Small Ground-independent Planar UWB Antenna. Zhi Ning Chen, Terence S. P. See, and Xianming Qing. Radio Systems Department, Institute for Infocomm ...
Small Ground-independent Planar UWB Antenna Zhi Ning Chen, Terence S. P. See, and Xianming Qing Radio Systems Department, Institute for Infocomm Research 20 Science Park Road, #02-21/25 TeleTech Park Singapore 117674 E-mail: {chenzn; spsee; qingxm}@i2r.a-star.edu.sg Introduction Ultra-wideband (UWB) is a promising technology for short-range high data-rate wireless communication applications. The UWB technology uses the short pulses or signals with extremely wide spectra to achieve wireless connection. So far, one of the main potential commercial applications may be in the area of consumer electronics. The UWB based systems may be embedded into a variety of portable devices. The issues of the antenna used in such UWB systems will be challenging, due to demands for ultra-wide operating bandwidths not only for parameters in frequency domain such as impedance and gain but also for time domain parameters such as phase and waveforms. One of the critical issues in UWB system design is the size of the antenna for portable devices, because the size affects the gain and bandwidth greatly. Therefore, to miniaturize the antennas capable of providing broad bandwidth for impedance matching and acceptable gain will be a challenging task [1]. The use of planar version can reduce the size of the antennas [2-7]. The antenna printed on a PCB can be of small size and easily integrated into RF circuits. In this paper, a small ground-independent planar UWB antenna is proposed. It is designed to cover the UWB band of 3.1-10.6 GHz. The antenna is etched onto a piece of PCB. A notch is cut from the radiator to reduce the size of the planar antenna. The simulations and measurements show that its impedance characteristics are ground-plane independent. The performance of the antennas is tested in time and frequency domains.
Antenna Design & Results Figure 1 shows the geometry of proposed antenna and the Cartesian coordinate system (x, y, z). The radiator and ground plane were etched on opposite sides of the PCB (RO4003, εr = 3.38 and 1.52 mm in thickness). The rectangular notch of ws × ls = 4 mm × 12 mm was cut close to the horizontal strip of wrs × lrs = 2 mm × 6 mm at a distance of ds = 6 mm. Two bevels were cut to improve the impedance matching at higher frequencies. The radiator is fed by a microstrip line of a 3.5 mm width located at d = 3.5 mm from the center with a feed gap g = 1 mm. The ground plane has a vertical length of lg = 9 mm. Figure 2 compares simulated and measured return losses, which show that the antenna achieved a 10dB bandwidth of 2.95-11.6 GHz, and in particular, the impedance characteristics feature ground-independence. In the simulations, the antenna was fed by a source at the end of the strip and close to the edge of the PCB. The excitation source with a 50-Ω internal resistance was directly located between the strip end and ground plane without any RF cable. In the measurements, a 50-Ω SMA was connected to the end of the
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strip and ground plane, and a Vector Network Analyzer by an RF cable. Usually, the RF cable significantly affects the performance of antenna under test. However, from Figure 2, it is found that the RF cable hardly affects the lower edge frequency at 2.9 GHz, which implies that the design is ground-independent in terms of impedance matching. This will make the design more robust and flexible when the antenna is used in various devices and environment. The radiation patterns were measured at frequencies of 3, 5, 6, and 10 GHz in the principal x-z, y-z, and x-y planes. For brevity, only the radiation patterns in the x-z plane are shown in Figure 3. The measurement shows that the antenna has dipole-like radiation characteristics, and the variation of the radiation patterns is slight across the frequency range of interest. The measured average gain is higher than -1.98 dBi, which can meet the usual requirement of -4 dBi for mobile applications. To examine time-domain performance of the antenna, Rayleigh pulses (monocycle) 2 v (t ) = te − (t / σ ) with σ = 35, 50, and 100 ps are selected to be the source impulses applied to the transmit antenna. Figure 4 shows the waveforms of the received impulses at a receive antenna identical to the transmit antenna at the distance of 30 mm, 200 mm, and 800 mm. It is seen that the waveforms for the longest source pulse duration have higher amplitudes, experience less distortion but more ringings. It is also concluded that for the same antenna, the pulse response in time domain varies with the choice of the source pulses. As such, the antenna design can be optimized according to the source pulses used in the system.
Conclusions A small planar PCB antenna designed for promising ultra-wideband applications has been presented. A notch has been cut from the radiator to reduce the size of the antenna and ease the effect of the ground plane on the impedance performance of the antenna by concentrating more electric currents on the radiator. As a result, the average gain of the antenna has been increased and the impedance response becomes more ground independent. Also, the performance of the antenna has been evaluated in both time and frequency domains.
References: [1] Z. N. Chen, et al, “Considerations for source pulses and antennas in UWB radio systems,” IEEE Trans. Antennas Propagat., vol. 52, no.7, pp.1739-1748, July 2004 [2] G. H. Brown and O. M. Woodward, “Experimentally determined radiation characteristics of conical and triangular antennas,” RCA Review, vol. 13, Dec. 1952 [3] M. J. Ammann, “Square planar monopole antenna,” IEE National Conf. Antennas & Propagat., York, England, pp.37-40, 1999 [4] Z. N. Chen, “Impedance characteristics of planar bow-tie-like monopole antennas,” Electronics Letters, vol. 36, no. 13, pp. 1100-1101, 2000 [5] M. J. Ammann, “Impedance bandwidth of the square planar monopole,” Microw. Opt. Techno. Lett., vol. 24, pp.185-187, 2000 [6] E. Antonino-Daviu, et al, “Wideband double-fed planar monopole antennas,” Electronics Letters, vol. 39, no. 23, pp.1635-1636, Nov. 2003 [7] Y. Zhang, et al, “Effects of finite ground plane and dielectric substrate on planar dipoles for UWB applications,” IEEE AP-S, pp.2512-2515, June 2004
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