Chang-Fu-Chen, Xiang-Jian Tian*, and Tang Yang
Ultrawideband (UWB) communications from 3.1 to 10.6 GHz, the band made unlicensed by the United States Federal Communications Commission (FCC), is attractive for a wide range of commercial applications.1 In support of UWB communications systems, antennas are an essential component. These antennas should provide high gain, wide-impedance bandwidths, and omnidirectional radiation patterns to make them suitable for UWB communications applications. Fortunately, all of these characteristics can be realized through the use of a coplanar-waveguide (CPW) feed and printed planar monopole structures.2-5
Some of these UWB antennas can achieve one of these properties by changing ground structures,6,7 while others can gain the necessary properties and performance by changing the shape of their radiators.8,9 The authors developed a two-horn planar monopole antenna structure for UWB applications which relies on a CPW feed. Computer simulations and measured results agree closely and indicate that the proposed two-horn antenna structure can achieve approximately omnidirectional radiation patterns at 5, 6, 7, 8, and 9 GHz, with high peak gain across the full bandwidth. The proposed antenna design provides an impedance bandwidth of about 10.9 GHz, from 3.0 to 13.9 GHz, with VSWR over that range of less than 2.0:1.
Figure 1 shows the geometry of the proposed two-horn antenna, which was fabricated on FR-4 circuit-board material with relative dielectric constant, er, of 4.4 and dielectric loss tangent of 0.0018. The size of the proposed antenna is 34 x 26 x 1 mm. The antenna consists of two rectangular ground planes with a horn-shaped slot, while the feed line is designed by using a rectangular patch to connect with the horn-shaped patch as the main radiator.
The optimized dimensions of the two-horn antenna are: W1 = 26 mm, W2 = 18 mm, W3 = 10.6 mm, W4 = 6 mm, W5 = 8 mm, W6 = W8 = 11.3 mm, W7 = 2.8, L1 = 34 mm, L2 = 13 mm, L3 = 15 mm, L4 = 10 mm, and L5 = 11 mm. Figure 2 shows a prototype of the proposed antenna, fabricated by hand according to these parameters.
The fabricated antenna was evaluated by means of measurements, as well as by using the commercial High-Frequency Structure Simulator (HFSS) electromagnetic (EM) simulation software from Ansoft Corp. Measurements were performed with a model 37269C microwave vector network analyzer (VNA) from Anritsu Co.
Figure 3 shows that the measured impedance bandwidth, which reached 10.9 GHz, is matched with the simulated results across a frequency range from 3.0 to 13.9 GHz (for a VSWR of less than 2.0:1). Figures 4(a) through 4(e) show the antenna’s far-field radiation patterns at 5, 6, 7, 8, and 9 GHz, respectively.
These results demonstrate that the co-polar and cross-polar radiation patterns in the X-Z (f = 0 deg.) and Y-Z (f = 90 deg.) planes are consistent with the properties required for omnidirectional radiation patterns. Finally, Figure 5 shows that the two-horn antenna achieves peak gains ranging from 2.8 to 12.5 dB, with gains across the entire impedance bandwidth always more than 2 dB.
Chang-Fu-Chen, Xiang-Jian Tian, and Tang Yang, College of Electronic Science and Engineering, Jilin University, Changchun, People’s Republic of China; e-mail (Tang Yang): firstname.lastname@example.org.