Compact Microstrip UHF-RFID Tag Antenna on Metamaterial Loaded with Complementary Split-Ring Resonators Joao P. S. Dias, Fernando J. S. Moreira and Glaucio L. Ramos GAPTEM, Department of Electronic Engineering, Federal University of Minas Gerais; Belo Horizonte - MG, 3127-91 Brazil Federal University of Sao Joao del-rei, Ouro Branco - MG, 3642- Brazil jpengtelecom@gmail.com,fernandomoreira@ufmg.br,glopesr@gmail.com Abstract This paper presents the design of a compact UHF RFID tag antenna with a bent and rectangular microstrip patch. Additionally, it utilizes Complementary Split-Ring Resonators (CSSRs) in the ground plane in order to reduce the size of the antenna. The RFID tag is composed by a RO33 Rogers Substrate with the electric permittivity of 3. and an UCODE 7 SL3S124 chip from NXP Semiconductor. The size of the is RFID tag is 36.5 x 22.5 x.795 mm designed to operate with a frequency of 915 MHz. The performance of the proposed antenna was analyzed in terms of antenna gain and return loss through simulations in CST Microwave Studio software. The results show that the use of complementary split ring resonator with the RFID tag reduce the size of the antenna in 3 percent. Index Terms Microstrip Antennas; Radio Frequency Identification (RFID); Metamaterial; Split-Ring Resonators. I. INTRODUCTION Radio frequency identification (RFID) is a technology used to track and identify objects [1]. The RFID system has been used in various applications including supply chain management, transportation system, security, animal detection, disease prevention, logistics, access point system, road tolling, and business transaction [2]. The UHF-RFID band is between 86-96 MHz defined by ITU (International Telecommunication Union), but each country has its own allocated frequency range. For example, the allocated frequency range in Brazil is between 92-928 MHz, while in Europe is between 866-869 MHz [3],[4]. A basic RFID system consists of RFID tag, RFID reader, computer host and communication network [5]. The RFID tag is the most relevant component in this system because it greatly impacts on the whole system. A typical RFID tag consists of an antenna and a microchip with internal read/write memory. The RFID system can be classified as active (with batteries), passive (batteryless) or semi-passive [6],[7]. The passive RFID system is based on backscattering method, in which the energy required to enable the chip comes from the system itself [8]. Most of the antennas for RFID tags are commonly fabricated as modified printed dipoles [8]. The chip has highly capacitive impedance. The antenna must be designed to have 978-19-6241-6/17/$31. c 217 IEEE highly inductive input impedance in order to maximize the power transfer [3]. The design of a RFID tag requires compact antennas, and microstrip antennas are usable because they have lowprofile planar configuration, compatibility with Integrated Circuits (IC), and low fabrication cost. Artificial substrate based on Complementary Split-Ring Resonators (CSSRs) are more common in this process in the miniaturization of microstrip antennas [9]. The idea is to use a technique that reduces the width of the antenna without increasing the thickness of the substrate. In this paper, it is proposed a compact dipole UHF RFID tag antenna. Its patch geometry is shown in Figure 1 and discussed in [1]. It is composed of a bent microstrip patch (Meander- Line Antennas - MLA) and a rectangular microstrip patch. In the MLA the resonant frequencies are achieved at much lower frequencies than in the case of a straight dipole of the same length, at the expense of a narrow bandwidth and a low efficiency due to the cancellation of the currents, which flow opposite to one another. Consequently, the antenna gain is reduced [1],[11]. The MLAs are used as well as to easily match the chip impedance with the antenna impedance for maximum power transfer [11]. Fig. 1: Layout of the compact dipole UHF RFID tag antenna from front and side view [1].
-1 TABLE II: Parameters and Dimension of the SRR Slot Parameter Description Dimension [mm] r inner radius 2.3 S Thickness of the inner and outer ring 1 G Gap.7 D Distance between the rings 1.2-2 -3.8.85.9.95 1 Fig. 2: Return loss of the initial Microstrip Antenna. TABLE I: Geometrical Parameters of the Microstrip Antenna Parameter Dimension [mm] Parameter Dimension [mm] a 43.5 e 4.5 b 19 g 3.5 c 1 h.76 d 4.6 gap 2 The novel feature in the present work is the design of the microstrip RFID tag on a metamaterial with complementary split rings in the ground plane. The resonant frequency is displaced and bandwidth has slight reduction. Those structures were implemented in order to reduce the size of the RFID tag. The proposed antenna is analysed by CST MWS (Computer Simulation Technology Microwave Studio) based on the Finite Integration Technique (FIT) [12]. This paper is organized as follows. Section II describes the antenna design, Section III explains the results from the simulation and discussions, and, finally, conclusions and future works are summarized in Section IV. II. DESCRIPTION OF THE MICROSTRIP RFID TAG This microstrip RFID tag was designed to operate in the frequency of 915 MHz. The initial dimensions of the antenna Fig. 4: Geometry of the Split-Ring Resonators (SRR) slot. are 43.5 x 27 x.795 mm. The substrate used in the microstrip antenna design is Rogers Substrate RO33 with the electric permittivity (ǫ r ) of 3., tangent loss (tanδ) of.1, substrate thickness (h) of.76 mm, and printed on both sides of the substrate cooper layer traces with thickness (t) of.175 mm. The RFID chip attached to the center is from NXP Semiconductor with the input impedance of 12.8-j248 Ω at 915 MHz and a threshold power sensitivity of -21 dbm over a 92-928 MHz band [13]. Table I shows the parameters and dimensions of the proposed initial microstrip RFID tag antenna. III. ANALYSIS AND DESIGN A. Antenna without Metamaterial Figure 2 presents the return loss (S-Parameters). In the return loss graph, it can be seen that the value minimum is of -28.25 db in 915 MHz. The bandwidth ( S-parameters less than -1 db) is about 14.8 MHz. Figure 3 shows that the directivity of the initial antenna is 2.19 dbi. The total radiantion efficiency is -22.6 db. Fig. 3: Radiation pattern of the initial antenna. Fig. 5: The back view of the four antennas with ground plane periodically etched CSRR. (a) Two SRR slot, (b) Three SRR slot, (c) Four SRR slot and (d) Six SRR slot.
-1-2 Antenna conventional Antenna with 2-SRR Antenna with 3-SRR Antenna with 4-SRR Antenna with 6-SRR -1-2 d = 3.2 mm d = 3.7 mm d = 4.2 mm -3.7.8.9 1 1.1 1.2-3.84.86.88.9.92.94.96 Fig. 6: Comparison of the simulated return losses between the conventional antenna and the four CSRR loaded antenna. Fig. 9: Parametric study of return loss with the parameter d. B. Complementary Split-Ring Resonators -1-1 -2-3 a = 35 mm a = 36 mm a = 37 mm -4.84.86.88.9.92.94.96-2 -3 g = 2 mm g = 3 mm g = 4 mm -35.84.86.88.9.92.94.96 Fig. 1: Parametric study of return loss with the parameter g. Fig. 7: Parametric study of return loss with the parameter a. -1-1 -2-3 e = 2 mm e = 3 mm e = 4 mm -2-3 b = 16.5 mm b = 17.5 mm b = 18.5 mm -4.86.88.9.92.94.96-4.84.86.88.9.92.94.96 Fig. 11: Parametric study of return loss with the parameter e. Fig. 8: Parametric study of return loss with different the parameter b. Figure 4 shows the geometry of the SRR slot. The parameters and dimensions are described in Table II. To evaluate the performance of the CSRRs, the ground plane of the initial antenna was notified, using four different configuration, as presented in Figure 5. Due to the insertion of the SRR structure in the ground plane, the resonance frequency of the microstrip antenna diminishes. Moreover, some physical parameters of the antenna are reduced in order to tune the antenna to the central frequency of the RFID band (915 MHz), favoring the
Fig. 14: Radiation pattern of the proposed antenna. Fig. 12: RFID tag antenna pieces: two metal faces and a substrate in the middle. miniaturization of the microstrip RFID tag antenna. Figure 6 shows the return loss for five different antennas. Four antennas loaded with CSRRs (Figure 5), as number of two, three, four and six SRR slot applied in the ground plane, respectively. And another antenna without the metamaterial structures (initial antenna). It is was noticed that the metamaterial structure shifts the antenna resonant frequency to the lower frequency, as expected. TABLE III: Parameters and Dimension of the CSRR loaded Antenna Parameter Dimension [mm] Parameter Dimension [mm] a 36.5 e 3 b 17.5 g 2 c 1 h.76 d 3.7 gap 2 C. Antenna with Metamaterial In order to tune the microstrip antenna, a parametric study is made with the relevant parameters of the microstrip RFID tag -1-2 -3.8.85.9.95 1 Fig. 13: Return loss of the microstrip RFID tag. (a,g,b,d and e) in the four antennas with CSRR but the one that showed greater capacity of miniaturization was the microstrip antenna with six SRR slot (Figure 12) and the result of their parametric analysis is illustrated from Figure 7 to Figure 11. The dimensions of the proposed antenna are described in the Table III. -1-2 -3 Antenna conventional on Metal Antenna with CSRR on Metal -4.7.8.9 1 1.1 1.2 Fig. 15: Return loss of the RFID tag in free space and on 1 x 1 mm aluminum plate. It can be seen from Figure 13 that the return loss (S- Parameters) is minimum in 915 MHz with the value of -26.58 db, and the bandwidth is about 13 MHz. Figure 14 shows that the directivity of the proposed antenna is 1.93 dbi. The total efficiency is -31.2 db. D. Applicability Analysis The RFID tags can be fixed on different types of objects. Therefore, it is analyzed the RFID tag mounted on four different surfaces (plastic, metal, paper and glass). For this simulation plates with the dimensions of 1 x 1 mm are used. Figure 15 shows a comparison of the return loss between antennas in free space and mounted on an aluminum plate. Figure 16 shows a comparison of the return loss between antennas in free space and mounted on a glass plate. Figure
-1-2 Antenna with CSRR on Glass Antenna conventional on Glass -3.7.75.8.85.9.95 1 Fig. 16: Return loss of the RFID tag in free space and on 1 x 1 mm glass plate. -1-2 Antenna with CSRR on paper Antenna conventional on paper -3.7.75.8.85.9.95 1 Fig. 17: Return loss of the RFID tag in free space and on 1 x 1 mm paper plate. -1-2 -3 Antenna with CSRR on plastic Antenna conventional on plastic -35.7.75.8.85.9.95 1 Fig. 18: Return loss of the RFID tag in free space and on 1 x 1 mm plastic plate. 17 shows a comparison of the return loss between antennas in free space and mounted on a paper plate. Figure 18 shows a comparison of the return loss between antennas in free space and mounted on a plastic plate. When the RFID tags was mounted on metal surfaces and glass it was observed a shift above 7 MHz at the resonant frequency. However the frequency displacement was more significant as the RFID tag was over aluminum plate, as expected. The RFID tag over plastic plate displaced the resonant frequency, in 4 MHz. When the RFID tag was mounted on surface of paper the shifted of the resonant frequency was insignificant. IV. CONCLUSION A compact dipole UHF-RFID tag antenna working at 915 MHz was presented. This proposed miniaturized RFID tag antenna by inserting a Complementary Split-Ring Resonators(CSRRs) in the ground plane. The antenna is composed of copper layer traces and Rogers 33 substrate with the dimension of 36.5 x 22.5 x.795 mm, and relative permittivity, ǫ r of 2.2. The simulation results proved that it is possible to reduce the size of the RFID tag antenna in 3 percent without compromising considerably the bandwidth. ACKNOWLEDGMENT This work was partidly supported by the brazilian agencies CAPES, CNPq, and FAPEMIG. REFERENCES [1] K. Finkenzeller, Introduction. RFID Handbook: Fundamentals and Applications in Contactless Smart Cards and Identification, 2nd ed (23) [2] K. Udo, and M. Fischer, Fully integrated passive UHF RFID transponder IC with 16.7-/spl mu/w minimum RF input power. IEEE Journal of Solid-State Circuits 38.1 (23): 162-168. [3] N. M. Faudzi, M. T. Ali, I. Ismail, H. Jumaat and N. H. M. Sukaimi, Metal mountable UHF-RFID tag antenna with meander feed line and double T-match. Technology Management and Emerging Technologies (ISTMET), 214 International Symposium on (pp. 33-38) IEEE, 214. [4] K. S. Rao, P. V. Nikitin and S. F. Lam, Antenna design for UHF RFID tags: A review and a practical application. IEEE Transactions on antennas and propagation, 53.12 (25): 387-3876. [5] N. M. Faudzi, M. T. Ali, I. Ismail, H. Jumaat and N. H. M. Sukaimi, Compact microstrip patch UHF-RFID tag antenna for metal object. Wireless Technology and Applications (ISWTA), 214 IEEE Symposium on. IEEE, 214. [6] E. W. T. Ngai, K. K. Moon, F. J. Riggins and Y. Y. Candace, RFID research: An academic literature review (199525) and future research directions. International Journal of Production Economics, 112.2 (28): 512. [7] Z. N. Chen and X. Qing, Antennas for RFID applications. Antenna Technology (iwat), 21 International Workshop on. IEEE, 21. [8] G. Marrocco, The art of UHF RFID antenna design: Impedancematching and size-reduction techniques. IEEE antennas and propagation magazine 5.1 (28). [9] Y. Lee and Y. Hao, Characterization of microstrip patch antennas on metamaterial substrates loaded with complementary splitring resonators. Microwave and Optical Technology Letters 5.8 (28): 2131-2135. [1] Y.U. Yanzhong, N.I. Jizhen and X. U. Zhixiang, Dual-band dipole antenna for 2.45 GHz and 5.8 GHz RFID tag application. Advanced Electromagnetics, v. 4, n. 1, p. 31-35, 215. [11] MWS, CST, Computer Simulation Technology: Microwave Studio. Computer Simulation Technology Std (216). [12] NXP Semiconductor, Product Data Sheet SL3S124 UCODE 7, 216.