Ying-Song Li, Yxiao-Dong Yang, Yu Bai, and Tao Jiang
Ultrawideband (UWB) technology shows great promise for high-data-rate, short-distance communications applications. Operating with short pulses or bursts of energy, UWB systems must also co-exist with current communications standards operating within the broad range of UWB devices. For this reason, filters and antennas capable of providing notches can be useful in the support of emerging UWB applications. To achieve that goal, the authors sought to develop a circular wide slot UWB antenna with dual notch-band characteristics using a coplanar-waveguide (CPW) feed. The bandwidth of the antenna is increased through the use of wide-slot technology. The dual notch-band functions are obtained by means of uniform-impedance resonator (UIR) and stepped-impedance resonator (SIR), which are etched on the antenna's circular radiation patch and embedded in the CPW-fed signal stripline, respectively. To demonstrate the approach, an UWB antenna was designed and fabricated with wide impedance bandwidth of 2.8 to 12.0 GHz and notch bands of 5.1 to 6.0 GHz for HIPERLAN/2 and IEEE 802.11a WLAN (5.1 to 5.9 GHz) and 8.2 to 9.3 GHz for satellite and military applications. The compact antenna measures only 32 x 24 mm.
The use of UWB technology may provide extremely fast data communications over short distancesparticularly for warless personal-area networks (WPANs), if some technical challenges can be overcome. With the UWB wireless protocol released by the United States' Federal Communications Commission (FCC) in 20021 covering 3.1 to 10.6 GHz, designers around the world started pursuing circuits and components for UWB communications applications. The antenna, which receives and transmits the UWB signals, is of course a key component, especially if it can be made small enough and with an even, omnidirectional radiation pattern for installation in a variety of end products.
Recently, a number of planar monopole antennas for UWB applications have been presented.2-6 However, some of these antenna designs are large in size and lack the wide impedance bandwidth needed for UWB communications. In addition, there are several existed narrowband communications systems in use for some time, which must be considered in the design of an UWB antenna. These include the HIPERLAN/2 bands (5.15 to 5.35 GHz and 5.470 to 5.725 GHz in Europe) and the IEEE 802.11a bands (5.15 to 5.35 GHz and 5.725 to 5.825 GHz in the US) for wireless-local-area-network (WLANs) communications and C-band communications, and X-band frequencies (8.2 to 9.3 GHz) for satellite and military communications.
To accommodate these existing applications while still performing across the UWB spectrum, several UWB antennas with notch-band7-12 or dual-notch-band characteristics13-21 have been studied. Although these antennas may reduce interference between UWB signals and existing narrowband communications systems, many of the proposed notch band antennas suffer from at least one of the following limitations: 1) a relatively low quality factor (Q) associated with the notch band, 2) complex geometry which is difficult to redesign and with a notch band that is not tunable, and 3) notch bands obtained by adding various slots which deteriorate the UWB radiation patterns. To overcome the limitations of these designs, stubs,11 and slots12-14 etched in the radiation patches and ground planes, parasitic elements along the radiation patch,18 and SIRs 21 have been used to design notch-band antennas. However, the notch band is also difficult to control.
Based on earlier research, a circular wide-slot UWB antenna with dual-notch-band characteristics was realized numerically and experimentally. Dual-notch-band functions are achieved by employing a radial UIR and SIR. The UIR, which is cut on the UWB antenna's circular radiation patch, is used to form the lower notch band. The SIR, which is embedded in the CPW-fed transmission signal stripline, produces the higher notch band. This experimental antenna design can not only cover the full UWB communications band but also provides two strong notch-band characteristics. The antenna was successfully optimized by using the High-Frequency Structure Simulator (HFSS) electromagnetic (EM) simulation software from Ansoft, fabricated on commercial microwave circuit laminate, and tested.
Figure 1 shows the geometry of the experimental UWB antenna with dual notch band characteristics using UIR and SIR. The antenna was printed on a high-frequency laminate with relative dielectric constant of 2.65 in the z direction at 10 GHz, a loss tangent of 0.002 in the z direction at 10 GHz, and thickness of 1.6 mm. The size of the antenna is 32 x 24 mm (length x width). The experimental antenna incorporates a circular wide slot structure, circular radiation patch, a radial UIR, an SIR, and a 50-Ω, CPW-fed structure. The 50-Ω, CPW-fed structure consists of the CPW transmission signal stripline with a signal strip width, W7, of 3.6 mm, and gap between the CPW ground plane and transmission signal strip with width S of 0.2 mm.
The dimensions of the circular wide slot, circular radiation patch, and gap, g, between the circular radiation patch and the CPW ground plane can be modified to improve the antenna's impedance bandwidth. The UIR and SIR help generate the two notch bands, designated the lower notch band near 5.5 GHz and the higher notch band near 8.5 GHz. The two notch bands can be controlled and tuned by adjusting the dimensions of the UIR and SIR. By properly selecting the parameters for the circular wide slot, the gap g, the radiation patch, the UIR and SIR, the antenna can be fabricated for UWB applications with two notch bands at key frequencies, such as WLAN and X-band for this project.
Current distributions along the experimental UWB antenna were investigated by simulations with HFSS based on finite-element-method (FEM) analysis. Figures 2(a), (b), (c), and (d) show simulated current distributions for the dual-notch-band antenna at 3.5, 5.5, 8.5, and 10.5 GHz, respectively. At 3.5 GHz Fig. 2(a)>, the current flows along the CPW-fed structure and the edge of the circular radiation patch, while the current distributions along the CPW ground plane and the UIR and SIR are small. At 5.5 GHz, as can be seen from the simulation of Fig. 2(b), the current flows mainly along the UIR to produce a resonant frequency for the lower notch band near 5.5 GHz; the current distribution along the CPW ground is small at 5.5 GHz. At 8.5 GHz Fig. 2(c)>, the current distributions on the CPW transmission signal line and the SIR are large, with the higher notch band near 8.5 GHz produced by the SIR. At 10.5 GHz Fig. 2(d)>, the current flows mainly along the CPW transmission signal stripline and the edge of the gap between the circular radiation patch and the CPW ground plane, while the current distributions at the UIR and SIR are small.
With the help of the HFSS software, the antenna's parameters were adjusted for optimum performance, with optimum values shown in the table. A prototype of the antenna was fabricated (Fig. 3) and characterized with the aid of a model 37347D vector network analyzer (VNA) from Anritsu Co. To study the impact of the UIR and SIR structures, the antenna was evaluated with and without those two structures. Measured VSWRs for the experimental antenna are shown at different frequencies in Fig. 4.
As can be seen from Fig. 4, the simulations agree closely with the measured results for the UWB antenna. Any differences between the two sets of data may be due to manufacturing tolerances and errors as well as the SMA connector to the CPW-fed transition, which was included in the measured results but not in the simulations. The measurement also shows that the dual notch bands are obtained by means of the UIR and SIR structures. It can also be seen in Fig. 4 that the experimental UWB antenna without the UIR and SIR has an impedance bandwidth of 131% ranging from 2.8 to 13.5 GHz.
Figure 5 shows measured radiation patterns at 3.5, 7.0, and 10.5 GHz. These patterns reveal that the antenna provides a nearly omnidirectional characteristic in the H-plane and a quasi-omnidirectional characteristic in the E-plane. Figure 5 shows that the radiation patterns in the E-plane deteriorate more or less with increasing frequency, also they still maintain quasi-omnidirectional behavior. Peak gain levels for the UWB antenna were determined by means of comparison to a double-ridged horn antenna. Stable gain was found throughout the UWB operating band, except at the notch frequencies.
For comparison purposes, the experimental antenna was also evaluated without UIR and SIR. Figure 6 plots peak gains for the UWB antenna with and without the UIR and SIR. The measured gain of the proposed antenna without the UIR and SIR increases from 1.86 dBi to nearly 5.1 dBi, as a result of the deteriorating radiation patterns of the experimental antenna at the high band. In the UWB operating band, the antenna without UIR and SIR has stable gains with less than 3.3 dBi fluctuation. But the gain of the UWB antenna with UIR and SIR drops quickly from 5.1 to 6.0 GHz and from 8.2 to 9.3 GHz. As designed, sharp gain decreases occur in the vicinity of 5.5 and 8.5 GHz. The gain drops to -4.8 dBi at the lower notch band and -3.9 dBi at the higher notch band.
Antenna dimensions D2 and D3 have a great deal of effect on the center frequency of the lower notch band, and changes to these two parameters were studied with HFSS and shown in Fig. 7 and Fig. 8, respectively. As Fig. 7 shows, the resonant frequency of the lower notch band moves down in frequency as the diameter D2 is increased, while the lower-notch-band Q improves. As Fig. 8 shows, the lower notch band moves lower in frequency as D3 is increased. This is caused by coupling between D3 and D2 which improves the parameters of the UIR. From this analysis, it can be seen that the center frequency of the lower notch band can be tuned by changing the dimensions of D2 and D3.
The center frequency of the higher notch band can be controlled by adjusting parameters L5, W5, and W6, as Fig. 9, Fig. 10, and Fig. 11 show, respectively. As Fig. 9 reveals, the resonant frequency of the higher notch band moves down in frequency with increasing L5, while the Q of the higher notch band is improved. Still, the bandwidth of the UWB antenna has deteriorated somewhat by the use of a fixed lower notch band. As Figure 10 shows the effects of changes in W5, where it can be seen that the center frequency of the notch band also moves lower in frequency with increased width of W5. Figure 10 indicates that the impedance bandwidth of the experimental antenna has decreased at the higher band since the increased width of W5 has altered the resonant frequency of the SIR. Figure 11 offers the effects of altering W6. As W6 increases, the higher notch band moves down in frequency, while the Q of the two notch bands is improved. This is due to the changes in dimensions which alter the parameters of the SIR.22 During design and optimization of the UWB antenna with dual notch bands, the effects of these key parameters on the UIR and SIR were carefully considered to meet the desired performance requirements.
In summary, the CPW-fed circular wide-slot UWB antenna with dual notch bands was designed and fabricated in a compact circuit by using the UIR and SIR to affect the lower and upper notch bands. Without the UIR and SIR, the experimental antenna achieves an impedance bandwidth of 131%. With the UIR and SIR, measured results show that the antenna not only has good dual notch band characteristic but also has large impedance bandwidth and good radiation patterns.
This work was supported by the National Nature Science Fund of China (grant No. 60902014) and the Nature Science Fund of Heilongjiang (grant No. QC2009C66). The authors are also thankful to Hebei VSTE Science and Technology Co. Ltd. for providing the test and measurement facility.