Compact CPW UWB Pattern Diversity Antenna with Dual Band-notched Characteristics

Transcription

1 Compact CPW UWB Pattern Diversity Antenna with Dual Band-notched Characteristics Rong Su 1,2, Peng Gao 1,2, Shuang He 3 and Peng Wang 1,2 1.Information Geoscience Research Center 2.Research Institute of Electronic Science and Technology University of Electronic Science and Technology of China Chengdu , China ( penggao@uestc.edu.cn) 3. TP-Link Technologies Co., Ltd. Abstract A compact printed CPW-fed ultra wideband (UWB) pattern diversity antenna for MIMO applications with dual band-notched characteristics is proposed. This antenna consists of two modified coplanar waveguides feeding rectangle radiating elements, with a stub is inserted at 45 axis to ensure high isolations. Two split-ring resonators on each radiating element are etched to generate notched bands for ceasing the potential interference from WiMAX systems and WLAN systems. Measured results show the antenna meets a bandwidth from GHz, except for two notched bands at GHz and GHz. The orthogonal radiation patterns and corresponding envelope correlation coefficient denote it is suitable for MIMO/diversity systems. Besides, it has a compact size of 50mm 50mm, which is suitable for mobile devices. 1. INTRODUCTION Ultra wideband (UWB) technologies have been proposed and demonstrated as good candidates for wireless high-data-rate communications. Nevertheless, many challenges are still being faced and improved. For instance, conventional UWB systems have met multi-path problems, which could be solved by multiple-input multiple-output (MIMO) technology [1]. As one of the key components in a typical transceiver, the requirement of designing UWB MIMO antennas includes wideband, high isolation, as well as diversity characteristics, which has been widely studied previously [2-5]. Meanwhile, in the commercial UWB band from GHz permitted by Federal Communications Commission (FCC) [6], there are other narrow band wireless systems, which may cause potential interference. It is possible to add regular resonate structures on the antennas to realize corresponding notched frequency [7-8], or multiple frequencies [9-11]. However, compact UWB diversity antenna with band-notched characteristics has seldom been reported, due to the rejecting resonators and decoupling structures usually strongly impact each other, which makes it very difficult to achieve these demands in limited space. Recently although one academic paper realized a printed monopole UWB MIMO antenna with band notched function, it has a size of 55mm*100mm. Meanwhile, only one band (WLAN at 5.5 GHz) is rejected [12]. In this case, there is still much scope and need for space reduction and enhanced performance, which is very important and more challenging [13]. In this paper, we propose a compact printed UWB pattern diversity antenna with dual bandnotched characteristics. Two split-ring resonator (SRR) slots on each radiating element are etched to generate notched bands for WiMAX system and WLAN system. Moreover, a rectangle stub is inserted at 45 axis to achieve high isolation performance. The detail is presented and discussed as follow. 2. ANTENNA DESIGN

2 2.1 Antenna Configuration The geometry of the proposed antenna is shown in Fig.1 and. This antenna has a compact size of 50mm 50mm, thickness of 0.8 mm, which is printed on an FR4 substrate, with a relative permittivity of 4.4 and loss tangent of It consists of two modified rectangle radiating elements, both fed by coplanar waveguides. By etching two right triangles, the exciting ports are designed to match 50-Ω load impedance. The ground plane is made by a 1/4 rectangle slot, with a rectangle stub is inserted to extend the effective current route, which efficiently enhances the isolation characteristics. Besides, there are two etched slots on each radiating element, to generate the band rejection characteristics. (c) Fig. 1. Geometry and photograph of proposed antenna Geometry from the top view and side view. Geometry of the radiation patch.(c) Photograph of fabrication The results are implemented and optimized by commercial software Ansoft HFSS V13. The dimensions of the proposed antenna are finalized as follows: w=l=50 mm, l1=8 mm, l2=36 mm, l3=32 mm, l4=30 mm, h=9.2 mm, h1=12 mm, h2=4.2 mm, h3=5 mm, w1=12 mm, w2=3 mm, wd=1 mm, g1=0.5 mm, g2=3 mm, g3=0.5 mm, g4=1.4 mm, r1=4.1 mm, r2=2.6 mm, α=52 deg, β=45.2 deg. 2.2 Antenna Design and Analysis A. Evolution of the proposed antenna Fig.2 shows the revolution of the proposed antenna and corresponding simulated return losses (S11/S22) and insertion loss (S12/S21). The antenna consists of two square radiating elements to achieve orthogonal radiation patterns, and the first split-ring is etched on radiation elements. The relationship between the length of the slots and the notched band can be acquired by (1): L 2 f center c 1 r 2 (1) Here f center is the frequency of resonance; ε r is the dielectric constant; c is speed of the light. To achieve a notched band at 3.5GHz, the length of ring1 is L1=26 mm. Fig.3 shows the simulated S- parameters. As illustrated, antenna has a notched band from 3.1GHz to 3.65GHz of WiMAX systems. Meanwhile, ring2 is introduced for ceasing WLAN system at 5.5GHz, whose length is 16.5 mm. The antenna has a impedance bandwidth from 2.32GHz to 12GHz expect for the two notched bands at GHz and GHz. Besides, the split-rings have little influence of

3 the isolation between two ports. The center frequency of notched bands is affected by the length of the split-rings which is determined by the radius of the rings. To better express the design process of the proposed antenna, the effects of radius are plotted in Fig.4 and Fig.5. Fig.4 shows the simulated S-parameters when the radius r1 of the ring1 changes. It is found that the notched band at WiMAX is shifted to lower frequencies when r1 increases. The change of r1 hardly impacts the center frequency of notched Fig.2 Evolution of the proposed antenna a and b band at 5.5GHz. In this case, the practical radius is set to 4.1 mm, to ensure best performances. Fig 4 plots the simulated current distributions of antenna at the frequency of 3.5GHz. It is clearly seen that the current is concentrated around the split-ring, where the band notched characteristics are obviously produced. Simulate results of ring2 is described in Fig.5 when the radius r2 varies from 2.4 mm to 2.8 mm. The results also show that the center frequency of notched band is lower when the radius increased. Finally, 2.6 mm is chosen as the radius of ring 2 to achieve better rejection around a center frequency of 5.5GHz. Fig.5 illustrates the simulated current distributions when ring 2 is introduced. Most current is blocked by two split-rings. Fig.3 Simulated S-parameters of antenna a and b B. Rectangle stub A rectangle stub is inserted on the ground at 45 to improve the scattering parameters. It is introduced as a reflector to reduce the mutual coupling by separating the radiation patterns of the two radiators. The result is shown in Fig.6. In Fig.6, when the stub is present, the start frequency of the operating band is shifted from 2.4GHz to 2.3GHz because the effective current path is lengthened. Fig.6 shows the change of the isolation between the two ports. The S-parameters across the

4 whole working band have reduced to less than -15dB. It is indicated that the proposed antenna is suitable for MIMO/diversity applications. Furthermore, the current distribution with stub and without stub at the resonant frequency of 6GHz is depicted in Fig.7. It is observed that the current flowing from port 1 to port 2 is blocked by the stub. This explains why the stub can reduce the mutual coupling between two ports. The effect is the same from port 2 to port 1. Both ports are matched with 50Ω impedance loads. Fig.4. Effect of the ring 1. Simulated return loss when r1 changes.. Simulated current when ring 1 is introduced Fig.5. Effect of the ring 2. Simulated return loss when r2 changes.. Simulated current when ring 2 is introduced. Fig. 6. Simulated S-parameters of proposed antenna when the stub varies: S11 and S22, S12 and S Results and Discussion

5 A. Impedance Bandwidth The proposed antenna is fabricated, as is shown in Fig.1 (c), and tested. The frequency re sponse is measured by Agilent E8363B vector network analyzer (VNA). Simulated and Fig.7. Simulated current distributions with and without rectangle stub at 6GHz. measured S-parameters of this antenna are given in Fig. 8. It is found that the antenna has an impedance bandwidth from GHz, namely when the return loss is better than 10dB, expect for two notched bands of GHz and GHz. Besides, the isolation between the two exciting ports is higher than 15 db in the whole working band. Measured return losses are with acceptable discrepancies with simulations, which are caused by soldering connectors and fabrication tolerance. Fig.8. Simulated and measured S-parameters. Measured S11/S22 of proposed antenna. Measured S12/S21 of the proposed antenna B. Radiation Performance Fig.9 shows the radiation patterns at 2.5, 6.5 and 10.5 GHz when port 1 is excited and port 2 is terminated by a 50Ω load, and vice versa. The radiation of each port is less directional in the Port1 XY-plane Port2 XY-plane Port1 XZ-plane Port2 XZ-plane

6 Port1 YZ-plane (c) Port2 YZ-plane Fig.9. Measured radiation pattern of the proposed antenna at 2.5, 6.5, 10.5 GHz. XY plane. XZ plane. (c)yz plane. H-plane (YZ-plane of port 1 and XZ-plane of port 2) with monopole-like radiation pattern in the E- plane (XZ-plane of port 1 and YZ-plane of port 2). The XZ-plane of port 1 and YZ-plane of port 2 are similar, as for the XZ-plane of port 2 and YZ-plane of port 1. Thus the two ports radiate a vertical polarization wave and a horizontal polarization wave respectively. In addition, the measured peak gain of proposed antenna is shown in Fig.10. As expected, this antenna has a stable peak gain through the working band, while two sharp decreases are observed in the vicinity of 3.5 and 5.5 GHz. This also proves they are significantly rejected. Fig.10.Measured peak gain of the proposed antenna. Fig.11. Measured envelope correlation coefficient C. Diversity Performance Furthermore, the envelope correlation coefficient (ECC) of the exciting ports is computed using (2) [14]. The ECC of the proposed antenna is below 0.01 across the whole operating band from 2.3 to 12 GHz as shown in Fig. 11, which also denotes the antenna is suitable for diversity system. ECC * * S S S S ( S11 S21 ) 1 ( S22 S12 (2) 4. Conclusion A compact CPW-fed UWB MIMO/diversity slot antenna with dual band-notched characteristics is

7 presented in this paper. This planar antenna has a compact area of 50*50 mm 2. By etching two SRR slots on radiating patch, two notched bands at GHz and GHz are realized. Meanwhile, a rectangle stub is inserted to achieve high isolations. Both measured and simulated return losses show an impedance bandwidth from GHz, with favorable orthogonal radiation patterns and flat gain in the operating band. Therefore, it is suitable for portable MIMO/diversity UWB application. References [1] T. S. P. See, and Z. N. Chen, An Ultrawideband Diversity Antenna, IEEE Trans. Antennas Propag., vol. 57, no. 7, Jun. 2009, pp [2] S. Zhang, Z. Ying, J. Xiong, and S. He, Ultrawideband MIMO/Diversity Antennas with a Tree- Like Structure to Enhance Wideband Isolation, IEEE Antennas Wireless Propag. Lett., vol. 8, Nov. 2009, pp [3] K. Wei, Z. Zhang, W. Chen, and Z. Feng, A Novel Hybrid-Fed Patch Antenna with Pattern Diversity, IEEE Antennas Wireless Propag. Lett., vol. 9, Apr. 2010, pp [4] E. Antonino-Daviu, M. Gallo, B. Bernardo-Clemente, and M. Ferrando-Bataller, Ultrawideband Slot Ring Antenna for Diversity Applications, Electron. lett., vol. 49, no. 7, 2010, pp [5] M. Gallo, E. Antonino-Daviu, M. Ferrando-Bataller, M. Bozzetti, J. M. Molina-Garcia-Pardo, L. Juan-Llacer, A Broadband Pattern Diversity Annular Slot Antenna, IEEE Trans. Antennas Propag., vol.60, no.3, 2012, pp [6] FCC, First Report and Order, revision of Part 15 of the Commission s Rules regarding Ultra Wideband Transmission Systems, FCC02-48, April 2002 [7] Y.J. Cho, K.H. Kim, D. H. Choi, S. S. Lee, and S.O. Park, A Miniature UWB Planar Monopole Antenna With 5-GHz Band-Rejection Filter and the Time-Domain Characteristics, IEEE Trans. Antennas Propag., vol.54, no.5, 2006, pp [8] M. Ojaroudi, Sh. Yazdanifard, N. Ojaroudi, and R. A. Sadeghzadeh, Band-Notched Small Square- Ring Antenna With a Pair of T-Shaped Strips Protruded Inside the Square Ring for UWB Applications, IEEE Antennas Wireless Propag. Lett., vol.10, 2011, pp [9] R. Zaker, C. Ghobadi, and J. Nourinia, Bandwidth Enhancement of Novel Compact Single and Dual Band-Notched Printed Monopole Antenna With a Pair of L-Shaped Slots, IEEE Trans. Antennas Propag., vol.57, no.12, 2009, pp [10] P. Gao, L. Xiong and S. He, Compact Printed Wide-Slot UWB Antenna With 3.5/5 GHz Dual- Notched Characteristics, IEEE Antennas Wireless Propag. Lett., vol.12,2013,pp [11] T.D Nguyen, D.H Lee and H.C Park. Design and Analysis of Compact Printed Triple Band- Notched UWB Antenna, IEEE antennas wirel. propag. Lett., vol.10, 2011, pp [12] J.M., Lee, K.B., Kim, H.K., Ryu, and J.M., Woo, A Compact Ultrawideband MIMO Antenna with WLAN Band-Rejected Operation for Mobile Devices, IEEE Antennas Wireless Propag. Lett., vol.11, 2012, pp [13] S. Lucyszyn, S. Pranonsatit, RF MEMS for Antenna Applications, 7th European Conference on Antennas and Propagation(EUCAP 2013), 2013, pp [14] S. Blanch, J. Romeu, and I. Corbella, Exact Representation of Antenna System Diversity Performance from Input Parameter Description, Electron. Lett., vol.39, no.9,2003, pp