Researches on Reconfigurable Antenna in CEMLAB at UESTC

Transcription

1 Sep Journal of Electronic Science and Technology of China Vol.4 No.3 Researches on Reconfigurable Antenna in CEMLAB at UESTC WANG Bing-zhong, IAO Shao-qiu, HANG ong, ANG ue-song, WU Wei-xia School of Physical Electronics, University of Electronic Science and Technology of China Chengdu China Abstract This paper summarizes the achievement and progress in the research on reconfigurable antenna since 2001, in Computational Electromagnetics Laboratory (CEMLAB) at University of Electronic Science and Technology of China (UESTC). Several typical reconfigurable antennas are introduced, which can realize frequency, pattern or frequency-pattern reconfigurability by electrically controlling methods. Some techniques involved in the design and analysis of reconfigurable antennas are reported. At last, the development trend of reconfigurable antenna is predicted in the conclusions. Key words reconfigurable antenna; frequency; pattern In modern radar and communication systems, various antennas are installed on a single platform, such as airplane, ship and satellite, for the purpose of communication, navigation, guidance, etc. Obviously, these antennas will increase the weight and cost of the systems. Furthermore, there are serious electromagnetic interferences between various antennas, which will severely impact the normal operation of the system. It is most desirable that all of the required functions can be achieved through a minimum number of antennas in order to reduce the weight and cost of systems, decrease radar cross section, and achieve a good electromagnetic compatibility performance [1-2]. The concept of reconfigurable antenna is presented due to the driving of the practical requirements. A common radiation aperture is used for multiple functions by reconfigurable operating states of the antenna. The reconfigurable capability can be achieved by using Micro Electromechanical System (MEMS) es, PIN diode es, or multi-port feeding. In 1983, the concept of reconfigurable antenna was proposed in an USA patent: frequency-agile, polarization diverse microstrip antenna and frequency scanned arrays [3]. In 1999, Defense Advanced Research Projects Agency (DARPA) started a research program: Reconfigurable Aperture Program (RECAP) [4]. During the last several years, researches on reconfigurable antenna have got a rapid and great progress. In 2001, the first reconfigurable antenna project in China was supported by the High-Technology Research Program ( 863 Program, under contract No. 2001AA123043). This project, Electromagnetic Theory and Key Techniques of Reconfigurable Antenna, has been carried out by researchers in Computational Electromagnetics Laboratory (CEMLAB) at University of Electronic Science and Technology of China (UESTC). In addition, since 2003, researches on reconfigurable antenna in CEMLAB have got the support from National Natural Science Foundation of China (NSFC) and other foundations (under contract No , No , and No D0219). The paper summarizes the achievement and progress in the research of reconfigurable antenna since 2001 in CEMLAB. 1 Some Typical Reconfigurable Antennas Reconfigurable antenna can be classified into three types. The first is frequency reconfigurable antenna, the second is pattern reconfigurable antenna, and the third is the antenna that can reconfigure both operation frequency and pattern simultaneously. In this section, some typical reconfigurable antennas designed in CEMLAB are depicted briefly. Each of them has good reconfigurable characteristics. More details about these antennas can be referred to the literatures in the references. 1.1 Frequency Reconfigurable Antenna Frequency reconfigurable antenna can reconfigure its operation frequency with almost fixed radiation patterns. It can be realized by changing antenna Received

2 226 resonant length. This reconfigurable antenna can be applied in multi-frequency or frequency-agile systems Rectangular Patch Antenna [5] It is well known that a rectangular patch antenna loaded on the nonradiating edges can operate at different frequencies when the depth of the loadedslots is changed [6]. According to this characteristic, reconfigurable patch antenna is proposed for multiband applications. The reconfigurable antenna is shown schematically in Fig.1. There are 6 and 8 es installed symmetrically in each short slit (slit1-1, slit2-2, slit6-6 ) and each long slit (slit3-3, slit4-4, slit5-5 ), respectively. By controlling these es on/off in different compages, the antenna can work with fifteen states and shift its operating frequencies over a one-octave bandwidth from 0.6 GHz to 1.2 GHz, while maintaining its radiation pattern stable. slot slit6 feed line w f slit6 slot slit1 slit2 slit3 slit4 slit5 d 1 d d d d d d d d d I slit1 slit2 slit3 slit4 1 d slit5 s b Fig.1 The frequency reconfigurable rectangular patch antenna E-shaped Patch Antenna [7] In the antenna presented above, the bandwidth for each state is so narrow that many states are used to cover a wide frequency band. This disadvantage can be overcome by widening the bandwidth of each reconfigurable state. Ref.[7] has proposed an improved frequency reconfigurable antenna. The antenna geometry is shown in Fig.2. The radiation patch consists of an E-shaped center patch and a U-shaped outer patch. There is a U-shaped slot between these two patches. Fifteen es are installed in the U-shaped slot and two es are installed in each parallel slits, respectively. State 1 is that all the es in the U-shaped slot are turned off and all the es in the two parallel slits are turned on. State 2 is opposite to State 1. A prototype has been made in the and Ku bands and the measured results demonstrate that the antenna s operation frequency band can shift between 9.2 ~15.0 Journal of Electronic Science and Technology of China Vol.4 slot L a GHz and 7.5~10.7 GHz with two alternative operation states. The two states can obtain relative bandwidths of 48% and 35%, respectively, and have good co-polarization radiation characteristics. This reconfigurable patch antenna is applicable to reconfigurable wideband communication and radar systems. U slot feed Fig.2 The frequency reconfigurable E-shaped patch antenna 1.2 Pattern Reconfigurable Antenna When radiation pattern of the antenna can be reconfigured while retaining it s operating frequency, it can be called pattern reconfigurable antenna. Pattern reconfigurable antenna can enable systems to avoid noisy environments, maneuver away from electronic jamming, improve security, and save energy by better directing signals toward intended users. Port 1 W s S Δ P L s W Shielding metal plate slit ε r ε 0 h perturbation slit h 1 h 2 center line O L center line Fig.3 The pattern reconfigurable CPW leak-wave antenna Port 2

3 No.3 WANG Bing-zhong, et al.: Researches on Reconfigurable Antenna in CEMLAB at UESTC CPW Leaky-wave Antenna [8] A pattern reconfigurable CPW leaky-wave antenna is presented for millimeter-wave application. The leaky-wave antenna structure is shown in Fig.3. The perturbation slits with distance ΔP are etched on the of CPW. A perturbation period P = pδ P (p is a positive integer) can be obtained by making the es opened every p es and the others closed. The maximum radiation direction of this periodic structure leaky-wave antenna can be determined by θ = arcsin( λ / λ λ / ) (1) m 0 g n 0 P λ 0 is the free-space wavelength, λ g is the guided wavelength inside the unperturbed CPW and n is a positive integer and denotes the order of the spatial harmonic fast-wave. It is easy to scan the antenna pattern by reconfiguring the period P. A mixed period structure is presented to reduce the scanning angle step and es are used to adjust the period. Combining the mixed periodic states and using two feed ports, the antenna can scan its pattern from -90 o ~90 o. When Port 1 is the feed port and Port 2 is loaded with a matched impedance, the main beam can scan from -90 o ~ 1 o through nine states. Due to the symmetry of the antenna structure, when Port 2 is the feed port, the single beam can scan from 1 o ~90 o by using the above nine states Loop Slots Antenna [9] A reconfigurable microstrip antenna etched with loop slots that change radiation pattern while maintaining operating frequency has been designed. The pattern reconfigurable loop slots antenna is shown in Fig.4. There are three 1mm wide metal loops and three 1mm wide slot loops placed by turn, surrounding a 2 mm 2 mm metal patch. There are 12 es symmetrically embedded in the right and left part of the slot loops. metal loop substrate coaxial feed slot loop Fig.4 The pattern reconfigurable loop slots antenna State 1 is that all the es in the left-side are turned off and others in the right-side are turned on. State 2 is opposite to State 1. Both of the two states operate at 8.0 GHz. State 1 can provide two quasiconical beams in the first and third quadrants of the upper half space. One of quasi-conical beams points at (θ, φ)=(40º, 22º), with 70º 43ºbeamwidth. The other points at (θ, φ)=(33º, 226º), with 61º 75ºbeamwidth. As State 1and State 2 are symmetric with respect to the - plane, radiation pattern of State 2 can be obtained according to State 1.The top patch area of the antenna is less than 0.38λ 0.38λ at 8 GHz. metal driven patch and side view B N S substrate (b) Different states of parasite Fig.5 The pattern reconfigurable agi microstrip antenna agi Microstrip Antenna [10] The scheme of the reconfigurable agi microstrip antenna is shown in Fig.5. The antenna has one driven element and four parasitic elements. Each EBG cell consists of one narrow slot and four rectangles, with 3 es installed along the slot symmetrically, one is in the middle and the other two are between the outer and the inner rectangles. By changing the states of the es in the EBG cell on the parasite, three states of the parasite can be obtained. The first state is B, in which all es in the cell are open. The second is N, in which all es are closed. The third is S, in which the middle is open and the other two es are closed. The modes of the antenna can be depicted by the states of the parasitic patches from the left to the right. In Mode-1, the parasite state is NNNN and the radiation pattern is broadside. In Mode-2, the parasite state is SSBN with one reflector at the right side. In Mode-3 the parasite state is NBSS with one reflector at the left side.

4 228 Journal of Electronic Science and Technology of China Vol.4 The three modes have a common bandwidth of 9.15~9.45GHz with S 11 below 10dB. The beam maximums of the three modes at frequency 9.3GHz are at elevations of 7º, +33.5º, and 40º, respectively, and the 3-dB bandwidths can cover the elevation range from 69ºto +67.5ºin the E-plane Fractal Microstrip Antenna Fractal shape antennas have drawn interests of many researchers [11]. These antennas usually have various advantages, such as wide-band, multi-band, and reduced antenna size. Fractal Hilbert curve is one of the most recent geometries to be studied for antennas. Some researchers have applied Hilbert line antennas in reconfigurable systems to reduce the size of antennas and change the radiation patterns [12]. The 3 rd -order Hilbert curve has been used to configure a microstrip antenna. By controlling the es integrated in the antenna structure, and introducing another feed point, two kinds of pattern reconfigurable fractal antennas can be achieved Single-port Hilbert Microstrip Antenna [13] As can be seen in Fig.6, the original patch size is 21 mm 22.5 mm and metal plate is 26 mm 27 mm. The coaxial probe feed is located at the symmetry axis of the geometry, 5 mm away from the bottom line of the patch. 8 slots are etched in the original antenna and 2 es are installed in each slot. (a) Original (b) Modified (c) Side view Fig.6 The pattern reconfigurable single-port fractal antenna By turning pairs a3, a4, a5 and a7 on while the others off, State 1 of the antenna can be obtained. State 2 is opposite to State 1. Both of the states have the same resonance frequency of 10 GHz. States 1 and 2 have the main beams of co-polarization in the a6 a5 feed point a1 a2 a3 a4 a7 a8 E-plane directing to θ= 32 and θ=+32 respectively, and the corresponding 3-dB beamwidths range from 64.2 to 6.1 and from +5.1 to Two-port Hilbert Patch Antenna [14] The patch is also modified from the original Hilbert patch above. As shown in Fig.7, two highly-isolated feed points are used and six narrow slots cooperated with some es are introduced. By controlling the states of the es, the antenna can be operated in Configs-1 and -2, respectively. substrate patch feed-2 feed-1 Fig.7 Pattern reconfigurable two-port fractal antenna When the antenna is fed by feed-1, by changing the states of the es to make the antenna in Configs-1 and -2 alternately, the working frequency can remain at 12.0 GHz and the radiation pattern can scan in the E-plane. Configs-1 and -2 have beam maximums at 44 and +29 in the E-plane respectively. The 3 db beamwidths cover the elevation range of 66 to 17 and +15 to +65, respectively. When the antenna is fed by feed-2 and in Config-1, it operates at 12.0 GHz and the main lobe of the antenna is in the H-plane. 1.3 Frequency and Pattern Reconfigurable Antenna [15-16] The ultimate target of reconfigurable antenna research is to design an antenna, which can reconfigure antenna frequency and pattern simultaneously. However, it is very difficult to do this. Ref.[15] attempts to explore the possibilities of microstrip antenna for reconfiguring the radiation characteristics over an extremely wide band using Genetic algorithm. The simple antenna structure in Ref.[16] is shown in Fig.8. Forty es have been installed in the reconfigurable radiation apertures. Code 0 or 1 is used to denote the off or on states of a, respectively. So a binary string composed of 0 and 1 with a length of 40 can describe the states of the array. Two directions, (θ 0 =60, φ 0 =0 ) and (θ 1 =120, φ 1 =30 ), are selected for the optimized goal.

5 No.3 WANG Bing-zhong, et al.: Researches on Reconfigurable Antenna in CEMLAB at UESTC 229 Three operation frequencies f 1 =10.0 GHz, f 2 =15.0 GHz and f 3 =20.0 GHz are selected randomly. For the first goal direction, the optimized return losses at three different frequencies are 15dB, 15.3 db and 13.5 db, respectively. For the second goal direction, the optimized return losses are 25 db, 14.3 db and 15.9 db, respectively. The results indicate that this antenna can obtain the required goals over an ultra-wide band through reconfiguring the states of the array installed in the shared aperture. To improve the design efficiency, a more effective optimization algorithm, micro-genetic algorithm, is applied to design the antenna whose structure is similar to that of Ref.[16]. Three goal directions of the main beam are chosen at the operation frequency f 0 =6.5 GHz. The results indicated that it is easy to get the desired goals. Choosing different operation frequencies, a frequency and pattern reconfigurable antenna can be designed easily feed line 30 substrate ε r 20 patch plane 10 W f (a) Three-dimensional view (b) Top view Fig.8 Reconfigurable microstrip antenna scheme 2 Techniques for Reconfigurable Antenna Design Several commercial EM simulation softwares are widely used in the design and analysis of reconfigurable antenna. It is very convenient to get performances of an antenna by simulation. However, only utilizing softwares will make the designing process difficult and time consuming, and will not help to understand the working principle of reconfigurable antenna. As for frequency reconfigurable antenna, through modifying antenna configuration, input impedance will be changed and different resonant frequencies will be achieved. Compared with frequency reconfigurable antenna, pattern reconfigurable antenna design is more complex and difficult. More attention should be paid on pattern reconfigurable antenna design. In this W r L y L z section, two kinds of design techniques for this aspect are introduced. The samples described in the following part have shown the efficiency of the techniques in designing pattern reconfigurable antennas. 2.1 Combining Floquet s Theorem with FDTD Method to Design Pattern Reconfigurable Leaky-wave Antenna [17] In Section 1.2.1, the pattern reconfigurable leaky-wave antenna is designed by using traditional FDTD method. The computational process needs a large amount of time. Combing Floquet s theorem with FDTD method and using a simple linear interpolation technique, an effective numerical technique is developed to re-design the reconfigurable CPW leaky-wave antenna. Based on Floquet s theorem, only one period cell needs to be simulated by using the periodic boundary condition. To a fixed β (the phase constant of the fundamental spatial harmonic inside the periodic structure) and the determined structure period, the corresponding operation frequency can be calculated by one time-domain simulation. When several different β are given to calculate their corresponding response frequencies, the f-β curve can be plotted by using a simple curve fitting method. According to the f-β curve, the β corresponding to a fixed frequency f 0 can be obtained easily by the linear interpolation technique. Then the main beam direction can be determined by Eq.(1). For simplifying the simulation of the mixed period structure in Section 1.2.1, a simple equivalent period formula is proposed for the mixed-period structure. The formula can be expressed as MP1 + NP P 2 eff = when P 1 P 2 << min( P 1, P 2 ) (2) M + N Combining the linear interpolation technique with Eqs.(1) and (2), the main beam directions of the mixed period states can be determined very simply. The design results indicate that the presented technique can predict the maximum radiation direction θ m of each reconfigurable state accurately. 2.2 Characteristic Mode Analysis Applied in Design of Pattern Reconfigurable Antenna The theory of characteristic modes, which was first formulated by Garbaz [18] and then refined by Harrington and Mautz [19], has long been used in various applications, such as analysis of radiation and scattering [20], pattern synthesis [21]. Characteristic mode analysis yields a set of so-called characteristic currents and characteristic fields that are related to its shape or

6 230 Journal of Electronic Science and Technology of China Vol.4 dimension, independent of any specific source or excitation. The characteristic currents form a weighted orthogonal set over the conductor surface, and the characteristic fields form an orthogonal set over the sphere at infinity. When applying characteristic modes in analysis and design of an antenna, the computational flows are as follows. According to the theory, enginvalues of [] should be calculated first. Then several predominant valid eigenvalues are chosen and corresponding characteristic currents can be worked out. Then far field patterns of different modes can be computed. In this way, the characteristic mode analysis of an antenna has been accomplished. Some contents can be found in Refs.[22-24]. According to the characteristic mode analysis, the diversity of different characteristic patterns provides the possibility for pattern reconfiguration. By combining relevant characteristic patterns in a weighted linear way to form the required patterns approximatively, a pattern reconfigurable antenna will be achieved. Those desired mode currents can be excited by appropriate sources based on the excitation factors of corresponding modes. When the feed ports are chosen and excited, the influences of excitation sources on patterns must be taken into account. If the feed system is chosen suitably, the desired modes become the principal contributors to the antenna, while other modes will be suppressed or not be excited. In Ref.[24], a pattern reconfigurable fractal antenna is described by using characteristic mode analysis. The patterns can be approximated by weighted linear combination of four key modes. The characteristic current distributions reveal why the probe feed can excite the four key characteristic modes effectively. Another example is the design of a square patch antenna with pattern reconfiguration. First, characteristic modes of the antenna without any excitation are given. Several modes are chosen and excited by two ports respectively to form a pattern reconfigurable antenna. 3 Conclusions During the last 5 years, some achievements in reconfigurable antenna have been attained in CEMLAB at UESTC. Several typical reconfigurable antennas, which can realize different reconfigurable characteristics, have been designed. Combination of Floquet s Theorem with FDTD method and the theory of characteristic modes are applied to the analysis and design of pattern reconfigurable antennas. However, there are still many problems and difficulties that should be solved and investigated. 1) Practical es with high performances and the techniques of integrating es with reconfigurable antennas should be investigated in depth. At present, compared with ideal es, practical es and their controlling networks will affect the expectant antenna characteristics. 2) Performances of reconfigurable antennas need to be improved for adapting to different applications. The reconfigurable characteristics of an antenna, especially the radiation patterns, are liable to be influenced by different surroundings or working platforms. It is necessary to study how to maintaining the antenna s reconfigurable characteristics in different cases. 3) Arrays with reconfigurable elements are worth investigating. In an array, not only the characteristics of reconfigurable elements should be considered, but also how to improve the performances of the array through the reconfigurable characteristics should be researched emphatically. 4) Electromagnetic theory and high performance design and optimization tools for analysis and design of reconfigurable antennas should be established and improved. Supported by proper theory and tools, evolutions and applications of reconfigurable antennas can be accelerated and some principles about reconfigurability can be expressly revealed. References [1] Golio M. RF and Microwave Handbook[M]. New ork: CRC Press, [2] Chang K. RF and Microwave Wireless Systems[M]. New ork: Wiley, [3] Schaubert D H, Farrar F G, Hayes S T, et al. Frequency-agile, Polarization Diverse Microstrip Antennas and Frequency Scanned Arrays[P]. USA, H01Q 001/38, [4] Potenziani E. The DARPA/CECOM REconfigurable Anatenna Program [EB/OL]. ~pbmuri/1999-review/potenziani/sld001.htm, [5] iao S Q, Wang B, ang S. A novel frequency reconfigurable antenna[j]. Microwave and Optical Technology Letters, 2003, 36(4): [6] Wong K L. Compact and Broadband Microstrip Antennas[M]. New ork: Wiley, 2002.

7 No.3 WANG Bing-zhong, et al.: Researches on Reconfigurable Antenna in CEMLAB at UESTC 231 [7] iao S Q, Wang B, Wang G F. Design of millimeter-wave reconfigurable patch antenna[j]. International Journal of Infrared and Millimeter Waves, 2002, 23(7): [8] iao S Q, Wang B, ang S, et al. A novel reconfigurable CPW leaky-wave antenna for millimeter-wave application[j]. International Journal of Infrared and Millimeter Waves, 2002, 23(11): [9] Wu W, Wang B, Sun S H. Pattern reconfigurable microstrip patch antenna[j]. Journal of Electromagnetic Waves and Applications, 2005, 19(1): [10] ang S. The Research on Microstrip Reconfigurable Antennas[D] Ph.D. Thesis, Chengdu: University of Electronic Science and Technology of China, (in Chinese). [11] Werner D H, Ganguly S. An overview of fractal antenna engineering research[j]. IEEE Antennas and Propagation Magazine, 2003, 45(1): [12] Vinoy K J, Varada V K. Design of reconfigurable fractal antennas and RF-MEMS for space-based systems[j]. 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Characteristic Mode Theory and Its Applications in the Researches of Pattern Reconfigurable Antennas[D]. Ph.D. Thesis, Chengdu: University of Electronic Science and Technology of China, 2005 (in Chinese). Brief Introduction to Author(s) WANG Bing-zhong ( 王秉中 ) was born in Sichuan, China, in He received the B.Sc., M.Sc., and Ph.D. degrees from University of Electronic Science and Technology of China (UESTC) in 1982, 1984, and 1988, respectively, all in electrical engineering. He is now a professor and the Head of the Institute of Applied Physics, UESTC. His current research interests include computational electromagnetics, numerical modeling and simulation of the electromagnetic behavior in high-speed integrated circuits and electronic packages, EMC analysis, electromagnetic modeling by artificial neural networks, computer-aided design for passive microwave and millimeter wave integrated circuits, and antenna design. IAO Shao-qiu ( 肖绍球 )was born in Hunan, China, in He received the B.Sc. degree in physics from Jishou University, Jishou, China, in 1997, and the M.Sc. and Ph.D. degrees in electromagnetic field and microwave engineering from UESTC in 2000 and 2003, respectively. His research interests include antenna design and computational electromagnetics. HANG ong ( 张泳 ) was born in Jiangsu, China, in He received the B.Sc. and M.Sc. degrees in Electrical Engineering and Radio Physics from UESTC in 2003 and 2006, respectively. He is currently pursing the Ph.D. degree in Radio Physics at UESTC. His research interests include microstrip antenna, antenna array and computational electromagnetics. ANG ue-song ( 杨雪松 ) was born in Hubei, China, in She received the B.Sc. degree in applied electronic technology from Huazhong University of Science and Technology, Wuhan, China, in She received Ph. D. degree in Radio Physics from UESTC in Her research interests include microstrip antenna, microwave circuit and computational electromagnetics. WU Wei-xia ( 吴炜霞 ) was born in Sichuan, China, in She received the B.Sc. and Ph. D. degrees from UESTC in 1994 and 2006, respectively. Her research interests include UWB antenna and antenna array.