covers U-NII1 (5150–5350 MHz)
band. The antenna also can cover U-NII2 (5470–5725 MHz) and U-NII3/ISM (5725–5825 MHz)
bands.
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