This paper proposes symmetrically beveled planar square monopole antenna (SBPSMA) to obtain the ultra-wideband impedance bandwidth for wireless ...
Symmetrically Beveled Ultra-Wideband Planar Monopole Antenna X. N. Qiu, H. M. Chiu and Ananda. S. Mohan* Microwave and Wireless Technology Research Laboratory, I&C Group, Faculty of Engineering, University of Technology, Sydney (UTS) PO Box 123, Broadway, N.S.W. 2007, Australia E-mail: {xnqiu, heng, ananda}@eng.uts.edu.au This paper proposes symmetrically beveled planar square monopole antenna (SBPSMA) to obtain the ultra-wideband impedance bandwidth for wireless applications. Parametric study on beveling parameters was conducted to identify the designs that provide maximum bandwidth. Introduction Recently, there is a renewed interest in Ultra-Wideband (UWB) technology for short range wireless communications due to new spectrum released by FCC. Accordingly, a UWB wireless communication system covers a very wide spectrum of frequencies that ranges from 3.1 to 10.6 GHz. As antennas are key components of any UWB wireless system, it is essential that they have ultra-wideband performance particularly with respect to impedance and radiation characteristics. For the sake of convenience, here we divide the whole UWB frequency band into three “sub” bands. They are (a) lower UWB Band covering frequencies between 3 and 5 GHz, (b) mid UWB Band covering frequencies between 5 and 8 GHz and (c) upper UWB Band covering frequencies between 8 and 10 GHz. Ideally, the impedance bandwidth at these three bands should have a minimum return loss of 10 dB, or lower. Previous studies have shown that UWB performance can be achieved by appropriately modifying the conventional monopole antenna by introducing planar circular and elliptical discs as well as square plates [1, 3]. The circular and elliptical disc planar monopole antennas can cover the whole UWB spectrum as defined by the FCC. Due to their simple geometry and the ease of fabrication, square planar monopole antennas are quite attractive for broadband application. However, the square planar monopole antennas can only achieve the required impedance bandwidth in the lower UWB band, with their overall impedance bandwidth being narrower than that obtained using circular or elliptic planar monopole antennas [1]. A number of authors [2-4] attempted to increase the impedance bandwidth of square planar monopole antennas, by employing techniques such as beveling of the lower side of the monopole [2], use of semi-circular base [3], and cutting of notches at the bottom side [4]. Most of these techniques employ changing of the distance between the lower part of the planar monopole antenna and its ground plane in order to tune the coupling thereby achieving wider impedance bandwidth. However, no one has attempted to study the effects of adding symmetrical beveling or other geometric modification to the upper part of the antenna on the antenna’s impedance bandwidth. In this paper, we aim at the bandwidth improvement of the square planar monopole particularly at the mid and upper UWB bands by modifying the shape of the antenna symmetrically both at lower and upper portions. The impedance characteristics for different cases including differing planar geometries were calculated by the Method of Moments based software named Feko [5]. The antennas were then fabricated and their respective impedance bandwidths
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are measured and compared with those obtained using Feko simulations. The simulated radiation patterns are also presented. Antenna Geometry Simulation and Discussion The geometry of a conventional square monopole antenna without any modification is schematically presented in Fig. 1(a). In order to validate Feko results, a square planar monopole antenna prototype with design parameters taken from [1] was fabricated and the measured results compared with those obtained from simulation. The comparison of the results is presented in the Fig. 1(b) and as shown in the figure, a reasonable agreement can be seen between each other. As can be seen from Fig. 1(b), the conventional square monopole is at best a dual band antenna. To improve its impedance bandwidth, many techniques were proposed which mostly concentrate on the geometry modification of the antenna at the lower side near the ground plane [1-4]. While the geometry modifications (beveling) at the bottom of the antennas improve overall antenna impedance bandwidth, they are often dual-band antennas as their mid UWB band performance is generally poor. In order to improve the mid band impedance bandwidth, we propose symmetrical antenna geometry modification (beveling) at lower and upper sides with respect to plane parallel to the ground plane. Fig. 2(a) shows the original antenna geometry with beveling applied only at the bottom of the antenna (hence forth called as asymmetrically beveled antenna) while in Fig. 2(b) the proposed symmetrical beveled antenna is shown. Fig. 3(a) shows the comparison of the impedance bandwidth performance between the asymmetrical and symmetrically beveled square monopole antennas, simulated using Feko. A prototype of the symmetrically beveled antenna was fabricated with the parameters a = b = 30mm, c = d = 100mm, H = 7mm and W = 15mm and the measured results are also included in Fig. 3(a). As can be seen from the Fig. 3(a), the symmetrical bevelling at the bottom and top portions of the antenna has greatly improved the overall impedance bandwidth. Also good agreement can be seen between the measured and simulated results in the case of the symmetrical beveling design (horizontal solid line indicates the -10dB RL). To further investigate the effect of symmetrical bevelling on impedance bandwidth, we have undertaken a parametric study using Feko and the results are plotted in the Figs. 3(b), (c) and (d) for both asymmetrical and symmetrically beveled antennas. The lower end of the bandwidth can be approximately determined by using the formulation in [6]. As shown in Figs. 3(b) and (c), the overall bandwidth performance of the symmetrically beveled antenna is better than the corresponding asymmetrical one, with the maximum bandwidth reaching as high as 8.4 GHz. However, for a particular choice of parameters the bandwidth becomes uniform: for e.g., when H = 6 to 9mm with fixed width W = 15mm or when the width W = 8 to15mm with the fixed height H = 7mm the bandwidth is maximum and stable at 8.4 GHz. This indicates that the bandwidth becomes insensitive to the beveling geometry for these ranges of parameters. In Fig. 3(d), we can see that over 85% of the UWB bandwidth within the defined FCC range (from 3.1 GHz to 10.6 GHz), a return loss of below -15 dB can be obtained, with the proposed symmetrically beveled monopole antenna for H = 7mm, W = 11mm, a = b = 30mm, c = d = 100mm.
Conclusion A Symmetrically Beveled Planar Square Monopole Antenna (SBPSMA) is proposed for ultra wideband wireless communications. By introducing symmetrical beveling at the top side of the monopole antenna, it is shown that improvement in impedance bandwidth as compared to the existing asymmetrically beveled antennas can be achieved. A parametric study has been performed to identify designs that can maximise the impedance bandwidth and cover the whole UWB spectrum as indicated by the FCC. Acknowledgement This work is partially supported by the Australian Research Council under the Discovery Grant Scheme. The authors would like to thank Dr. Perez Moses for his help with the fabrication and measurement of the antennas. References: [1] M. J. Ammann, “Impedance Bandwidth of the Square Planar Monopole”, Microwave And Optical Technology Letters, Vol. 24, No. 3, Feb 5 2000. [2] M. J. Ammann, “Control of the Impedance Bandwidth of Wideband Planar Monopole Antennas Using A Beveling Technique”, Microwave And Optical Technology Letters, Vol. 30, No. 4, Aug 20 2001. [3] P. V. Anob, K. P. Ray, and Girish Kumar, “Wideband Orthogonal Square Monopole Antennas with Semi-Circular Base”, IEEE Antennas and Propagation Society International Symposium, (2001), Volume: 3, 8-13 July 2001. [4] Saou-Wen Su, Kin-Lu Wong and Chia-Lun Tang, “Ultra-Wideband Square Planar Monopole Antenna For IEEE 802.16a Operation in the 2-11 GHz Band”, Microwave And Optical Technology Letters, Vol. 42, No. 6, Sep 20 2004 [5] Feko User’s Manual, EM Software & Systems –S.A. [6] N. P. Agrawall, G. Kumar, and K. P. Ray, “Wide-band Planar Monopole Antennas”, IEEE Trans. Antennas Propagat, vol. 46, pp. 294-295, 1998. -2 -4 -6
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(a) (b) Fig. 1: (a) the geometry of the planar monopole antenna with a = b = 30mm, c = d = 100mm. (b) Comparison of the simulated and measured results for the planar monopole antenna.
(a) (b) Fig. 2: the geometry of the planar monopole antenna with a = b = 30mm, c = d = 100mm. (a) original antenna geometry with beveling technique applied at the bottom [2], (b) proposed antenna geometry based on [2]. The beveling geometry is defined by H (Height) × W (Width) 0
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(c) (d) Fig. 3: (a) Reflection coefficient comparison of asymmetrical and symmetrical beveling antenna design. (b) Bandwidth comparison of asymmetrical and symmetrical beveling antenna design with W = 15mm and H varying from 1 to 15mm. (c) Bandwidth comparison of asymmetrical and symmetrical beveling antenna design with H = 7mm and W varying from 1 to 15mm. (d) Percentage comparison of impedance bandwidth lower than -15dB in UWB bandwidth of asymmetrical and symmetrical beveling antenna design with H = 7mm and W varying from 1 to 15mm.
