Planar Antenna Aids UWB Communications

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Wireless communications channels are being overloaded with voice, video, and data information, making ultrawideband (UWB) strategies very appealing. In support of UWB systems, a printed planar monopole antenna with stepped gradient structure and novel coplanar-waveguide (CPW) feed was developed. Analysis of the antenna design shows a wide impedance bandwidth of 3.1 to 13.5 GHz with VSWR of less than 2.0:1. The antenna provides a quasi-omnidirectional radiation pattern at lower frequencies and is a candidate for a variety of applications in UWB communications systems.

Commercial use for UWB communications applications from 3.1 to 10.6 GHz was approved in 2002 by the United States’ Federal Communications Commission (FCC).^1-3 UWB systems were meant to coexist with other, more-narrowband wireless standards, such as wireless local area networks (WLANs) operating from 5.15 to 5.35 GHz and from 5.725 to 5.825 GHz. The antenna is a critical component in these UWB systems, compared to traditional, more-narrowband transmit/receive antennas, since it must handle a broad frequency range with high gain, uniform directivity, and compact size.^4-6 A variety of antennas has been developed for UWB systems.^7-11 These antennas feature many differently shaped planar elements, such as rectangular, disc, elliptical, and triangular geometries.^12,13

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1. This diagram shows the configuration and basic dimensions of the proposed UWB antenna.

Figure 1 shows the configuration of the novel antenna. It was printed on low-cost FR-4 microwave substrate measuring 32 x 26 mm and featuring 1-mm thickness, with relative dielectric constant of 4.4. The antenna features two rectangular ground planes. The CPW feed line is designed by using a rectangular patch to connect with the stepped-gradient-structure patch of the main radiator. Both patch structures are printed on the same side of the FR-4 substrate. The CPW feed line is designed for a characteristic impedance of 50 Ω with fixed 2.8 mm (W₁₀ = 2.8 mm) feed line width and 0.3-mm ground gap. The antenna provides a wide bandwidth with omnidirectional radiation pattern. Optimal dimensions for the antenna are as follows:

L = 32 mm;

L₁ = 8.0 mm;

L₂ = 4.0 mm;

L₃ = 2.5 mm;

L₄ = 2.5 mm;

L₅ = 2.0 mm;

L₆ = 11.0 mm;

L₇ = 10.0 mm;

L₈ = 11.25 mm;

L₉ = 12.5 mm;

W = 26.0 mm;

W₁ = 23.0 mm;

W₂ = 17.0 mm;

W₃ = 12.0 mm;

W₄ = 9.0 mm;

W₅ = 6.0 mm;

W₆ = 10.0 mm;

W₇ = 8.0 mm;

W₈= 9.0 mm;

W₉ = 11.3 mm; and

W₁₀ = 2.8 mm.

Figure 2 shows a prototype of the antenna, fabricated by hand and based on the optimal dimensions. The antenna design was analyzed by changing one parameter at a time with all other parameters fixed. A commercial simulation software was used for the analysis: the High Frequency Structure Simulator (HFSS) from Ansoft. Figure 1 shows the dimensions for the antenna as a result of this optimization approach. The antenna appearing in Fig. 2 was fabricated to verify the design.

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2. This photograph reveals the fabricated antenna.

The antenna’s impedance bandwidth was simulated with HFSS and measured for the prototype using a model 37269C vector network analyzer (VNA) from Anritsu Co. The results for the simulation and measurements are shown in Fig. 3, with close agreement between the sets of data. Discrepancies between simulated and measured results are likely caused by measurement environmental effects, SMA connector effects, fabrication imperfections, and inappropriate quality of the microwave substrate, not all of which were included in the simulations. The results show an impedance bandwidth of 3.1 to 13.5 GHz for the antenna, or more than 10 GHz for a voltage standing wave ratio (VSWR) of less than 2.0:1. It is clear that the antenna can be used for frequencies above the FCC band in various applications.

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3. The plot compares the measured and simulated VSWRs for the proposed antenna as a function of frequency.

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Radiation Revelations

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To study the antenna’s radiation patterns, two principle planes were selected. These are referred to as the x-z plane (H-plane) and the y-z plane (E-plane). Figures 4(a) through (d) show the normalized radiation patterns in the H- and E-planes for the antenna at 4, 6, 7, and 8 GHz, respectively. The results reveal that the antenna provides an acceptable quasi-omnidirectional radiation pattern suitable for receiving signals from all directions at lower frequencies. Due to antenna surface currents, however, its cross-polarization performance deteriorates with increasing frequency. Figure 5 plots the antenna’s measured peak gain as a function of operating frequency. Gain ranges from 3.3 to 10.1 dB in the measurement range from 2 to 14 GHz; gain across the full impedance bandwidth of 3.1 to 13.5 GHz always exceeds 3 dB, indicating that the antenna design is suitable for use in UWB systems.

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4. These far-field radiation patterns show the antenna’s behavior at (a) 4 GHz, (b) 6 GHz, (c) 7 GHz, and (d) 8 GHz.

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5. Measured peak gain for the proposed antenna is plotted as a function of frequency.

In short, this compact monopole microstrip antenna shows promise for UWB applications. Following simulations with HFSS software, a prototype measuring just 32 x 26 mm was fabricated on FR-4 substrate. Simulations and measurements show an impedance bandwidth of 3.1 to 13.5 GHz, a bandwidth of more than 10 GHz for a VSWR of less than 2.0:1. The antenna has an acceptable quasi-omnidirectional radiation pattern required to receive information signals from all directions at lower frequencies. It is thus an attractive candidate for UWB communications applications.

Bo Gao, Lecturer

College of Communication Engineering, Jilin University, Changchun 130012, Jilin Province, People’s Republic of China.

Ge Wu, Engineer

College of Electronic Science and Engineering, Jilin University, Changchun 130012, Jilin Province, People’s Republic of China.

Jia-Yu Huo, Doctoral Student

College of Communication Engineering, Jilin University, Changchun 130012, Jilin Province, People’s Republic of China.

Xiao-Jian Tin, Professor

College of Electronic Science and Engineering, Jilin University, Changchun 130012, Jilin Province, People’s Republic of China.

Acknowledgments

This work was supported in part by the National Natural Science Foundation of China under Grant No. 60372061, and the Changchun Science and Technology Support Program under Grant No.11KZ36.

References

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