SLOTTED ULTRA WIDEBAND ANTENNA FOR ... - IEEE Xplore

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Abstract, In choosing an antenna topology for ultra wideband (UWB) design, several factors must be taken into account including physical profile, compatibility, ...
2008 Loughborough Antennas & Propagation Conference

17-18 March 2008, Loughborough, UK

SLOTTED ULTRA WIDEBAND ANTENNA FOR BANDWIDTH ENHANCEMENT Yusnita Rahayu 1, Tharek A. Rahman 2, Razali Ngah 3, Peter S. Hall 4 1,2,3

Wireless Communication Centre (WCC), Faculty of Electrical Engineering Universiti Teknologi Malaysia, Johor Bahru, Malaysia (1) [email protected] (2) [email protected] (3) [email protected] 4

Department of Electronic, Electrical and Computer Engineering University of Birmingham Edgbaston Birmingham, B15 2TT United Kingdom (4) [email protected] Abstract, In choosing an antenna topology for ultra wideband (UWB) design, several factors must be taken into account including physical profile, compatibility, impedance bandwidth, radiation efficiency, and radiation pattern. In this paper, a small compact T slots UWB antenna is presented. It originates from conventional rectangular monopole and is realized by adding T slots for both patch and feeding strip. The wideband behaviour is due to the fact that the currents along the edges of the slots introduce an additional resonance, which, in conjunction with the resonance of the main patch, produce an overall broadband frequency response characteristic. The slots also appear to introduce a capacitive reactance which counteracts the inductive reactance of the feed. Thus, the bandwidth broadening comes from the patch and T-slots, coupled together to form two resonances. The effect of current distributions of the T slots was simulated by using Zeland FDTD. The measured return loss of this proposed antenna is also presented as well.

1. Introduction Over the past few years, considerable research efforts have been put into the design of UWB antennas and systems for communications. Accordingly, many techniques to broaden the impedance bandwidth of small antennas and to optimize the characteristics of broadband antennas have been widely investigated in many published papers as listed in [1–17]. Some examples of the techniques used to improve the impedance bandwidth of the planar monopole antenna include the use of beveling technique [1], [2], [11], semi-circular base [13], cutting notches at bottom [6], [12], an offset feeding [1], [2], [15], a shorting pin [1], [16], and a dual/triple feed [1], [8], [10], [12], [17], magnetic coupling [3], folded-plate [5], [9], hidden stripline feed [14]. The radiators may be slotted to improve the impedance matching, especially at higher frequency [1], [4]. Planar monopole antennas are good candidates owing to their wide impedance bandwidth, omni-directional radiation pattern, compact and simple structure, low cost and ease of construction. In this paper, a small compact T slots UWB antenna is presented. To design this UWB antenna, three techniques have been applied to the proposed antenna: (i) two steps of notches placed at the two lower corners of the patch, (ii) a partial ground plane and (iii) T slots, which can lead to a good impedance matching. By selecting these parameters, the proposed antenna can be tuned to operate in the 3.1–10.6 GHz frequency range. Both simulation and experimental results are discussed as well. The simulation results in this paper are obtained from Zeland simulation, which are based on the FDTD method.

2. Antenna Design Fig. 1(a) and Fig.1(b) show the geometry and photograph of the proposed UWB antenna, respectively. As shown in Fig.1(a), the antenna has a compact dimension of 30 mm x 30 mm (Wsub x Lsub), designed on FR4 substrate with thickness of 1.6 mm and relative dielectric constant (εr) of 4.7. The radiator is fed by a microstrip line of 3 mm width (wf). On the front surface of the substrate, a rectangular patch with size of 15 mm x 12 mm (w x l) is printed. The distance of h between the rectangular patch to ground plane printed on the back surface substrate is 1 mm, and the length (lgrd) of truncated ground plane of 11 mm. The excitation is a 50 Ω microstrip line printed on the partial grounded substrate.

978-1-4244-1894-7/08/$25.00 ©2008 IEEE

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2008 Loughborough Antennas & Propagation Conference

17-18 March 2008, Loughborough, UK

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Fig. 1: (a) Geometry and Coordinate System of the T Slots Antenna, (b) Photograph of the T Slots Antenna

The modified truncated ground plane acts as an impedance matching element to control the impedance bandwidth of a rectangular patch. This is because the truncation creates a capacitive load that neutralizes the inductive nature of the patch to produce nearly-pure resistive input impedance [7]. The slot size of ws, ws1, ws2, ws3, ls1, ls2, ls3 are 1, 5, 3, 6, 11, 7, 2 mm, respectively. The two notches size of 1.5 mm x 12 mm (w1 x l1) and 1 mm x 9 mm (w2 x l2) are at the two lower corners of radiating patch. Cutting notches at the bottom techniques are aimed to change the distance between the lower part of the planar monopole antenna and the ground plane in order to tune the capacitive coupling between the antenna and the ground plane, thereby wider impedance bandwidth can be achieved. This technique is confirmed by the simulation result shown in Fig. 2. Any reshaping of this area strongly affects the current path. Fig. 3 shows the effect of T slots to the antenna performance. From the graph, at upper frequency of 10.5 GHz, the │S11│ reaches -30 dB. The bandwidth enhancement is due to much more vertical electrical current achieved in the patch through the T slots resulting in much regular distribution of the magnetic current in the slots. Fig. 4 shows the simulated current distribution of the proposed antenna at three different frequencies. The use of slot embedded on the microstrip patch shows as the most successful technique utilized a coupled resonator approach, in which the microstrip patch acts as one of the resonator and slot as the second resonator near its resonance. To validate the simulation results, an antenna prototype was fabricated and tested. In this prototype, measurements are done by using a coaxial port which is soldered at the bottom edge of microstrip line. However, some differences in the simulated and measured results are expected, since in the simulation model discrete and not coaxial port is used. In reality the coaxial cable has a considerable effect, especially the length of its inner conductor, which is connected to the input of the antenna, creating an additional inductance.

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Fig. 3: Simulated Return Loss of Antenna for with and without T Slots

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2008 Loughborough Antennas & Propagation Conference

17-18 March 2008, Loughborough, UK

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Fig. 4: Simulated Current Distribution at (a) 3GHz, (b) 6.5 GHz, (c) 10.5 GHz

The simulated and measured return losses are plotted in Fig. 5(a). The resonance frequencies are shifted from the simulated result but they are still covering the UWB bandwidth requirement of 3.2 to 10.5 GHz with respect to -10 dB. The return loss curves of frequency ranges above 10.5 GHz are getting worst. Simulated VSWR is shown in Fig. 5(b). The measured 3D radiation patterns of the antenna are shown in Fig. 6 at frequencies 4 GHz, 5.8 GHz, and 10.6 GHz. The radiation patterns are nearly omni directional. Simulated vs Measured Return Loss

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Fig. 5: (a) Measured and Simulated Return Loss, (b) Simulated VSWR

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Fig. 6: Measured Radiation Pattern at (a) 4GHz, (b) 5.8GHz, (c)10.6 GHz

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2008 Loughborough Antennas & Propagation Conference

17-18 March 2008, Loughborough, UK

3. Conclusion In this paper, a T slots antenna has been presented. This antenna uses two notches at the bottom of patch and T slots with a partial ground plane for bandwidth enhancement. An experimental prototype has been designed, fabricated and tested. The measured return losses cover the UWB bandwidth requirements of 3.2 GHz – 10.5 GHz with respect to -10 dB. The measured radiation patterns of this prototype are also presented at frequencies 4, 5.8, and 10.6 GHz, respectively.

References [1] Zhi Ning Chen, et al, “Planar antennas,” IEEE Microwave Magazine, Vol. 7, No. 6, Page(s): 63-73, December 2006. [2] Giuseppe R. and Max J. Ammann, “A novel small wideband monopole antenna,” Loughborough Antennas & Propagation Conference (LAPC), Loughborough University, UK, 11-12th April 2006. [3] Nehdar Behdad and Kamal Sarabandi, “A compact antenna for ultra wideband applications,” IEEE Transactions on Antennas and Propagation, Vol. 53, No. 7, pp. 2185-2192, July 2005. [4] Zhi Ning Chen, Terence S. P. See, and Xianming Qing, “Small ground-independent planar UWB antenna,” IEEE Antennas and Propagation Society International Symposium, pp. 1635-1638, 9-14th July 2006. [5] Daniel Valderas, et al, “Design of UWB folded-plate monopole antennas based on TLM,” IEEE Transactions on Antennas and Propagation, Vol. 54, No. 6, pp. 1676-1687, June 2006. [6] Seok H. Choi, et al, “A new ultra-wideband antenna for UWB applications,” Microwave and Optical Technology Letters, Vol. 40, No.5, pp. 399-401, March 5th 2004. [7] A. A. Eldek, “Numerical analysis of a small ultra wideband microstrip-fed tap monopole antenna,” Progress In Electromagnetic Research, PIER 65, pp.59-69, 2006. [8] Serge Boris, Christophe Roblin, and Alan Sibille, “Dual stripline fed metal sheet monopoles for UWB terminal applications,” ANTEM, Saint Malo, France, 15-17th June 2005. [9] Z. N. Chen, M.Y.W. Chia, and M. J. Ammann, “Optimization and comparison of broadband monopoles,” IEE Proceeding Microwave Antenna Propagation, Vol. 150, No. 6, pp. 429-435, December 2003. [10] Kin Lu Wong, Chih Hsien Wu, and Saou Wen Su, “Ultrawide-band square planar metal plate monopole antenna with trident-shaped feeding strip,” IEEE Transactions on Antennas and Propagation, Vol. 53, No. 4, pp. 1262-1269, April 2005. [11] Marta Cabedo Fabres et al, “On the influence of the shape of planar monopole antennas in the impedance bandwidth performance, “Microwave and Optical Technology Letters, Vol. 44, No. 3, Feb 2005. [12] Hassan Ghannoum, Serge Bories, and Raffaele D’Errico, “Small-size UWB planar antenna and its behaviour in WBAN/WPAN applications,” IET seminar on Ultra wideband System, Technologies and Applications, April 2006. [13] Xiao Ning Qiu, H. M. Chiu, and A. S. Mohan, “Investigation on a class of modified planar monopole antennas for ultra-wideband performance,” 9th Australian Symposium on Antennas, Sydney, Australia, 1617th Feb 2005. [14] E. Gueguen, F. Thudor and P. Chambelin, “A low cost UWB printed dipole antenna with high performance,” IEEE International Conference on Ultra Wideband (ICU), Zurich, 5-8th Sept, 2005. [15] M. J. Ammann and Z. N. Chen, “An asymmetrical feed arrangement for improved impedance bandwidth of planar monopole antennas,” Microwave Optical Technology Letters, Vol. 40, No. 2, pp. 156-158, 2004. [16] E. Lee, P. S. Hall, and P. Gardner, “Compact wide-band planar monopole antenna,” Electronics Letters, Vol. 35, No. 25, pp. 2157-2158, December 1999. [17] E. Antonio-Daviu et al, “Wideband double-fed planar monopole antennas,” Electronics Letters,” Vol. 39, No. 23, pp. 1635-1636, November 2003.

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