Miniaturized cpw fed planar monopole antenna for dual band wlan applications

Scientific Journal of Information Engineering June 2013, Volume 3, Issue 3, PP.46-49

Miniaturized CPW-fed Planar Monopole Antenna for Dual-band WLAN Applications Gaoli Ning Department 1, Systems Engineering Division of CALT, Beijing 100076, China Email: gln429@126.com

Abstract A compact and simple design of CPW-fed planar antenna with dual-band operation for wireless local area networks (WLAN) applications is presented. Using a modified fork-shaped radiation element, multiple impedance bandwidths covering 2.4/5.2/5.8 GHz WLAN bands are obtained. In addition, by meandering the middle strip resonating at the lower band, the antenna is significantly miniaturized. The antenna is successfully designed and measured, exhibiting broadband matched impedance, stable and fairly good gain and omni-directional radiation patterns. To the authors’ knowledge, its overall performance is among the state of the art.

Keywords: Monopole Antenna; Miniturized; Dual-band

1 INTRODUCTION Recent developments of modern wireless and mobile communications have evoked increasing demands for novel antennas with multi-band operation [1]. For instance, wireless local area networks (WLAN), a viable, cost-effective and high speed data connectivity solution, operate in 2.4-2.48 GHz, 5.15-5.35 GHz and 5.725-5.825 GHz according to the IEEE 802.11b/a/g/n standards. Various kinds of antennas suitable for WLAN applications have been developed. Among these antennas, the printed monopole antennas have particularly received much more interests than others owing to their potential in low profile, easy fabrication, good impedance matching, and dual- or multiband operation [2-6]. However, most of the recently presented dual- or multi-band antenna designs are either complex in structure [2] or large in antenna size [3,4] or have large cross-polarization and poor radiation pattern performance especially at high frequencies for practical applications [4-6].

In this article, a novel coplanar waveguide (CPW)-fed planar fork-like monopole antenna is proposed to achieve dual-band operation, covering the WLAN bands. By meandering the middle strip resonating at 2.4 GHz, the overall antenna is miniaturized. In addition, the proposed antenna is simple in structure and has excellent radiation performance which is omni-directional in the H-plane and close to bidirectional in the E plane within the bands of operation due to the regular distribution of the current on the radiation element. Details of the antenna design and experimental results are presented and discussed.

2 ANTENNA DESIGN The configuration and photograph of the proposed antenna are depicted in Fig.1 and Fig.2, respectively. The antenna has a simple structure by etching it on one side of an inexpensive FR-4 dielectric substrate with relative dielectric constant of 4.4, loss tangent of 0.02, and thickness of 1.6 mm, while the other side is without any metallization. The proposed antenna is composed of the feeder and the radiation element. The radiation element consists of three strips with different lengths, where the meander strip resonates at 2.4 GHz-band (because it has the longest math for the current distribution) and the straight ones operate at 5.2/5.8 GHz-band, respectively. Note that the radiation elements are close to each other so that the coupling exists and the “a quarter wavelength” law doesn’t apply here. The overall size of the antenna is only 27×16 mm2 thanks to the proper meander configuration while good electrical performance is still retained. The coplanar waveguide is adopted as the feeder, which has many advantages such as simple - 46 http://www.sjie.org/

structure of a single metallic layer, easy to fabricate, wide bandwidth, and easy integration with active devices. Note the gap formed between the ground plane and radiation element significantly affects the impedance performance and was optimized by simulation. The ground plane is bevelled, so a smooth transition of the resonant mode from one frequency to another results and good impedance match over a broad frequency range is obtained [7]. The design parameters of the proposed antenna are listed as follows: L = 27 mm, W = 16 mm, Lg = 8 mm, Wg = 2.85 mm, dg = 4.1 mm, W1 = 8 mm, W2 = 3 mm, L1 = 4.5 mm, L2 = 4.3 mm, L3 =16 mm, S = 1 mm, L4 = L5 = 6 mm, L6 = 2 mm, W3 = 5 mm, W4 = 6.7 mm, g = 0.5 mm, Wf = 3.7 mm.

FIG. 1 STUCTURE OF PROPOSED ANTENNA

FIG. 2 THE FABRACATED ANTENNA

3 EXPERIMENTAL RESULTS AND DISCUSSION By using the 3D full-wave electromagnetic simulation software Ansoft HFSS, the antenna was successfully designed and a prototype was fabricated and measured.

Simulated and measured return losses are shown in Fig.3 for comparison. As observed, the measured result agrees well with the simulated one, and the measured bandwidth (S11<-10 dB) ranges from 2.36 to 2.5 GHz in the lower band, while the upper band covers 1.2 GHz from 4.8 to 6 GHz. Obviously, the achieved bandwidths can cover the WLAN bands of 2.4/5.2/5.8 GHz according to the IEEE 802.11b/a/g/n standards. Meantime, it doesnâ&#x20AC;&#x2122;t cover over too much as to interfere with other communication bands. Fig.4 shows the performance for return loss resulting from using three different lengths of the branch W4 with other parameters fixed. It can be seen that the 2.4-GHz resonance frequency shifts from 2.5 GHz to 2.3 GHz as W4 increases. At the same time, variations in W4 have little effect on performance at 5.5-GHz resonance. Fig.5 presents the effects of changes in the length of the shorter strip L1. By increasing the length of L1, the central resonant frequency of the 5.5-GHz band shifts downward towards 5.0 GHz. Since the length of the current path increases by an increase in L1, the resonant frequency decreases.

FIG. 3 MEASURED AND SIMULATED FIG. 4 RETURN LOSS WITH DIFFERENT

FIG. 5 RETURN LOSS WITH

RETURN LOSSES OF PROPOSSED ANTENNA

DIFFERENT LENTHS OF L1

LENTHS OF W4

Fig.6 illustrates the far-field radiation patterns in H-plane and E-plane at 2.45/5.2/5.8 GHz, respectively. It is observed that the antenna possesses fairly good omni-directional radiation characteristic in the H-plane and the - 47 http://www.sjie.org/

patterns in the E plane are close to bidirectional. Note that the proposed antenna also has low cross-polarization levels in the two principal planes in both operating bands. All these good radiation characteristics are attributed to the quasi-symmetric geometry of the antenna and the regular distribution of the current on the radiation element. The peak gains over the two operational bands are plotted in Fig.7. For the 2.4-GHz band, the antenna gain level is 1.81.6 dBi with variation of 0.2 dBi while for the 5.5-GHz band, the antenna gain varies from 2.2 to 3.1 dBi. When a higher antenna gain is desired, one can use some other low-loss microwave substrates in place of the FR-4 substrate used here, and an increase of the antenna gain level of about 2-3 dBi is possible [8].

(a) E-PLANE AT 2.45 GHZ

(d) H-PLANE AT 5.2 GHZ

(b) H-PLANE AT 2.45 GHZ

(c) E-PLANE AT 5.2 GHZ

(e) E-PLANE AT 5.8 GHZ

(f) H-PLANE AT 5.8 GHZ

FIG. 6 RADIATION PATTERNS OF PROPOSED ANTENNA

FIG. 7 PEAK GAINS IN DUAL SEPRATE BANDS

4 CONCLUSIONS A novel compact CPW-fed printed monopole antenna, suitable for dual-band WLAN applications, has been presented, implemented and measured. In this design, by meandering the middle strip resonating at the lower band, the antenna is significantly miniaturized. With its simple structure and stable and decent radiation pattern performance, the proposed antenna emerges as an eminent competitor for multi-band applications.

ACKNOWLEDGMENT Thanks are given to the colleagues in the Systems Engineering Division of CALT for their sincere help.

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Dietrich, Barts R., Stutzman W., and Davis W. “Trends in antennas for wireless communications.” Microwave Journal. 46 (2003): 22-28

[2]

Lu, and Cai Y. “Planar Compact Multi-band C-shape Monopole Antenna with Inverted L-shape Parasitic Strip for WiMAX Applications.” Progress in Electromagnetics Research C. 20 (2011): 17-29

[3]

Xu, Yan Z. and Wang C. “Multi-band Modified Fork-shaped Monopole Antenna with Dual L-shaped Parasitic Plane.” Electronics Letters. 47 (2011):364-365

[4]

Thomas, and Sreenivasan M. “Compact Triple Band Antenna for WLAN/WiMAX Applications.” Electronics Letters. 45 (2009): 811-813

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Pan, Horng T., Chen W., and Huang C. “Dual Wideband Printed Monopole Antenna for WLAN/WiMAX Applications.” IEEE Antennas Wireless Propagation Letters. 6 (2007):149-151

[6]

Zhang, Jiao Y., Song Y., Zhang T., Ning M., and Zhang F. “A Modified CPW-fed Monopole Antenna with Very Small Ground for Multiband WLAN Applications.” Microwave and Optical Technology Letters. 20 (2010): 463-466

[7]

Chu, and Yang Y. “3.5/5.5 GHz Dual Band-notch Ultra-wideband Antenna.” Electronics Letters. 44 (2008):172-174

[8]

Pan, Wong K. “A Broadband Slot-loaded Trapezoid Microstrip Antenna.” Microwave and Optical Technology Letters. 14 (2000): 16-19

AUTHORS Lining Gao was born in Baoji, Shaanxi province, China, in 1987. He received his bachelor and master degree in electromagnetic field and microwave technology at Xidian University, Xi’an, China, in 2009 and 2012, respectively. His research mainly focuses on RF and microwave amplifiers, RF and microwave active circuits and systems, microstrip antennas.

He is now a wireless C&T engineer at Systems Engineering Division of CALT in Beijing.

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