With the development of modern wireless communications, the ultra-wideband (UWB) systems have attracted much attention recently because of their advantages, including high speed data, small size, low cost and low complexity. The antenna of the ultra-wideband systems plays an important role in the receiving and transmitting of ultra-wideband radiation. Many types of UWB antennas have been presented for these applications, such as a spline-shaped antenna,1 diamond antennas,2,3 annular ring antenna,4 bow-tie antennas,5,6 triangular patch antennas7 and wide slot antennas.8,9 However, a narrow band used by WLAN operating at 5 to 6 GHz coexists within the required UWB bandwidth. A UWB antenna, with a notched band in the 5 to 6 GHz range is desired to reduce the potential interference. Recently, several UWB antennas with frequency band-rejection function have been proposed.10-12 Most of the proposed antennas have a complex structure and the notched bands are not tunable. In another design, a small square monopole antenna, with two inverted U-shaped slots in the radiation patch and an H-shaped slot in the ground plane,13 is realized with a dual notched band characteristic. However, the structure is complex and is difficult to design. A good design of the notched band characteristic with a stub14 is investigated, with an operating bandwidth range from 3.1 GHz to more than 10.6 GHz for a voltage standing-wave ratio (VSWR) less than 2, and a notch band at the expected frequency of 5.12 to 6.08 GHz for WLAN applications. The dimension of the antenna is large and has a non-tunable rejection band. In addition, the previously proposed antenna has always various slots cut in the radiation patch or the ground.

In this article, a CPW-fed ultra-wideband antenna with a tunable band-notch and good characteristics is proposed and investigated in detail. The antenna consists of a CPW ground with a wide slot, a fork-like radiation patch, a stub and the 50 Ω CPW-fed structure. By inserting a stub between the two branches of the fork-like radiation patch, a notched band, which is variable with frequency, is achieved. The proposed antenna has an impedance bandwidth of 111.4 percent range from 3.06 to 10.9 GHz with good impedance matching and an approximately constant gain is achieved. It also has a notch band frequency of 5 to 6 GHz for WLAN applications. The proposed antenna has been manufactured and measured. Numerical and experimental results for the frequency characteristics, current distributions, radiation patterns and gain of the proposed UWB antenna are also presented and discussed.

Figure 1a Geometry of the proposed antenna.

Figure 1b Photograph of the antenna.

Antenna Design

Figure 1 illustrates the geometry and the configuration of the proposed band notch antenna with a fork-like radiation patch and a tuning stub. The antenna is printed on a substrate with a relative permittivity of 2.65, a loss tangent of 0.002 and a thickness of 1.6 mm. The size of the antenna is 21×28 mm, and a 50 Ω CPW feeding structure is employed. It consists of a CPW ground with a wide slot, which has a length L1 = 15 mm and width W1 = 16.8 mm, a fork-like radiation element where the distance between the two branches of the fork is 8 mm, a stub with a length L2 = 7 mm and width W2 = 1 mm and a 50 Ω CPW feeding structure. The 50 Ω CPW feeding structure consists of a microstrip signal strip with a width W4 = 1.4 mm and the CPW ground, which has a gap s = 0.3 mm from the microstrip signal strip. All the dimensions have been obtained using Ansoft High Frequency Structure Simulator (HFSS) v.11.0 based on the finite element method (FEM).

Figure 2 VSWR vs. frequency as a function of L2.

Parameters Study

In the design, the distance between the CPW ground and the fork-like radiation element and the dimension of the stub play an important role in the notched band frequency and the impedance. Then, the length L2 and the width W2 of the stub and the distance between the CPW ground and the fork-like radiation element g are considered to optimize the proposed UWB antenna. Figure 2 shows the simulated results of VSWR vs. frequency with various values of L2. The length L2 of stub has an obvious effect on the center frequency of the notch band. By increasing the length of the stub, the center of the resonance frequency moves to a lower frequency. The bandwidth of the notched band is also broadened by increasing the length of the stub. This is due to the coupling between the fork-like radiation element and the stub and the resonance of the stub. In this design, the resonance frequency can be postulated from Equation 1:13

Figure 3 VSWR vs. frequency as a function of W2.

Where L is the total length of the stub, which here is described as L2, εre is the effective dielectric constant, and c is the speed of light. Taking Equation 1 into consideration, the length of the stub at the beginning of the design is determined and then later adjusted for the final design.

The effect of the width W2 of the stub is illustrated in Figure 3. With the width of the stub increasing, the center of the notched band frequency is moved slightly and the bandwidth of the notched band is almost invariable, while the impedance bandwidth of the antenna is increased, which is caused by the capacitive and inductive changes between the fork-like radiation patch and the stub.

Figure 4 VSWR vs. frequency as a function of g.

Figure 4 shows the effect of the distance g between the CPW ground and the fork-like radiation element. The impedance of the antenna is getting wider and the bandwidth of the notched band is broadened, while center of the notched band is changed slightly. This is due to the coupling effect and the capacitive and inductive changes between the CPW ground and the fork-like radiation element.

Figure 5 Current distribution in the proposed antenna.

The simulated current distribution of the proposed antenna without the tuning stub at 5.5 GHz and with the tuning stub at 3.5, 5.5 and 9 GHz are calculated and shown in Figures 5a, 5b, 5c and 5d, respectively. It can be seen from Figure 5a that the current flows mainly on the CPW-fed structure and the fork-like patch, while around the ground it is small. The current distribution at 3.5 and 9 GHz, shown in Figures 5b and d, is mainly along the wide slot in the CPW ground, CPW-fed structure and the fork-like radiation patch. On the contrary, in Figure 5c, the current distribution around the tuning stub and the CPW-fed structure is obtained. Therefore, the surface current can excite the notch band frequency.

Figure 6 VSWR vs. frequency for the proposed antenna.

Results and Discussions

In order to estimate the design antenna, the proposed antenna is optimized. The optimized parameters are as follows: L = 28 mm, W = 21 mm, L1 = 15 mm, W1 = 16.8 mm, L2 = 7 mm, W2 = 1 mm, W3 = 1.4 mm, m = 10.8 mm, g = 1.2 mm, W4 = 1.4 mm, s = 0.3 mm, h = 1.6 mm and the distance between the two branches of the fork is 8 mm. The proposed antenna has been manufactured and tested, using the above parameters. The measured results were obtained with an HP8757D network analyzer with the antenna placed in an anechoic chamber. The VSWR of the proposed antenna with and without the tuning stub is shown in Figure 6. The proposed antenna can cover the whole UWB band without the tuning stub. It also appears that the antenna can satisfy the UWB (3.1 to 10.6 GHz) applications for a VSWR < 2, while rejecting the 5 to 6 GHz used in WLAN applications. The differences between the simulated and measured values may be due to dimensional errors in the manufactured antenna and the SMA connector to the CPW-fed transition, which is included in the measurements but not taken into account in the calculated results. The measured radiation patterns at 3.5, 6.5 and 9.5 GHz are shown in Figure 7. It shows that the antenna can give a nearly omni-directional characteristic in the H-plane and quasi omni-directional pattern in the E-plane. The gain of the proposed antenna, with and without the tuning stub, is shown in Figure 8. As desired, the gain sharply decreases in the vicinity of 5.5 GHz and the gain of the notch band drops to -4.6 dBi.

Figure 7 Gain of the proposed antenna vs. frequency.

Figure 8 Radiation patterns of the proposed antennas.

Conclusion

A CPW-fed ultra-wideband antenna with a band-notch characteristic is proposed for UWB applications. The band notch frequency is obtained by using a tuning stub in the middle of the fork-like radiation patch. The antenna is successfully optimized, fabricated and tested. The results show that the antenna not only has a band notch characteristic, but also has a good radiation pattern. The antenna also has compact dimensions of 28 × 21 × 1.6 mm, which makes it attractive for UWB applications.

Acknowledgment

This work was partially supported by the National Nature Science Fund of China (No.60902014).

References

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Cheng-yuan Liu received his bachelor's degree in electrical and information engineering in 2006 and his master's degree in electromagnetic field and microwave technology from Harbin Engineering University, China. He is a doctoral candidate at the Harbin Engineering University, China. His research interests are mainly in microwave theory, UWB antenna and UWB filters.

Ying-song Li received his bachelor's degree electrical and information engineering in 2006 and his master's degree in electromagnetic field and microwave technology from Harbin Engineering University, China. He is a doctoral candidate at the Harbin Engineering University, China. His research interests are mainly in microwave theory, electromagnetic compatibility and microwave antenna design.

Tao Jang received his bachelor's degree in electrical engineering in 1994, his master's degree in information and signal processing in 1999 and his doctorate in communication and information systems in 2002 from Harbin Engineering University, China. He worked in the Harbin Institute of Technology, China as a post-doctoral fellow in 2003 and worked in the National University of Singapore as a research fellow in 2004. He is a professor in the Harbin Engineering University, China. His research interests are mainly in computational electromagnetics, microwave engineering, radio wave propagation and navigation and EMC.

Xiao-dong Yang received his bachelor's degree in electrical engineering from the Harbin Science Technology University, China, in 1985. He received his master's degree and doctorate from Meisei University, Japan, in 1991 and 1995, respectively. He joined the department of communication system design in HITACHI, Japan. In 1999, he worked in the Advanced Technique Research center, Meisi University, Japan, as a research fellow. Since 2000, he has been working in the Research Centre of Electronic Science and Technology Engineering at HEU, China, where he is a professor. His research interests are mainly in microwave theory, electromagnetic compatibility and antenna design.