A triple-band Microstrip Antenna for WLAN Applications

Kuang Fuqiang1 , Shen Dongya1,2, Xu Jie1，Shuai Xinfang1，Ren Wenping1

1School of Information Science and Engineering Yunnan University, Kunming, 650091, China 2State Key Lab of Millimeter Waves, Nanjing, 210096, China

Email: aichuan20@126.com shendy@ynu.edu.cn

Abstract—A novel compact triple-band monopole microstrip antenna for WLAN applications is proposed. The proposed antenna is made up of two one-quarter wavelength resonating components, fed by a 50ohmic microstrip line. The configuration is simple. The resonating frequencies of the proposed antenna are 2.5GHz and 4.8GHz, respectively, the bandwidth is about 500MHz (2.25~2.75GHz) and 2.63GHz (3.82~6.45GHz) (return loss less than -10dB), covering the triple bands of WLAN system. The lower and higher resonant frequencies can be easily tuned by adjusting a few parameters of the presented antenna. This antenna satisfies the requirements of wireless local area network(WLAN) 802.11(2.4~2.484GHz, 5.15~5.35GHz, 5.725~5.825GHz) applications. The softwares of Ansoft HFSS11 and CST Microwave Studio are used to simulate and optimize the antenna.

Keywords- triple-band; microstrip antenna; WLAN

I. INTRODUCTION The interest in researching and designing of multiband

and broadband antenna has increased dramatically in recent years, with the boost in modern wireless communication systems. Meanwhile, the design of multiband antenna, which covers 2.4GHz, 5.2GHz and 5.8GHz WLAN bands, becomes the main topic in the antenna field [1]. Technological progresses propose new requirements for antenna designs, such as low profile, wide impedance bandwidth, light weight, multiband and easy to manufacture, and so on [2]. Monopole microstrip antennas have those advantages and therefore get many applications [3].

Monopole microstrip antennas easily realize dual-band and multiband by constructing different resonant lengths of radiator. The proposed antenna satisfies the requirements of wireless local area network (WLAN) 802.11 (2.4~2.484GHz, 5.15~5.35GHz, 5.725~5.825GHz) applications, by using the methods of expanding the bandwidth [4].

II. DESIGN OF ANTENNA CONFIGURATION Fig. 1 shows the configuration of the triple-band

microstrip antenna. The proposed antenna is printed on FR4

substrate (36×25mm2), and the substrate thickness is 1.6mm, relative permittivity 4.4. The proposed antenna consists of two microstrip lines with different width and length. The longer microstrip line is made up of w1, w2 and w3, the total length 1 1 2 3 3.2L w w w mm= + + − , operating at lower resonate frequency. The shorter microstrip line consists of w4, w5 and w6, also, the total size 2 4 5 6L w w w= + + , operating at higher resonate frequency.

The optimized antenna sizes are shown in TableⅠ , units: mm. From the table, obviously, 1 30.7L m m= ,

2 11.8L mm= . The width of length w1 component and w6 component are 1.2mm and 1.5mm, respectively. The length of 1L can be determined approximately from one-quarter wavelength of the lower operating frequency (2.4GHz in this design). Also, to fine-tune the monopole’s second resonant frequency to around 5.2GHz, the length of 2L is chosen to increase from the section having the feed point to innermost section having the open end. A 50ohmic microstrip line is used to feed the antenna, in which the appropriate width w7= 2.3mm and length 20mm are fixed as shown in Fig.1.

Figure 1. Configuration of the proposed antenna. (a) Top view, (b) Side view.

2010 International Conference on Communications and Mobile Computing

978-0-7695-3989-8/10 $26.00 © 2010 IEEE

DOI 10.1109/CMC.2010.66

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TABLEⅠ. THE PARTIAL DIMENSIONS OF THE ANTENNA

Parameters W1 W2 W3 W4 W5 W6 W7

Unit(mm) 13 15 5.9 3.3 4 4.5 2.3

III. PARAMETER STUDY

This section presents the important parameters, such as w3, w6 and w7, which influence the operating frequency bands of 2.5GHz and 5.3GHz. Then, the parameter effects of the proposed antenna on impedance bandwidth are illustrated.

A. Effect of parameter w3 The antenna configuration parameters, as illustrated in

TableⅠ, have been chosen and the parameter w3 has been varied (w3=5.9mm, 4.9mm, 3.9mm, 2.9mm).The return losses of the designed antenna are shown in Fig.2. As the w3 decreases, the return loss of the first resonant frequency becomes worse and the resonant frequency increases. The first resonant frequency is affected largely between the frequencies of 2GHz and 3GHz. The second resonant frequency is varying slightly. In this case, the appropriate parameter w3=5.9mm is chosen.

Figure 2. Simulated return losses (S11) for various w3 with other

parameters unchanged.

B. Effect of parameter w6 In order to improve the bandwidth, the parameter

w3=5.9mm has been selected from the previous step, and the parameter w6 is varied to be 5.5mm, 4.5mm, 3.5mm and 2.5mm, with other parameters unchanged. The return losses

with variation of w6 are depicted in Fig.3. As the w3 decreases, the first and the second two resonant frequencies is shifted to the right and left, respectively. The parameter has largely affected the amplitude of S11 parameter at the second and third resonant frequencies. Therefore, considering the three resonant frequencies, the suitable parameter value of w6=4.5mm is preferred to cover the frequency range.

Figure 3. Simulated return losses for various w6 with other parameters

unchanged.

C. Effect of parameter w7 Additional parameter w7 is still investigated. The

significant effect on the impedance matching of the proposed antenna, as illustrated in Fig.4, can be clearly seen. The parameter w7 affects obviously the locations of the latter two resonant frequencies. As the parameter increases, the latter two resonant frequencies shift to the low frequency. Therefore, in order to cover the operating frequency band, the appropriate parameter w7=2.3mm should be chosen.

Figure 4. Simulated return losses for various w7 with other parameters

unchanged.

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IV. SIMULATED RESULTS AND ANALYSIS

A. Simulated E-field distribution of the antenna surface

Fig.5, Fig.6 and Fig.7 illustrate the E-field distribution of the proposed antenna surface at 2.4GHz and 5.2/5.8GHz. It is evident from the E-field distribution the resonant frequency of 2.4 GHz is contributed by the longer resonant element and those of 5.2/5.8GHz by the shorter element.

Figure 5. Simulated E-field distribution of the antenna at 2.4GHz.

Figure 6. Simulated E-field distribution of the antenna at 5.2GHz.

Figure 7. Simulated E-field distribution of the antenna at 5.8GHz.

B. Return Loss Fig.8 shows the simulated return losses of the

constructed prototype using commercial software HFSS and CST Microwave Studio. The results agree well with each other, so the results are reliable.

Figure 8. Simulated return losses of proposed antenna.

As shown in Fig.8, the second impedance bandwidth is

expanded obviously, covering the third band of 5.8GHz, which satisfy the normal operating bandwidth of triple bands WLAN system.

C. Radiation Pattern The simulated radiation patterns of the proposed antenna

in the XOZ plane and YOZ plane at 2.4GHz, 5.2GHz and 5.8GHz are depicted in Fig.9, Fig.10, and Fig.11 respectively. In general, broadside radiation patterns are clearly seen. The proposed antenna shows great radiation characteristic from the patterns.

Figure 9. Simulated radiation patterns at 2.4GHz.

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Figure 10. Simulated radiation patterns at 5.2GHz.

Figure 11. Simulated radiation patterns at 5.8GHz.

D. Voltage Standing Wave Ratio (VSWR) Fig.12 shows the simulated VSWR of the proposed

antenna. For the operating frequency range of the designed antenna, VSWR is less than 3, which satisfies the practical requirements of WLAN applications.

Figure 12. Simulated VSWR of the proposed antenna.

V. CONCLUSION

A microstrip-line-fed triple-band antenna is presented in this paper. It covers the WLAN 2.4/5.2/5.8GHz frequency ranges and has 500MHz and 3.3GHz of impedance bandwidth (return loss less than -10dB) for the lower and upper operating bands, respectively, or 20.8% and 63.5% referenced to 2.4GHz and 5.2GHz. Impedance matching can be obtained by adjusting the parameters w6 and w7 of the proposed antenna. The proposed antenna with excellent performance has a simple structure and low profile, making it promising for WLAN applications.

ACKNOWLEDGMENT This paper is supported by National Natural Science

Foundation of China (No. 60662004), National 863 Project (2007AA01Z2B4), State Key Lab of Millimeter Waves Foundation (No.K200814) and Key Project of International Scientific and Technical Cooperation of Yunnan Province (No. 2009AC010).

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