Tang Yang, Xiao-Jian Tian, Chang-Fu Chen, and Wen Gao
Low-power communications by means of ultrawideband (UWB) technology holds great promise for effective short-range transfer of large amounts of data at high data rates. Since the United States' Federal Communications Commission (FCC) allocated the frequency range from 3.1 to 10.6 GHz for UWB applications in 2002,1 the technology has developed very quickly in terms of amplifiers, antennas, and other key communications system components. Of course, the antenna is one of the more critical components in such a system, since it can impact both transmit and receive performance.
Antennas designed for UWB use must meet a variety of requirements. These include ease of manufacturing; being low in cost and profile; and having broad bandwidth, high gain, and an omnidirectional radiation pattern. Numerous planar monopole antennas have been designed and developed in recent years to meet these requirements for UWB applications.2-7 But due to the fact that a portion of the UWB bandwidth (from 5.0 to 5.8 GHz) has been reserved for existing wireless applications, such as wireless local area networks (WLANs), an effective UWB antenna design must serve to reduce interference between UWB communications systems and 5-GHz WLAN systems.8-10
With that goal in mind, a novel band-notched coplanar-waveguide (CPW) antenna with U-shaped slot was developed. The notch band is achieved by forming a U-shaped slot into a step-shaped monopole patch. The antenna has two rectangular ground planes, and both of its structures are printed on the same side of the microwave substrate and fed by CPW.
Simulations of the antenna design using a commercial computer-aided-engineering (CAE) software tool match well with experimental test results, both revealing that the antenna design provides a strong notch within the band from 5.0 to 5.8 GHz while still achieving a wide impedance bandwidth with VSWR of better than 2.0:1 from 3.1 to 15.6 GHz, an impedance bandwidth of better than 126%.
Figure 1 shows the configuration of the UWB antenna. It is fabricated on low-cost FR-4 circuit-board laminate with relative dielectric constant (er) of 4.4 in the z-direction at 10 GHz, dielectric loss tangent of 0.0018 in the z-direction at 10 GHz, and thickness (h) of 1.6 mm. The UWB antenna measures just 24 x 34 mm. The antenna consists of a step-shaped monopole patch, with U-shaped slot to provide the band-notch function and two rectangular patches serving as the ground planes. Both of the structures are printed on the same side of the FR-4 substrate and fed by a 50-Ω microstrip feed line which is connected to an SMA connector; the width of this feed line is W10 = 2.4 mm. The antenna was optimized with the following dimensions: L1 = 2 mm, L2 = 14 mm, L3 = 8 mm, L4 = 2.5 mm, L5 = 1.5 mm, L6 = 12.5 mm, L7 = 2.8 mm, L8 = 2.4 mm, L9 = 6.1 mm, W1 = 9 mm, W2 = 18 mm, W3 = 24 mm, W4 = 10 mm, W5 = 8.4 mm, W6 = 5 mm, W7 = 12 mm, W8 = 7 mm, and W9 = W10 = W11 = 11.4 mm. Figure 2 shows a prototype of the proposed antenna, fabricated according to these optimized dimensions.
The proposed UWB antenna was fabricated and simulated by means of the commercial electromagnetic (EM) computer simulation software High Frequency Structure Simulator (HFSS) from Ansoft Corp. Measurements were performed using a model 37269C microwave vector network analyzer (VNA) from Anritsu Co., which has a standard measurement frequency range of 40 MHz to 40 GHz.
Figure 3 shows that the results of the simulation and the measurement agree closely. The impedance bandwidth extends from 3.1 to 15.6 GHz (with a VSWR of less than 2.0:1), which exceeds 126%. By introducing the U-shaped slot on the radiating element, a band-notch characteristic from 5.0 to 5.8 GHz can be achieved.
Figure 4 shows the antenna's far-field radiation patterns at 4, 6, 7, and 9 GHz. It also shows the copolar and cross-polar patterns in the x-z (f = 0 deg.) and y-z (f = 90 deg.) planes. As can be seen from these plots, the proposed antenna design provides an approximately omnidirectional radiation pattern. Figure 5 shows that the proposed antenna can obtain high peak gains of about 3.2 to 9.8 dB from 3.1 to 15.6 GHz; the peak gain decreases at frequencies in the vicinity of the band-notched area around 5.3 GHz.
In summary, a novel band-notched CPW antenna with U-shaped slot has been designed for UWB applications. By introducing a U-shaped slot into the radiating element, a strong band-notched function can be obtained from 5.0 to 5.8 GHz, serving to minimize interference of the UWB antenna with existing wireless applications in the 5-GHz band. The results of computer simulations and measurements show that the impedance bandwidth of this antenna design extends from 3.1 to 15.6 GHz, and exhibits a VSWR across that frequency range of less than 2.0:1 (a greater than 126% impedance bandwidth). It delivers nearly omnidirectional radiation patterns across its full operating bandwidth, with high gain except in the notched frequency band. As the measurements and simulations demonstrate, this proposed antenna design is a strong candidate for UWB communications applications.
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