Yang Zhu, Fu-Shun Zhang, Peng Fei, Yong-Chang Jiao
Ultrawideband (UWB) communications technology offers great promise for handling impressive data rates at low transmit power levels. Since the United States Federal Communications Commission has cleared the way for UWB communications within the frequency band from 3.1 to 10.6 GHz,1 there has been heightened interest among researchers and manufacturers in the RF/microwave industry to develop practical hardware transmit/receive solutions based on this technology.
One of the key components in a UWB system is the antenna, and this component has been of particular interest to designers and researchers over the last decade. A number of different antennas have been designed and implemented for UWB applications, including the disc monopole2 and butterfly-shaped monopole3 configurations. Their ground planes are perpendicular to the radiators, which leads to a large volume for these antennas.
To reduce the size and volume of antennas for UWB applications, numerous microstrip-fed, planar-printed monopole antennas have been developed, including dumbbell-shaped monopoles,4 hybrid antenna designs,5 and monopoles with multiple steps.6 In addition, many UWB antenna designs featuring coplanar-waveguide (CPW) feeds have been published in the literature. For example, elliptical, hexagonal, and fractal monopole designs are presented in refs. 7, 8, and 9, respectively, while a monopole antenna with two symmetrical strips to create multiple resonances was proposed in ref. 10. But most of the aforementioned UWB antennas are relatively large and the designs complex.
Compared to microstrip feed lines, CPW transmission lines suffer less loss and make it possible to print antenna circuitry on one side of a printed-circuit-board (PCB) substrate material, making CPW the preferable way to feed the antenna and integrate it with active devices. Hence, a simple and compact monopole antenna fed by CPW with a shape based on a half-periodic sinusoid is proposed here. By curving the lower edge of the radiating patch, it was possible to maintain a good degree of miniaturization for the current path of the monopole at the lowest frequencies of interest. The total dimensions of proposed antenna are only 24.5 21.7 mm, or dimensions in wavelength, λ0, of 0.25λ0 0.22λ0. The effect of key design parameters on the behavior of this antenna will be described in order to aid those wishing to apply this design approach to the development of a similar UWB antenna. The correlative simulated and measured results will be presented and discussed in detail.
Figure 1 shows the configuration of the proposed antenna. The antenna is fed by a 50-Ω CPW transmission line with line spacing of g and signal line width of W1. Both the radiating patch and the CPW feed line are printed on the same side of a low-cost FR-4 substrate with dimensions of L0) W0, thickness of 1.6 mm, and relative dielectric constant, &lambdar, of 4.4. The shape of the radiating element is based on an area enclosed by a horizontal beeline and a half-periodic sinusoid.
The lowest frequency, fL, of the antenna corresponding to a reflection coefficient of less than or equal to -10 dB is determined by the length, k, of designed antenna. To satisfy the impedance bandwidth over a frequency range of approximately 3.1 to 10.6 GHz, the ratio n (where n = W0/k) must be allocated within a certain range. Additionally, the in-band reflection coefficient can be slightly adjusted by the distance, d, between the upper edge of the ground plane and the bottom of the sinusoid. According to the above three points, it is easy and quick to accomplish the design of antenna with any type of substrate (and different relative dielectric constant, thickness, and other characteristics) in any operating band. The final optimized dimensions of the designed antenna are as follows: L0 = 245 mm, W0 = 21.7 mm, L1 = 95 mm, W1 = 2 mm, k = 14.5 mm, d = 0.5 mm, and g = 0.3 mm.
To better understand the effects of various parameters on the impedance bandwidth of the proposed UWB antenna, a three-dimensional electromagnetic (EM) simulator, Version 11.1 of the High-Frequency Structure Simulator (HSFF) software from Ansoft, was used to simulate the design while changing the values of different parameters to view the effects of the changes. The first step involved studying the effects of changes in length, k. The initial value of k was about one-quarter wavelength at the lowest operating frequency, fL. As Fig. 2(a) shows, changes to the length k will affect fL. The frequency fL shifts towards the lower band as k is increased, illustrating that it is possible to choose a value of k that is appropriate for a desired lowest operating frequency, fL.
When the value of k was selected as 14.5 mm, the simulated value for fL was 3.1 GHz. Figure 2(b) shows the frequency response of the simulated reflection coefficient for the proposed antenna with various values of ratio n when the other parameters are fixed at their optimized values. The simulation results indicate that the impedance bandwidth can cover the full UWB band when the value of ratio n is set within the range of about 1.2 to 1.8. Figure 2(c) illustrates the influence of the distance d on the antenna's impedance matching. As shown, smaller values of d are better than the larger ones for achieving an in-band impedance match.
To verify the design, the prototype of the proposed UWB antenna was fabricated and measured using a model 37269A vector network analyzer (VNA) from Wiltron Co. (now Anritsu) in an anechoic chamber. The simulated and measured reflection coefficients for the designed antenna are shown in Fig. 3. It is clear that the measured impedance bandwidth (for a reflection coefficient of less than or equal to -10 dB) is 7.9 GHz from 3.1 to 11 GHz and the simulated and measured results are in good agreement. The measured gains are also shown in Fig. 3. Clearly, stable gain for the designed antenna has been achieved over the full UWB frequency band.
Figure 4 shows the radiation patterns for the fabricated prototype antenna at 3, 5, 7, and 9 GHz. Dipole-like radiation patterns are observed in the E-plane. Co-polarized radiation patterns in the H-plane remain nearly omnidirectional across the full operating bandwidth. To measure the transmission response, two identical antennas were mounted face to face at a distance of 30 cm as shown in Fig. 5. The low-variation magnitude of the transfer function was measured and shown in Fig. 5(a). The group delay fluctuates between 1 and 3 ns throughout the operating band as shown in Fig. 5(b). These results demonstrate that the proposed antenna possesses excellent phase linearity and offers superior pulse-handling capabilities for modern UWB communication systems.
In summary, a simple and compact printed monopole antenna with sinusoidal edge fed by CPW was analyzed and designed for use in UWB communications applications. The antenna measures only 24.5 21.7 mm (0.25λ0 0.22λ0). The measured results for a prototype antenna confirm that the impedance bandwidth (for a reflection coefficient of less than or equal to -10 dB) of the antenna design ranges from 3.1 to 11 GHz. In addition, the designed antenna has good omnidirectional radiation characteristics with stable gain. Measured transfer functions and group-delay performance indicate that the compact antenna design should perform well with typical UWB signals, especially in small, integrated devices.
YANG ZHU, Doctor, FU-SHUN ZHANG, Professor, PENG FEI, Doctor, YONG-CHANG JIAO, Professor, Xidian University, Mailbox 385, No.2, South Taibai Road, Xi'an 710071, Shaanxi Province, People's Republic of China; e-mail: zhuyang940@163.com, fshzhang@mail.xidian.edu.cn, pfei@mail.xidian.edu.cn, ychjiao@xidian.edu.cn
Acknowledgment
This work was supported by the RIM Advanced Antenna Research Program.
References