I. INTRODUCTION
The rapid development of wireless communication technology has led to an increasing demand for dual-frequency antennas. Also, microstrip antennas have found extensive applications due to their light weight and small size. In recent years, numerous microstrip antennas with dual-frequency characteristics have been successfully designed for wireless applications [Reference Maci and Gentili1]. Usually, the dual-frequency operation is realized by the use of two resonators [Reference Hu, Li, Chen, Tan and Long2], two resonant modes of one patch [Reference Zhang, Zhang, Hong and Wu3–Reference Jan5], shorting pins [Reference Esfahlani, Tavakoli and Dehkhoda6], and reactive loading technique [Reference Sieber and Werner7]. However, the impedance bandwidth of the first band is usually narrow. Bandwidth enhancement can be obtained by using thick air or foam substrate [Reference Liu, Wang and Jin8], parasitic patch [Reference Sung9–Reference Anguera, Puente, Borja, Delbene and Soler11], reactive loading technique [Reference Peng, Chen, Hung, Shen, Chien and Tseng12, Reference Anguera, Puente and Borja13], photonic band-gap or electromagnetic band-gap structure [Reference AbuTarboush, Al-Raweshidy and Nilavalan14, Reference Hadarig, de Cos and Fernando15], matching network [Reference Liu, Zhong and Esselle16], suitable slots in the radiating patch or the ground plane [Reference Yang, Yang, Hong and Li17], and a reflecting board [Reference Han, Zhang, Chen, Han and Ma18].
In this paper, a compact dual-frequency antenna is proposed. The dual-frequency operation is achieved by embedding an open-ring slot in the radiating patch. Moreover, a method for enhancing the bandwidth of dual-frequency antenna is studied by using a partial ground. Simulated results show that the antenna with a partial arc-shaped ground can obtain a larger bandwidth for two bands than that with a partial rectangle ground. Experimental results show that the antenna with a partial ground can operate in 2.4 and 5 GHz bands, with good agreement between the simulated and measured results.
II. ANTENNA DESIGN
The geometry of the compact dual-frequency antenna is shown in Fig. 1. It consists of three layers, the top layer is the elliptical patch, the middle layer is the substrate, and the bottom layer is the ground plane. An open-ring slot is etched in the radiating patch to realize dual-frequency operation. The antenna is designed to operate at 2.4 and 5.2 GHz bands, and is simulated using Ansoft High Frequency Structure Simulator. The substrate is FR4 with a thickness of 1.6 mm, relative permittivity of 4.4 and loss tangent of 0.02. The optimized parameters are, a = 9.1, b = 7, l s = 37.6, w s = 0.3, and d = 1.2 mm, respectively. When the center of the ground plane is chosen as the origin of coordinates, the feeding point is at (−2.5 mm, 0). The proposed antenna has a compact size of 24 × 20 mm2 (0.38 λ 1 × 0.31 λ 1).
Geometry of antenna, (a) side view; (b) top view.
Figure 2 shows the simulated |S 11| with and without the open-ring slot in the patch. It can be seen that when a slot is etched in the patch, a new frequency (f 1) is excited. As a result, a dual-frequency antenna can be obtained. The numerical analysis of slot size is performed. Figure 3 shows the |S 11| for different slot sizes. It is obvious that as l s increases, f 1 decreases significantly while f 2 decreases slightly. Also, f 1 and f 2 are not sensitive to w s . In the case of l s = 37.6 mm and w s = 0.3 mm, the resonant frequencies of the antenna are 2.38 and 5.18 GHz, respectively. Further research indicates, f 1 is controlled by the dimensions of the patch and the length of the slot, and f 2 is determined by the dimensions of the patch, that is,
$${\lambda _1} = \pi \left[ {1.5(a + b) - \sqrt {ab}} \right] + 2d + {l_s},$$
$${\lambda _2} = \pi \left[ {1.5(a + b) - \sqrt {ab}} \right],$$
where λ i is the guided wavelength at f i . From Figs 2 and 3, we can see that the bandwidth at f 1 is narrow, and it cannot meet the requirements for wireless communications.
|S 11| with and without slot in the patch.
|S 11| for different slot sizes.
III. BANDWIDTH ENHANCEMENT
To enhance the bandwidth at f 1, the method of using a partial ground is adopted. We study the dual-frequency antenna with a partial rectangle ground and a partial arc-shaped ground. Figure 4 shows the bottom view of antenna.
Bottom view of antenna with partial ground, (a) rectangle ground; (b) arc-shaped ground.
For the antenna with a partial rectangle ground, the effect of the partial ground dimensions on the antenna performance is investigated. Figure 5 shows the |S 11| for different l g . It indicates that the slot in the ground can change the input impedance of antenna, and introduce two new resonant frequencies (f 1′ and f 2′). By properly adjusting the value of l g , two new frequencies close to f 1 and f 2 can be obtained, and that the impedance bandwidth of two bands can be improved. Also, it is clear that as l g increases, the impedance matching in the first band becomes better, while that in the second band becomes worse. In the case of l g = 14.3 mm, the antenna can obtain a better bandwidth of 10.8% (2.27–2.53 GHz) and 11.2% (5.08–5.68 GHz) in two bands, respectively.
|S 11| for different l g .
In the case of the partial arc-shaped ground, let x g denote the x-coordinate of the center of arc in the ground. The numerical analysis for different x g in case of r = 11 mm is performed, and the result is shown in Figure 6. It is clear that the new frequencies decrease with the decreasing of x g . In the case of x g = −11.9 mm (62% patch overlaps with the ground plane), the maximum bandwidth of 10% (2.29–2.53 GHz) and 13.8% (5.13–5.89 GHz) in two bands is obtained, respectively. It can cover the 2.4, 5.2, and 5.8 GHz bands for wireless local area network (WLAN). Therefore, the partial arc-shaped ground is adopted in this paper. Additionally, the radiation efficiency of the antenna is 65% at f 1 and 89% at f 2. Compared with that of antenna with whole ground, it is increased by 24 and 21%, respectively.
|S 11| for different x g .
In order to illustrate the mechanism of bandwidth improvement using partial ground, the simulated surface current distribution at the second band is shown in Fig. 7. It can be seen that, at f 2, strong current appears on the patch for both antennas with whole ground and partial ground. At f 2′, the current on the patch is weak for the antenna with whole ground, but it is strong for the antenna with partial ground. Also, it is obvious that the current path length of the antenna with whole ground is greater than that with partial ground, as shown in Fig. 7(b) and 7(d). That is to say, when a partial ground is employed, a higher resonance (f 2′) is excited and the bandwidth of the antenna is improved.
Surface current distribution at the second band, (a) at f 2 with whole ground; (b) at f 2 with partial ground; (c) at f 2′ with whole ground; (d) at f 2′ with partial ground.
Furthermore, the effects of arc slot dimension, feeding point position and nearby metal object on the performance of antenna are investigated. We found that the bandwidth of two bands increases as the dimension of arc slot in the ground increases. In order to cover the 2.4, 5.2, and 5.8 GHz bands for WLAN, the dimension of arc slot should be properly adjusted. Figure 8 shows the |S 11| for different x f , which denotes the x-coordinate of the feeding point. It is observed that, when x f increases, the impedance matching becomes poor. Also, when metal object is placed below the antenna, both the impedance matching and the radiation performance become poor.
|S 11| for different x f .
IV. EXPERIMENTAL RESULTS
The proposed antenna is fabricated on inexpensive FR4 substrate with the relative permittivity of 4.4 and thickness of 1.6 mm. Figure 9 shows the photograph of antenna. It is fed by a coaxial probe through a SubMiniature version A (SMA) connector. The external conductor of SMA is soldered together with the ground plane. The dimensions of the antenna are, a = 9.1, b = 7, l s = 37.6, w s = 0.3, d = 1.2, x g = −11.9, and r = 11 mm, respectively. The N5230A vector network analyzer and the 8092 antenna measurement system are used to measure the antenna.
Photograph of antenna, (a) top view; (b) bottom view.
The simulated and measured |S 11| of the antenna is shown in Fig. 10. Good agreement between the simulated and measured results is obtained. It can be seen that the antenna can operate in 2.4, 5.2, and 5.8 GHz bands. The measured bandwidths of two bands are 9.5% (2.31–2.54 GHz) and 13.6% (5.14–5.89 GHz), respectively. The difference between the simulated and measured results is mainly caused by the SMA connector and the fabrication tolerances.
Simulated and measured |S 11|.
Figure 11 shows the normalized measured radiation patterns at two bands. Here, the E- and H-plane are in the xz- and yz-plane, respectively. We can find that broadside radiation patterns in E- and H-planes are obtained. Also, the direction of maximum radiation at f 1 has about 10° of deviation from the normal direction due to the asymmetry of the ground. Figure 12 shows the measured gains in two bands. The peak gain in two bands is 0.43 and 4.15 dBi, respectively.
Measured radiation patterns, (a) at f 1; (b) at f 2.
Measured gains, (a) in f 1 band; (b) in f 2 band.
V. CONCLUSION
A compact dual-frequency antenna is proposed. Compared with the conventional half-wave antenna, the antenna has a compact size of 24 × 20 mm2, which equals to 0.38 λ 1 × 0.31 λ 1. Also, a bandwidth enhancement method of using a partial ground for dual-frequency antenna is studied. A dual-frequency antenna with a partial rectangle ground and a partial arc-shaped ground is investigated using electromagnetic simulations. Simulated results show the antenna with a partial arc-shaped ground provides a larger bandwidth for two bands than that with a partial rectangle ground. Experimental results indicate that the antenna with a partial arc-shaped ground can operate at 2.4 and 5 GHz bands for WLAN. The bandwidths of two bands are 9.5 and 13.6%, respectively. Also, good radiation performances have been achieved at both bands. The antenna is of low cost and suitable for wireless communication application.
ACKNOWLEDGEMENTS
This work was supported by the National Science Foundation of China (61172045, 61271160), the Natural Science Foundation of Shanxi province (2012011013-3), and the National Fundamental Fund of Personal Training (J1210036).
Liping Han received her B.S., M.S., and Ph.D. degrees in Electronic Engineering from Shanxi University, Taiyuan, China, in 1993, 2002, and 2011, respectively. Presently, she is an Associate Professor with the College of Physics and Electronics, Shanxi University. Her research interests include microwave and millimeter-wave integrated circuits and microstrip antenna.
Longfei Hao received his B.S. degree in Electronic Engineering from Shanxi University, Taiyuan, China in 2009. Presently, he is a graduate in Xidian University, Xi'an, China. His research interests include information retrieval and data mining.
Liyun Yan received her B.S. and M.S. degrees in Electronic Engineering department from the Beijing Institute of Technology, Beijing, China, in 1999 and 2006, respectively. She is presently working toward the Ph.D. degree with the College of Physics and Electronics Engineering, Shanxi University, Taiyuan, China. Her research interests include microstrip antennas and chipless RFID system.
Runbo Ma received his B.S., M.S., and Ph.D. degrees from Shanxi University, Taiyuan, China, in 1993, 2002, and 2011, respectively, all in Electronic Engineering. Presently, he is an Associate Professor with the College of Physics and Electronics, Shanxi University. His research interests include microwave and millimeter-wave integrated circuits, RFID, and microstrip antenna.
Wenmei Zhang received her B.S. and M.S. degrees in Electronic Engineering from Nanjing University of Science and Technology, Nanjing, China, in 1992 and 1995, respectively, and the Ph.D. degree in Electronic Engineering from Shanghai Jiao Tong University, Shanghai, China, in 2004. Presently, she is a Professor with the College of Physics and Electronics, Shanxi University, Taiyuan, China. Her research interests include microwave and millimeter-wave integrated circuits, EMC, and microstrip antenna.
