Frequency-Reconfigurable Antenna using Metasurface

Transcription

1 Title Frequency-Reconfigurable Antenna using Metasurface Author(s) Zhu, HL; Liu, XH; Cheung, SW; Yuk, TTI Citation IEEE Transactions on Antennas and Propagation, 2014, v. 62 n. 1, p Issued Date 2014 URL Rights IEEE Transactions on Antennas and Propagation. Copyright IEEE.; 2014 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.; This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

2 80 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 1, JANUARY 2014 Frequency-Reconfigurable Antenna Using Metasurface H. L. Zhu, X. H. Liu, S. W. Cheung, Senior Member, IEEE, and T.I.Yuk,Member,IEEE Abstract A frequency-reconfigurable antenna designed using metasurface (MS) to operate at around 5 GHz is proposed and studied. The frequency-reconfigurable metasurfaced (FRMS) antenna is composed of a simple circular patch antenna and a circular MS with the same diameter of 40 mm (0.67 ) and implemented using planar technology. The MS is placed directly atop the patch antenna, making the FRMS antenna very compact and low profile with a thickness of only mm (0.05 ). The MS consists of rectangular-loop unit cells placed periodically in the vertical and horizontal directions. Simulation results show that the operating frequency of the antenna can be tuned by physically rotating the MS around the center with respect to the patch antenna. The MS placed atop the patch antenna behaves like a dielectric substrate and rotating the MS changes the equivalent relative permittivity of the substrate and hence the operating frequency of the FRMS antenna. Measured results show that the antenna has a tuning range from 4.76 to 5.51 GHz, a fractional tuning range of 14.6%, radiation efficiency and a realized peak gain of more than 80% and 5 dbi, respectively, across the tuning range. Index Terms Frequency reconfigurable antenna, metasurface, metasurfaced antenna, source antenna. I. INTRODUCTION DUE to the demand for integrating multiple wireless standards into a single wireless platform, reconfigurable antenna, also known as tunable antenna, is attracting much attention. In general, the mechanism used to reconfigure the frequency bands of reconfigurable antennas can be mechanical or electrical. Electrically reconfigurable antennas, which are far more popular, can be classified into band switching and continuous tuning. Band switching can be achieved using PIN-diode switches and the operating frequency band is switched among different frequency bands, depending on the switching states [1] [3]. Continuous tuning can be accomplished using varactor diodes and the antennas can be frequency tuned smoothly within the operating frequency bands [4]. In designing these antennas, direct-current (DC) biasing circuits are needed to bias the PIN or varactor diodes. However, there are drawbacks in electrically reconfigurable antennas. For example, the electronic components and circuits in the biasing circuits may have adverse effects on Manuscript received May 06, 2013; revised September 10, 2013; accepted October 23, Date of publication November 01, 2013; date of current version December 31, The authors are with the Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong ( zhuhl@eee.hku.hk; h @eee.hku.hk; swcheung@eee.hku.hk; tiyuk@eee.hku.hk). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TAP the antennas performances, DC electrical sources are needed to drive the PIN or varactor diodes, and the antenna operation is depending on the reliability of the electronic components and the DC sources [4] [12]. In mechanically reconfigurable antennas, the antenna structures consist of movable parts. Frequency tuning is obtained by adjusting the movable parts [13], [14]. The main drawback of such designs was that the overall antenna size required varied with the tuned frequency. Moreover, the actuatorusedtoproduce the mechanical movements was very complicated and occupied much space, which led to a bulky and expensive structure. In fact, change of size and/or shape is the common problem for most mechanically tunable antennas. Metasurface (MS), a two-dimensional equivalent of metamaterial, is essentially a surface distribution of electrically small scatterers [15]. With its succinct planar structure and low cost, MS has wide applications and one of which is on the design of planar antennas. In [16], [17], it was shown that the performance of a patch antenna could be enhanced significantly by placing a MS atop of it. In our previous study in [17], it was demonstratedthatbyaddingaspecially designed MS atop a simple patch/slot antenna, not only the realized gain and return-loss bandwidth ( )would be significantly improved, but also polarization conversion from linear polarization (LP) to circular polarization (CP) could be achieved. The patch/slot antenna (known as the source antenna in such design) together with the MS was called MS antenna. In these designs [16], [17], an air gap was used between the MS and the source antenna, which substantially increased the volume of the MS antenna. In this paper, a novel frequency-reconfigurable metasurfaced (FRMS) antenna constructed by placing a MS atop a patch antenna is proposed. To make the FRMS antenna more compact and low profile, the patch antenna and MS are placed together in direct contact, eliminating the air gap between them. Thus the proposed design has the apparent advantages of compact size, low cost and simple construction. Results of studies show that the operating frequency of the antenna can be continuously tunedbyrotating the MS around the center and relative to the patch antenna. For easy mechanical operation, the shapes of the patch antenna and the MS are made circular with identical size. The frequency-reconfigurable property of the FRMS antenna is also analyzed and explained in this paper. The main differenceintheuseofmsbetweentheproposed FRMS and our previous design in [17] is that, in the proposed FRMS, the MS is used to tune the frequency and polarization of the antenna remains linear (same as the source antenna), while in our previous design in [17], the MS converts LP from the source antenna to CP. The proposed FRMS antenna is X 2013 IEEE

3 ZHU et al.: FREQUENCY-RECONFIGURABLE ANTENNA USING METASURFACE 81 Fig. 1. Geometries of (a) patch antenna (source antenna), (b) metasurface, and (c) unit cell. Fig. 2. Assembly schematic of FRMS antenna. studied and designed to operate at about 5 GHz using the EM simulation tool CST [18]. For verification of simulated results, the FRMS antenna is fabricated and measured using the antenna measurement equipment, Starlab system [19]. Simulated and measured results show good agreements. The antenna can achieve a large tuning range and fractional bandwidth, high gain, high efficiency and good radiation pattern. II. DESIGN OF FREQUENCY-RECONFIGURABLE METASURFACED ANTENNA The FRMS antenna proposed here consists of a patch antenna (as the source antenna) and a metasurface (MS). For easy reconfigurable operation of the antenna, the patch antenna and MS are designed to have a circular shape and occupy the same area. The patch antenna is designed on a double-sided substrate using planar technology as shown in Fig. 1(a), while the MS is designed on a single-sided substrate, as shown in Fig. 1(b), and composing of a number of rectangular-loop unit cells as shown in Fig. 1(c). The unit cells are placed periodically along the -axis and -axis directions on the - plane as shown in Fig. 1(b). As will be seen later, frequency reconfigurability of the antenna can be achieved by rotating the MS around the center relative to the patch antenna. The rotation angle is measured from the -axis as shown in Fig. 1(b). Due to horizontal and vertical symmetries of the MS, and the maximum rotation angle without repeating is 90. In assembling the FRMS antenna, the non-copper side of the MS is placed in direct contact with the radiator of the patch antenna as shown in Fig. 2. This leads to a very compact and low profile structure. The antenna is microstrip-fed using a 50- feed-line. An SMA connector is fed to the feed-line through the ground plane and substrate material. The FRMS antenna together with the SMA connector is studied and designed on a Rogers substrates RO4350B, having a thickness of mm and a relative permittivity of. The dimensions are listed in Table I which is used to fabricate the FRMS antenna as shown in Fig. 3 for measurement. III. ANALYSIS OF FRMS ANTENNA As will be shown later by the simulated and measured results, rotating the MS with respective to the source antenna will change the resonant frequency of the FRMS antenna. Here we attempt to analyze such frequency-reconfigurable property. The Fig. 3. Prototypes of (a) patch antenna and metasurface and (b) FRMS antenna. TABLE I DIMENSIONS OF FRMS ANTENNA (UNIT: mm) source antenna used in our FRMS antenna is a patch antenna which radiates linearly polarized signal and has a fixed fundamental resonant frequency. In our analysis, we start with using an MS with infinite size, as shown in Fig. 4(a). A linearly polarized plane wave, with the polarization direction making an angle with the -axis of the unit cell as shown in Fig. 4(b), is applied onto the non-copper side of the MS. Computer simulation is used to obtain the parameters of the MS. The equivalent impedance and reflective index of the MS are then calculated, respectively, using [20] [22] where, in (2),, is the equivalent thickness of the MS, and is the wave number. Note that the calculated and are complex. For practical dielectric materials such as the Rogers substrate used in our design, the signs in (1) and (2) are determined by the requirements that and [20], where and denote the real-part and imaginary-part operators, respectively. Using (1) and (2), the (1) (2)

4 82 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 1, JANUARY 2014 Fig. 4. (a) Simulation model of MS with infinite size incident by linear -field, and (b) -field having angle with -axis of unit cell. Fig. 6. Calculated equivalent (a) and (b) for different. Real part: red 10, green 35,blue 55 and pink 80,and imaginary part: red 10, green 35, blue outline star on a solid line 55 and pink 80. Fig. 5. Simulated (a) and (b) for different. Magnitude, red :10, green :35,blue :55 and pink :80, and Phase, red :10, green :35, blue outline star on a solid line:55 and pink :80. equivalent relative permittivity and permeability of the MS can be calculated using [20] Since the MS assumed here has an infinite size, periodic boundary has to be used in computer simulation to evaluate the parameters for (1) and (2). By setting the appropriate boundary conditions in CST, the infinitely large periodic structure shown in Fig. 4(a) can be represented by using only one single unit cell [23], which significantly speeds up the simulation process. The simulated and with,35,55 and 80 for the frequency band from 4 to 6 GHz are shown in Fig. 5. (3) Using (1), (2) and (3) and the results of Fig. 5, the computed and using (3) for different are shown in Fig. 6. It can be seen in Figs. 5 and 6 that affects and and hence the calculated (or equivalent) and as indicated in (1) (3). Fig. 6(a) shows that, for different values of, is relatively stable at approximately 1, just like any practical dielectric substrates. However, Fig. 6(b) shows that, as increases from 10 to 80, the equivalent drops from around 14.5 to 7.2, respectively. Note that is in fact equivalent to the rotation angle in our proposed FRMS antenna as shown in Fig. 1. Thus in our proposed FRMS antenna, rotating the MS by with respective to the source antenna will reduce the equivalent relative permittivity of the MS, hence increase the equivalent signal wavelength and shift up the resonant frequency of the antenna. This will be verified by the measured and simulated results showed later. These results show that the MS could be regarded as a special dielectric slab which has a variable relative permittivity when illuminated by a linearly polarized plane wave. The value of depends on the polarization direction of the plane wave. To further study this, we compare the resonant frequency of our proposed FRMS antenna with that of a patch antenna having a dielectric slab atop of it. The computer model used for this study is shown in Fig. 7, which is obtained from Fig. 1 by removing the copper layer of MS. The dielectric slab has a thickness of mm (same as that of the MS substrate used in our FRMS antenna) and is denoted as substrate in Fig. 7. In the study, the simulated of our proposed FRMS antenna with different

5 ZHU et al.: FREQUENCY-RECONFIGURABLE ANTENNA USING METASURFACE 83 TABLE II SIMULATED AND CALCULATED EQUIVALENT RELATIVE PERMITTIVITY FOR DIFFERENT MS ROTATION ANGLES Fig. 7. Computer model for obtaining equivalent. Fig. 9. Simulated and measured with different rotation angles. 10 :red sim red mea; 25 : green outline pentagon on a dashed line sim green pentagon on a solid line mea; 35 :brown outline star on a dashed line sim brown mea; 55 :blue sim blue mea; and 80 pink sim pink mea. Fig. 8. Simulated with different rotation angles and. rotation angles are first obtained as shown by the dash lines in Fig. 8. It can be seen that the rotation angles of 80,55,35 and 10 lead to the resonant frequencies of 4.77, 5.07, 5.27 and 5.51 GHz, respectively. Then the relative permittivity of substrate in the model of Fig. 7 is adjusted in order to obtain the same resonant frequencies in. In the study, the relative permittivity of the substrate for the patch antenna remains constant at. Results are also shown by the solid lines in Fig. 8 for comparison. It can be seen that rotating the MS from to 55,35 and 10 is equivalent to changing the relative permittivity of substrate from to 7.5, 9.5, and 13, respectively. For further comparison, we also calculate of the MS at these rotation angles using the MS model shown in Fig. 4(a) and (1) (3). The results, which have already been shown in Fig. 6(b), are listed in Table II for comparison. There are slightly differences between the simulated and calculated, which can be explained as follows. A plane wave is assumed as incident wave in Fig. 4(a), which is not the case in our FRMS antenna, andthemsisassumedtobeinfinitely large in Fig. 4(a) while the MS in our proposed FRMS antenna has a finite size. Despite the differences, the method using the parameters to calculate the equivalent permittivity is still very instructive in the design of our FRMS antenna. IV. SIMULATION AND MEASUREMENT RESULTS A. Frequency Reconfigurability The frequency reconfigurability of the FRMS antenna is studied using the reflection coefficient. The simulated and measured of the antenna with different rotation angles are shown in Fig. 9. It can be seen that the simulated and measured resonant frequencies agree very well. The resonant frequency is proportional to the rotation angle for the reason explained previously. As the rotation angle increases from 10 to 25,35,55 and 80, the resonant frequency shifts up from 4.77, to 4.9, 5.07, 5.31 and 5.51 GHz, respectively. The FRMS antenna has the best matching condition when the rotation angle is about 55 which corresponds to of the MS (see Table II). As moves away from 55, the matching condition degrades due to the change of the equivalent, showed in Table II. To investigate the relationship between the rotation angle and the resonant frequency, study is carried out for to 90 at a step of 5. The simulated and measured results in Fig. 10 again show good agreements. The resonant frequency is proportional to the rotation angle, which can be explained as follows. In the design of MS, one of the important parameters determining the property is periodicity of unit cells on the MS. In our proposed MS shown in Fig. 1, the unit cells have a rectangular shape and are placed periodically along the -and -axes. In the co-polarization direction of the patch antenna (i.e., along the -axis), the unit cells have the lowest periodicity at and the highest periodicity at. As the rotation angle changes from 0 to 90, the periodicity changes from the lowest to the highest, which corresponds to changing from maximum to minimum. As a result, the resonant frequency of the FRMS antenna is proportional to the rotation angle. It can be seen that the resonant frequency is quite linearly related to the rotation angle. Using linear regression, the relationship can be approximated by a straight line with a slope of 10.1 MHz/degree as shown in Fig. 10. With the rotation angle increased from to 90, the resonant

6 84 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 1, JANUARY 2014 Fig. 10. Resonant frequency vs. rotation angle. Fig. 11. Simulated and measured efficiencies at different rotation angles. 10 : red sim red mea; 35 :green outline star on a dashed line sim green mea; 80 :blue sim blue mea. frequencyshiftsupcontinuouslyfrom4.76to5.51ghz,with a fractional tuning range of 14.6%. B. Efficiency, Radiation Pattern and Realized Gain The antenna efficiency, radiation pattern and realized gain of the FRMS antenna at the resonant frequencies of 4.77, 5.07 and 5.51 GHz (which span the whole reconfigurable-frequency range) corresponding to,35 and 80, have been studied using simulation and measurement. The simulated and measured efficiencies are shown in Fig. 11. Here the efficiency is the total efficiency defined as the ratio of the radiated power from the antenna to the input power to the antenna. It can be seen that the simulated and measured efficiencies agree well and are above 80% at the resonant frequencies. The simulated and measured radiation patterns of the FRMS antenna at the resonant frequencies of 4.77, 5.07 and 5.51 GHz are shown in Fig. 12. It can be seen that the antenna has directional radiation patterns, same as a typical patch antenna. The front-to-back ratios are all large than 15 db. Co-polarization (linear polarization along the -axis) is much stronger than cross polarization (linear polarization along the -axis), with polarization isolation of more than 10 db. Thus the MS simply shifts the resonant frequency without affecting much the shape of radiation pattern or polarization. The simulated and measured realized gains of co-polarization at the same resonant frequencies are shown in Fig. 13. Again Fig. 12. Simulated and measured radiation patterns at resonant frequencies (a) & (b) 4.77 GHz, (c) & (d) 5.1 GHz, and (e) & (f) 5.5 GHz, corresponding to,35 and 80.pink : sim co-pol, pink : sim X-pol, black : mea co-pol, black : mea X-pol). (a) 4.77 GHz. (b) 4.77 GHz. (c) -plane@5.07ghz.(d) 5.07 GHz. (e) 5.51 GHz. (f) 5.51 GHz. Fig. 13. Simulated and measured realized gains at different rotation angles. 10 :red sim red mea; 35 :blue sim blue mea; 80 : green outline star on a dashed line sim green mea. very good agreements are observed. The measured gains are 5.3, 5.5 and 5.4 dbi at the resonant frequencies of 4.77, 5.07 and 5.51 GHz, respectively.

7 ZHU et al.: FREQUENCY-RECONFIGURABLE ANTENNA USING METASURFACE 85 V. CONCLUSIONS A FRMS antenna designed using a patch antenna and a MS has been presented. The resonant frequency of the antenna can be mechanically reconfigured by rotating the MS around the center with respect to the patch. The frequency-reconfigurable property has been analyzed and explained. The simulated and measured performances in terms of frequency reconfigurability, efficiency, gain and radiation pattern, have been presented. Results have shown that the resonant frequency can be tuned continuously over a range of 750 MHz at around 5 GHz. REFERENCES [1] H. F. AbuTarboush et al., Areconfigurable wideband and multiband antenna using dual-patch elements for compact wireless devices, IEEE Trans. Antennas Propag., vol. 60, pp , [2] Z.J.Jin,J.H.Lim,andT.Y.Yun, Frequencyreconfigurable multiple-input multiple-output antenna with high isolation, IET Microw., Antennas Propag., vol. 6, pp , [3] A. Sheta and S. F. 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Zhu received the B.Eng. degree in information engineering and the Master s degree in electromagnetic field and microwave engineering from Beijing Institute of Technology, Beijing, China, in 2009 and 2011, respectively. He is currently working toward the Ph.D. degree in electrical and electronic engineering at the University of Hong Kong, Hong Kong, China. His research interests include antenna design and study of metasurface. X. H. Liu received B.Eng. degree in photoelectric information engineering from Shenzhen University, Shenzhen, China, in 2012 and the M.Sc degree in electrical and electronic engineering (communications engineering stream) from The University of Hong Kong, Hong Kong, China, in His research interests include antenna design and study of metasurface. S. W. Cheung (SM 08) received the B.Sc. degree (with First Class Honours) in electrical and electronic engineering from Middlesex University, U.K., in 1982 and the Ph.D. degree from Loughborough University of Technology, U.K., in From 1982 to 1986, he was a Research Assistant in the Department of Electronic and Electrical Engineering, Loughborough University of Technology, where he collaborated with Rutherford Appleton Laboratory and many U.K. universities to work a project for new generations of satellite systems. He is now an Associate Professor at the University of Hong Kong and in charge of the Microwave, RF Frequency and Telecom Laboratories. His current research interests include antenna designs, 2G, 3G and 4G mobile communications systems, MIMO systems and satellite communications systems. Dr. Cheung has been serving the IEEE in Hong Kong for the past twenty years. In 2009 and 2010, he was the Chairman of the IEEE Hong Kong Joint Chapter on Circuits and Systems and Communications. He was the Honorary Treasurer and currently the Chair-Elect of the IEEE Hong Kong Section. T. I. Yuk (M 86) received the BS degree from Iowa StateUniversity,Ames,IA,USA,in1978 and the MS and Ph.D. degrees from Arizona State University, AZ, USA, in 1980 and 1986, respectively. Since 1986, he has been a Lecturer at the University of Hong Kong. His current research interests include: wireless communications and antenna designs.