In this paper, we present a low-cost compact Radio Frequency Identification (RFID) tag antenna for the toll collection system in the ultra-high frequency (UHF) bands. The antenna is a printed dipole antenna, which is built on the top-side of a low-cost FR-4 substrate (εr = 4.4 and a thickness of 0.8 mm).
12 Thi Ngoc Hien-Doan, Van Khang-Nguyen, Ngoc Chien-Dao A LOW-COST COMPACT RFID TAG ANTENNA FOR TOLL-GATES ĂNG TEN THẺ RFID GIÁ THÀNH THẤP DÀNH CHO TRẠM THU PHÍ Thi Ngoc Hien-Doan1, Van Khang-Nguyen1, Ngoc Chien-Dao2 School of Electronics and Telecommunications Hanoi University of Science and Technology; hien.doanthingoc@hust.vn, khang.nguyen@hust.edu.vn Ministry of Science and Technology; dnchien@most.gov.vn Abstract - In this paper, we present a low-cost compact Radio Frequency Identification (RFID) tag antenna for the toll collection system in the ultra-high frequency (UHF) bands The antenna is a printed dipole antenna, which is built on the top-side of a low-cost FR-4 substrate (εr = 4.4 and a thickness of 0.8 mm) A meanderline with E-shaped ending is inserted into each-arm of dipole for the compactness The antenna is fed by a modified T-matching network for impedance matching with the UCODE G2XM chip The final design with overall size of 70 mm x 20 mm x 0.8 mm yields a |S11| < –10-dB bandwidth of 250 MHz (800 – 1050 MHz), which covers entire UHF RFID bands (860 – 960 MHz) Also, the antenna yields an omnidirectional radiation pattern with a directivity of 1.3 dB across its operational bandwidth Tóm tắt - Trong báo này, giới thiệu ăng ten dành cho thẻ sử dụng công nghệ nhận dạng tần số vô tuyến RFID giá thành thấp dải tần số siêu cao UHF Đây loại ăng ten lưỡng cực in đế FR-4 có giá thành thấp (εr = 4,4 có độ dày 0,8mm) Một đường gấp khúc có dạng chữ E thêm vào hai nhánh ăng ten lưỡng cực nhằm làm nhỏ kích thước ăng ten Ăng ten cấp nguồn mạng phối hợp trở kháng chữ T với chip UCODE G2XM Ăng ten thu có kích thước 70 mm x 20 mm x 0,8mm, hệ số phản xạ |S11| < –10-dB, băng thông 250 MHz (800 – 1050 MHz), phủ toàn băng tần UHF RFID (860 – 960 MHz) Thêm nữa, ăng ten có đồ thị xạ đẳng hướng với độ định hướng 1,3dB Key words - Radio Frequency Identification; toll collection system; antenna; T-matching network; meander-line; E-shape Từ khóa - nhận dạng tần số vơ tuyến; hệ thống trạm thu phí; ăng ten; mạng phối hợp trở kháng chữ T; đường gấp khúc; dạng chữ E Introduction The intelligent transportation system (ITS) with electronic toll collection (ETC) enables the electronic collection of toll payments These systems allow reducing road congestion and increasing road safety Manual toll collection causes vehicles to pile up in queues at collection stations, hampering free flow of vehicles [1] The ETC systems are usually developed based on RFID technologies RFID systems are composed of at least three core components: RFID tags, RFID readers, and databases that associate arbitrary records with tag identifying data It is obvious that a tag antenna plays a key role in overall RFID system performance factors because passive tags obtain energy from the incoming radio frequency communication signal Therefore, the tag antenna has substantial effects on the reading distance, the overall size, and the compatibility with the tagged object of RFID systems Especially, for the ETC systems, RFID tags can be detected with high accuracy even when vehicles are moving at high speed and drivers not want to waste time waiting in a long queue to pay their toll Therefore the tag antenna of ETC system needs to satisfy some requirements as follows [2]: • Large bandwidth to get enough data transferring velocity • High gain, the tags can be read by the reader from big distance • High directivity, the antenna should have proper main lobe width and low side-lobe level To date, several RFID-tag antennas, e.g [3] – [5], have been reported for the ETC systems Nuttaka Homsup (2016) developed an ETC system by using semi-passive RFID at frequency of 5.8 GHz in order to communicate between the transceiver and the transponder The performance of transponder was improved by adding a top and a bottom parasitic element with the loop antenna [3] Shunbo Zhang (2008) proposed a Slot-coupled circularly polarized square patch antenna for electronic toll collection system Wide bandwidth is obtained by using two suitable coupling slots in the ground plane of the stripline structure [4] Jae Su Jang (2013) presented a planar array antenna with special beam shaping for ETCS-RSE (Electronic Toll Collection System-Road Side Equipment) in an operating band of 5.79 – 5.85 GHz The proposed 10 x 10 array antenna achieved flat-topped radiation pattern by using Woodward and Lawson pattern synthesis [5] Also, various different types of passive RFID tag antennas have been developed including designs based on planar circular patch antennas [6], square microstrip patch antennas [7]–[9] However, the above antennas have some limitations such as large size, complex structure and high cost This paper presents a low-cost RFID tag antenna for the toll collection systems in UHF bands The antenna is a printed dipole, which employs meander-line with E-shaped d ending in each-arm to achieve a compact size A modified T-matching network is used to match the impedance between the antenna and UCODE G2XM chip, and consequently, achieves a broad operational bandwidth The antenna is characterized by using the commercially available electromagnetic simulation software Ansoft High-Frequency Structure Simulator (HFSS) Antenna design and characteristics 2.1 Antenna Geometry Figure illustrates the geometry of the passive RFID tag antenna, which is built on a FR4 substrate with a dielectric constant of 4.4, a loss tangent of 0.025, and a thickness of 0.8 mm The proposed tag antenna is ISSN 1859-1531 - THE UNIVERSITY OF DANANG, JOURNAL OF SCIENCE AND TECHNOLOGY, NO 11(120).2017, VOL constructed by two major below sections: • Two capacitive loadings at the end of antenna arms for the antenna miniaturization • A T-matching network in the feeding structure for the impedance matching with the complex impedance of the tag chip Lt1 13 hen, 𝐼(𝑧) = 𝐼𝑐 (𝑐𝑜𝑠𝑘𝑧 + 𝜔𝐶𝜌 𝑠𝑖𝑛𝑘𝑧) C: Capacitance of the load If we put: 𝐴𝑠𝑖𝑛 𝜃 = 1 𝐴𝑐𝑜𝑠𝜃 = 𝜔𝐶𝜌 Rt1 Ws1 Lb1 Lb2 Wb1 Lb1 Wb2 Lf Lsub Wff Chip The of an arm with capacitive load is replaced by an arm without load having the length as follows: Sm Ht Wm Wf Figure The geometry of the proposed antenna with T-match network After the optimization, the designed antenna has the dimensions as follows: Wsub = 20 mm, Lsub = 70 mm, Wb1 = Wb2 = mm, Lb1 = Lb2 = 18 mm, Ls1 = Ls2 = 10 mm, Ws1 = Ws2 = 0.4 mm, g1 = g2 =1 mm, Rt1 = mm, Wt = mm, Lt1 = mm, Wf = 0.4 mm, Lf = 15 mm, Lt = 2.5 mm, Wm = 0.5 mm, Sm = 0.6 mm, and Wff = 0.6mm 2.2 Antenna Miniaturization As mentioned above, the proposed antenna employs meander-line with E-shaped d ending in each-arm to achieve a compact size For better understanding this issue, this subsection undertakes an analysis of the current on the dipole with meander-line with E-shaped ending, which can be considered as the capacitive loading The Figure illustrates a dipole antenna with two capacitive loadings at the end of each arm ltd ~ l/2 L/2 Figure Dipole antenna with capacitive load at the end of two arms Because of this capacitive load, the terminal impedance has finite value, the terminal current is not zero, which means that 3the current distribution will be similar to the case where the terminal is extended by one segment Then the current distribution function is determined by the following formula [10]: I(z) = Ic coskz + i ρ sin kz (1) Ic, Uc: Current, voltage at the end of the arm ρ: Wave impedance z: The distance from the point of view from the end of the arm With, 𝐼 𝑈𝑐 = 𝑐 𝑖𝜔𝐶 l θ k = + So the greater the capacitance of the load, the longer the arm length is The geometry of the proposed antenna without T-matching network is shown in Figure Two capacitive loadings have the shape of T letter Lf Uc L Lt Lm 𝐼(𝑧) = 𝐼𝑐 𝐴𝑠𝑖𝑛(𝑘𝑧 + 𝜃) 𝑙 g1 I0 Then Wt Ls1 Wsub Figure The geometry of the proposed antenna without T-match network 2.3 T-Matching Network Because of the complex impedance of the tag chip, a T-matching network loaded with a meander line is used in the feed to obtain the impedance matching The main role of an impedance matching is to force load impedance being nearly equal the complex conjugate of the source impedance and maximum power can be transferred to the load The reactance between source impedance and load impedance reduces the current and dissipates the power in the load To restore the dissipation to the maximum that occurs when Rs is equal to RL, the reactance of the loop must be zero It means that the load and source are made to be complex conjugates one another, so they have the same real parts and opposite type reactive parts The input impedance of the antenna can be adjusted by using the modified T-matching network in the feeding structure The dimension of T-matching network is reduced by using the meander lines IC Model Antenna Resonant Inductor Cchip Rchip Antenna Figure The UCODE G2XM application model The UCODE G2XM chip, which has an input impedance of 24 –j195 (Ω) at 915 MHz is selected to attach to the proposed antenna So, the input impedance of the proposed antenna must be approximately 24 + j195 (Ω) for 14 Thi Ngoc Hien-Doan, Van Khang-Nguyen, Ngoc Chien-Dao Simulated results The simulated input impedances of the antenna are illustrated in Figure Good conjugate matching between the input impedances of the antenna and the UCODE G2XM chip is received The resistance and reactance components of the input impedance are fairly close to those of the chip in the 890–925 MHz range It has value of 22+j202 (Ω) at 915MHz The simulated input impedance of the antenna is shown in Figure The simulated results yield a –10-dB reflection coefficient bandwidth of 250 MHz (800-1050 MHz) Lm=2.3 mm Lm=3.0 mm Lm=3.5 mm 120 100 Resistance (Ohm) the complex conjugate matching between the tag chip and the antenna The UCODE G2XM application model is shown in Figure 4, the input capacitance and resistance are in parallel [11] The input impedance of the antenna can be adjusted by using the modified T-matching network in the feeding structure 80 60 40 20 0.70 0.75 0.80 0.85 0.90 0.95 Frequency (MHz) Tag chip reactance Simulated reactance Tag chip resistance Simulated resistance 280 240 Lm= 2.3 mm Lm= 3.0 mm Lm= 3.5 mm 300 280 Reactance (Ohm) Impedance (Ohm) 260 200 160 120 80 240 220 200 180 160 140 40 120 0.70 0.75 0.80 0.85 0.90 100 0.70 0.95 Frequency (MHz) Sm Wm Wf 0.90 0.95 -2 -4 |S11| (dB) -6 Lt Lm Ht 0.85 Figure Simulated input impedances of the proposed antenna with different value of Lm Figure illustrates the geometry of T-matching network As mentioned above, the input impedance of the antenna is easily adjusted by using the modified T-matching network in the feeding structure This is observed in Figure 7, which shows the input impedance of the antenna for different values of Lm Chip 0.80 Frequency (MHz) Figure The simulated input impedances of the proposed antenna Wff 0.75 -8 -10 -12 -14 -16 Lf -18 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 Figure The geometry of T-matching network As shown in Figure 7, the input reactance significantly increases with an increment of Lm of the meander line whereas the input resistance is changed insignificantly The |S11| value is determined by using the basic formula as follows: S11 = −20log | Za −Z∗c Za +Zc | where Za and Zc are the input impedances of the antenna and the tag chip, respectively Frequency (MHz) Figure The simulated reflection coefficients of the proposed antenna Figure shows the simulated radiation patterns of the proposed antenna in the E-plane and H-plane It is observed that the antenna yields a nearly perfect omnidirectional pattern This is further confirmed in Figure 10, which illustrates the 3D radiation pattern of the antenna It has a gain of 1.3 dB at the frequency of 915MHz ISSN 1859-1531 - THE UNIVERSITY OF DANANG, JOURNAL OF SCIENCE AND TECHNOLOGY, NO 11(120).2017, VOL (dBic) -5 -10 300 -15 -20 -25 -30 270 -25 -20 -15 240 -10 -5 330 30 60 90 120 210 150 180 15 Conclusions A compact, UHF–RFID tag antenna has been designed and simulated Two capacitive loadings are applied at the end of two arms of proposed antenna for the antenna miniaturization A meander-line T-match has been used to conjugate matching for the desired input resistance and reactance by tuning design parameters The optimized structure achieves a good antenna characteristic; its impedance bandwidth is 250 MHz (800–1050 MHz) Due to flow-cost, compact size, broadband, easy fabrication, the proposed antenna can be widely used in RFID applications This antenna structure solves the disadvantages of the previously introduced antennas in section REFERENCES (a) E plane (dBic) -5 -10 300 -15 -20 -25 -30 270 -25 -20 -15 240 -10 -5 330 30 60 90 120 210 150 180 (b) H plane Figure Simulated radiation pattern at 915 MHz: (a) E plane and (b) H plane Figure 10 The 3D radiation patterns of the proposed antenna [1] K Kamarulazizi, W Ismail, “Electronic toll collection system using passive”, Journal of Theoretical and Applied Information Technology, 2005-2010 [2] L.Wenming, N.Huansheng, W Baofa, “RFID Antenna Design of Highway ETC in ITS”, 7th International Symposium on Antennas, Propagation & EM Theory, Guilin, pp 1-4, 2006 [3] H Nuttaka, K Vuttichai, S.Winyou and B Pravit, “Simulation and analysis of an antenna in a transponder for the electronic toll collection system of Expressway in Thailand”, 13th International Conference on Electrical Engineering/Electronics, Computer, Telecommunications and Information Technology (ECTI-CON), Chiang Mai, pp 1-4, 2016 [4] Z Shunbo, Z Yuan, and Z Shouzheng, “Slot-coupled Circularly Polarized Square Patch Antenna for Electronic Toll Collection System”,, International Conference on Microwave and Millimeter Wave Technology, Nanjing, pp 1210-1213, 2008 [5] J Jae-Su, K Nyung-Hak, K YeongWoo, H Jae-Kwon, “Planar Array Antenna Design with Beam Shaping for ETCS-RSE”,, AsiaPacific Microwave Conference Proceedings (APMC), Seoul, pp 1185-1187, 2013 [6] J H Lu and B S Chang, “Planar circularly polarized tag antenna with compact operation for UHF RFID application”, Journal of Electromagnetic Waves and Applications, vol 27, no 15, pp 1882–1891, 2013 [7] H D Chen, C Y D Sim, and S H Kuo, “Compact broadband dual coupling-feed circularly polarized RFID microstrip tag antenna mountable on metallic surface”, IEEE Trans Antennas Propagation, vol 60, no 12, pp 5571 –5577, Dec 2012 [8] H D Chen, S H Kuo, C Y D Sim, and C H Tsai, “Couplingfeed circularly polarized RFID tag antenna mountable on metallic surface”, IEEE Trans Antennas Propagation, vol 60, no 5, pp 2166–2174, May 2012 [9] C Cho, I Park, and H Choo, “Design of a circularly polarized tag antenna for increase reading range”, IEEE Trans Antennas Propagation, vol 57, no 10, pp 3418–3422, Oct 2009 [10] Phan Anh, “Antenna Theory and Technoloy”, 2007 [11] UCODE G2XM Datasheet [12] P V Nikitin, K V S Rao, S F Lam, V Pillai, R Martinez, H Heinrich, “Power reflection coefficient analysis for complex impedances in RFID tag design”, IEEE Trans Microwave Theory Tech., vol 53, no 9, pp 2721 –2725, Sep 2005 (The Board of Editors received the paper on 11/09/2017, its review was completed on 13/10/2017) ... Conclusions A compact, UHF? ?RFID tag antenna has been designed and simulated Two capacitive loadings are applied at the end of two arms of proposed antenna for the antenna miniaturization A meander-line... 1185-1187, 2013 [6] J H Lu and B S Chang, “Planar circularly polarized tag antenna with compact operation for UHF RFID application”, Journal of Electromagnetic Waves and Applications, vol 27, no 15,... Choo, “Design of a circularly polarized tag antenna for increase reading range”, IEEE Trans Antennas Propagation, vol 57, no 10, pp 3418–3422, Oct 2009 [10] Phan Anh, ? ?Antenna Theory and Technoloy”,