Yin-Song Li, Xiao-Dong Yang, Yu Bai, and Tao Jiang

Dual-band antennas are useful for serving multiple wireless applications, such as WiMAX and wireless local area networks (WLANs), with a single structure. One such dual-band (2.5 and 5.5 GHz) antenna design was developed for those two applications in a circuit as small as 20 x 35 x 1.6 mm. It is a U-shaped monopole structure with two inverted, L-shaped lateral patches and coplanar-waveguide (CPW) feed line. The antenna provides 10-dB impedance bandwidths of 64% and 22.2% at the lower- and higher-frequency bands, respectively.

With the increasing use of wireless communications, there is a growing need for multiband antennas that can serve multiple applications, such as WLANs and WiMAX. Such antennas should be small in size and with low profiles, with omnidirectional radiation patterns. A number of printed monopole antennas have been reported with dual-band characteristics.1-14 However, most of them have narrow bandwidths and are addressed to the needs of WLAN1-10 and personal communication system (PCs) applications.11-14 Recently, several dual-band planar antennas for WLAN/WiMAX applications have been realized.15-18 but these antennas have narrow bandwidths and nonplanar feed structures15 and tend to be large in size,16-18 which makes them unsuitable for modern wireless facilities applications.

In another dual-band antenna design approach, a wide-slot antenna with fractal slot and U-slot dual antenna was realized.19 Also, a dual antenna with a semicircular ground using a rectangular patch with four round corners was reported,16 with compact dimensions of 35 x 35 mm. To form a dual-band antenna, two rectangular slots are etched on the radiation patch to produce a notch in the wideband antenna. However, these designs are complex and etching the slots on the radiating patch deteriorates the quality of the resulting radiation patterns.

For this article, a U-shaped monopole antenna with symmetrical CPW feed was developed for both WLAN and WiMAX applications. The antenna consists of a U-shaped patch and two inverted L-shaped lateral patches that are driven by a 50-Ω CPW transmission line on one side. The distinct frequency bands can be determined by controlling the parameters of the U-shaped patch, the inverted L-shaped lateral patches.

Figure 1 illustrates the geometry and the configuration of the proposed dualband antenna for WLAN/WiMAX applications. The antenna is printed on a dielectric substrate with relative permittivity (er) of 2.65, a loss tangent of 0.002, and a thickness of 1.6 mm. The size of the antenna is 20 x 35 x 1.6 mm. A CPW transmission line with a fixed signal-strip width of 1.4 mm (parameter W6) and a gap distance of 0.3 mm (parameter S) between the signal strip and the ground plane is used as the feed line for the antenna. The dual-band antenna consists of a U-shaped patch which can be regarded as a dipole antenna, two inverted L-shaped lateral patches, and the ground plane. Two finite ground patches of the same size (9 x 10 mm) are situated symmetrically on each side of the CPW transmission line; the U-shaped monopole is connected to the end of the CPW transmission line. Two inverted L-shaped stripline structures are added on each side of the transmission line as lateral patches.

By properly selecting the parameters for the U-shaped patch and the two inverted lateral patches and the partial ground plane numerically and experimentally, the dimensions required for precisely fabricating the antenna for WLAN/WiMAX applications can be found. The antenna's various geometric parameters were adjusted and optimized by means of the High Frequency Structure Simulator (HFSS) electromagnetic (EM) simulation software from Ansys. Optimal values for the various antenna parameters are listed in the table. Figure 2 shows the fabricated prototype of the dual-band antenna.

Figure 3 shows the simulated and measured return-loss-versus-frequency performance for the dual-band antenna. The antenna's performance was simulated using HFSS software while the actual performance of the prototype was measured by means of a model 37347D microwave vector network analyzer (VNA) from Anritsu Co. As Fig. 3 reveals, the antenna has two distinct frequency bands, from 2.0 to 3.7 GHz and from 4.5 to 5.9 GHz, according to the simulation data, and from 1.9 to 3.7 GHz and from 4.8 to 6.0 GHz according to the measured results. The differences between the simulated and measured values may be due to the errors and manufacturing tolerances in fabricating the antenna and to the SMA-connector-to-CPW-fed transition, which is included in the measurements but not taken into account in the simulations.

Figure 4 and Figure 5 show measured radiation patterns for the dual-band antenna at two different frequencies: 2.5 and 5.5 GHz, respectively. The measured radiation patterns reveal an omnidirectional H-plane radiation pattern and a dipole-like E-plane radiation pattern for the compact dual-band antenna design. Since the measurements were not performed in free space, the transmitted energy from the antenna may have been absorbed in part by the test instruments and surrounding equipment in the laboratory; this is the reason why the measured radiation patterns show distortion. Figure 6 shows the peak gains of the fabricated antenna compared to a horn reference. The measured average gains were 2.05 dBi (1.4 to 2.71 dBi) and 3.55 dBi (2.3 to 4.8 dBi) at the lower- and upper-frequency operating bands.

The length of the branches of the U-shaped patch and the two inverted L-shaped patches and the gap between the patch and the ground are important parameters for controlling the impedance bandwidth of the dual-band antenna. Figure 7 shows the simulated performance for return loss resulting from using three different lengths of the branches of the U-shaped patch L1 with other parameters fixed. It can be seen that the 2.5-GHz resonance frequency shifts to 2.2 GHz and the bandwidth around 2.5 GHz becomes narrower as a function of L1 length. At the same time, variations in L1 have little effect on performance at 5.5 GHz.

Figure 8 presents the effects of changes in the length of the lateral patch, L2. By increasing the length of L2, the 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 L2, the resonant frequency decreases.

Not shown, but measured, was the effects of changes in parameter g on the return-loss performance of the dual-band antenna. It was found that the return-loss performance deteriorates with increases in g, with the bandwidths of both the lower- and higher-frequency bands decreasing in the process. This is caused by coupling between the patches and ground, which is reduced with increasing g.

In summary, the antenna provides good performance over both wireless bands, showing a true dual-band characteristic. In addition, by adjusting the various parameters listed in the table, it is possible to individually tune the characteristics of the antenna's two frequency bands, to achieve stable gain and consistent omnidirectional radiation patterns in the two frequency ranges for both WLAN and WiMAX applications.