A thin card-sized on-metal UHF-RFID tag using a radiative mushroom structure with an IC chip mounted on a small magnetic loop Takayoshi Konishi 1,2a), Atsushi Sanada 2, Hiroshi Kubo 2, and Yoshitaka Hori 3 1 New Business Promotion Division, NEC TOKIN Co. 3 8 1 Nishi-Kanda, Chiyoda-ku, Tokyo 101 8362, Japan 2 Graduate School of Science and Engineering, Yamaguchi University 2 16 1 Tokiwadai, Ube, Yamaguchi 755 8611, Japan 3 Capacitor Division, NEC TOKIN Co. 3 8 1 Nishi-Kanda, Chiyoda-ku, Tokyo 101 8362, Japan a) t-konishi@ieee.org Abstract: In this paper, a novel thin card-sized on-metal 953 MHz UHF RFID tag is presented. The RFID tag operates in a unique configuration with an IC chip mounted on a small metallic loop and a radiative mushroom structure. A card-sized 54 86 0.94 mm 3 tag is designed successfully with a reasonable IC impedance matching with appropriate loop dimensions. It is shown that the reading range performance is tolerant of the environment and the measured reading range is 2.6 m on a large 200 300 2mm 3 metal board. The reading range is 0.1 m longer than a commercial card-sized on-metal tag with much larger thickness of 1.8 mm. Keywords: RFID, tag, mushroom structures, high-impedance surfaces, metamaterials Classification: Microwave and millimeter wave devices, circuits, and systems References [1] M. Stupf, R. Mittra, J. Yeo, and J. R. Mosig, Some novel design for RFID antennas and their performance enhancement with metamaterials, Microwave and Optical Technical Letters, vol. 49, no. 4, pp. 858 867, April 2007. [2] P. Raomonen, M. Keskilammi, I. L. Sydänheimo, and M. Kivikoski, A very low profile CD EBG antenna for RFID reader, 2004 IEEE Antennas and Propagation Society, AP-S International Symp., (Digest) 4, pp. 3808 3811, 2004. 276
[3] L. Ukkonen, L. Sydänheimo, and M. Kivikoski, Patch antenna with EBG ground plane and two-layer substrate for passive RFID of metallic objects, 2004 IEEE Antennas and Propagation Society, AP-S International Symposium, (digest)1, pp. 93 96, 2004. [4] L. Ukkonen, L. Sydänheimo, and M. Kivikoski, Effect of metallic plate size on the performance of microstrip patch-type tag antennas for passive RFID, IEEE Antennas Wireless Propag. Lett., vol. 4, pp. 410 413, 2005. [5] W.-K. Tsai and K.-H. Lin, Design of a novel tag antenna with EBG structure for RFID of metallic object, 2006 Proc. Third IASTED International Conf. Antennas, Radar and Wave Propagation, pp. 7 10, 2006. [6] D.-U. Sim, D.-H. Kim, J.-I. Choi, and H.-D. Choi, Design of novel dipoletype tag antennas using Electromagnetic Banbgap (EBG) surface for passive RFID applications, 2007 IEEE Antennas and Propagation Society, AP-S International Symposium, pp. 1333 1336, 2007. [7] B. Gao, C. H. Cheng, M. M. F. Yuen, and R. D. Murch, Low cost passive UHF RFID packaging with Electromagnetic Band Gap (EBG) substrate for metal objects, Proc. Electronic Components and Technology Conference, pp. 974 978, 2007. [8] T. Konishi, T. Miura, Y. Numata, S. Sato, A. Sanada, and H. Kubo, An Impedance Matching Technique of a UHF-Band RFID on a high- Impedance Surface with Parasite Elements, Proc. IEEE Radio and Wireless Symposium, San Diego, MO3A-3, Jan. 2009. [9] A. Sanada, C. Caloz, and T. Itoh, Planar distributed structures with negative refractive index, IEEE Trans. Microw. Theory Tech., vol. 52, no. 4, pp. 1252 1263, April 2004. 1 Introduction Reading range performances of passive UHF RFID tags are degraded in the vicinity of a metallic object due to the physical limitation caused by the inverse electric image. Thus, generally speaking, the miniaturization and the long reading range performance are inconsistent for RFID tags. However, there still are great demands for RFID tags to be thin and to operate on a metal simultaneously for commercial use [1, 2, 3, 4, 5, 6, 7, 8]. Several on-metal UHF RFID tags have been developed so far. Most of them are comprised of an RFID antenna with a metal-backed thick spacer just to avoid an influence by any adjacent object. Even with this technique, a typical thickness of the on-metal tags is limited to a couple of millimeters or more with less reading range degradation. However, further drastic reduction is strongly desired. Another approach for the thickness reduction has been reported using a ready-made RFID tag inlet on a planar mushroom structure. Here, the mushroom structure is expected to work as a high-impedance surface (HIS) or an artificial magnetic conductor (AMC) to minimize the inverse electric image of the inlet antenna. However, the tags do not always exhibit the best reading range performance due to an IC impedance mismatch brought by the fact that the operations both of the tag and the mushroom HIS/AMC are changed by the mutual couplings. 277
Fig. 1. Proposed UHF RFID tag. (a) Prototype. (b) Mushroom unit cell. (c) Top view. (d) Cross sections. In this paper, we propose a novel thin card-sized on-metal 953 MHz UHF RFID tag with a better reading range performance. It consists of an IC chip mounted on a small metallic loop and a planar mushroom structure. Although the proposed tag also uses the mushroom structure, the design and operation are totally different from the conventional ones; i.e., the mushroom structure is intentionally designed to have a propagation mode at the operation frequency and acts as a radiator by itself. The IC chip is mounted on a loop being small enough to suppress the radiation by itself but large enough to perform a magnetic coupling between the IC to obtain the best impedance matching. The IC impedance matching design is simple and can be done by optimizing the loop dimensions. 2 UHF RFID tag configuration The proposed UHF RFID tag is shown in Fig. 1. The RFID tag is composed of a rectangular 4 4 cell mushroom structure and an IC chip mounted on a metallic loop as shown in Fig. 1 (a). It operates at the Japanese standard UHF tag frequency 953 MHz and the total dimension of the tag is 54 86 0.94 mm 3 including the surface plating. The unit cell of the mushroom structure consists of a metallic top patch and a via connecting the patch at the center to the ground plane. Asymmetric floating electrodes are introduced underneath the top metallic top patch to overlap the adjacent top patch to achieve a drastic enhancement of the MIM capacitance as shown in Fig. 1 (b) [9]. Note that the enhancement is concentrated in one direction of the polarization (the longer unit-cell direction) to implement unit cells within the card size. The unit cell size 278
is 13.25 21.25 mm 2. The top patch size is 12.95 20.95 mm 2 and the via diameter is 1.0 mm. The floating electrode is placed 0.1 mm below the top patch with a 0.5 mm spacing from the via contour. The relative permittivity and tanδ of the substrate are 3.5 and 0.003, respectively. The electrodes are made of 18 μm thick copper. The total thickness of the mushroom structure is 0.71 mm. It is experimentally confirmed that the structure has a propagation band from 801 MHz to 1049 MHz in the longer unit-cell direction by transmission measurements using coaxial probes mounted at the edges of the structure. An Impinj s class 1 IC chip, MONZA2, is used for the prototype. The IC chip is based on the EPC global generation 2 standard. The IC is mounted on a small loop patterned with 35 μm thick copper on a 0.1 mm thick substrate. The line width of this loop is 1.0 mm and the loop dimension is optimized to have the best impedance matching as shown in the next section. The loop is put at the center of the mushroom structure as shown in Fig. 1 (c). The loop is positioned on the slit of the top patch so that the magnetic flux can interlink to the mushroom structures. The cross section of the tag is shown in Fig. 1 (d). The total size of the prototype tag is within the card size of 54 86 mm 2 with the thickness of 0.94 mm including a 0.1 mm thick polyester sheet on the backside for a DC isolation. The polyester sheet is supposed to be replaced with an adhesive layer in a real product. 3 IC impedance matching In order to obtain the best reading range performance, the loop dimension is optimized experimentally. The input impedance looked from the IC land without the IC is measured. The measurements are carried out with the tag on an aluminum board with the size of 200 300 2mm 3. A 0.1 mm thick polyester sheet is put on the backside of the tag as mentioned in the foregoing section. Figure 2 shows the measured input impedance Z in of the optimized prototype tag and the measured IC impedance Z IC on the Smith chart normalized by 50 Ω with markers at the operation frequency of 953 MHz. As shown in Fig. 2, Z in and Z IC are 17.7+j72.5 Ω and 30.7 j78.5 Ω, respectively, both at 953 MHz. In this prototype, the reflection due to the mismatch between Z in and Z IC is calculated as (Z in Z IC)/(Z in + Z IC ) =0.295 ( 10.6dB). Here, the inside area of the optimized loop is as small as 2.0 20 mm 2. 4 Reading range performance evaluation The reading range performance of the optimized prototype tag is evaluated using a commercial RFID system. The measurements are carried out in an anechoic chamber. In the evaluation, the HIS tag is also backed by an aluminum plate with the dimension of 200 300 2mm 3 as in the impedance measurements. 279
Fig. 2. Measured tag input impedance Z in and IC impedance Z IC. The sweep range is from 900 to 1000 MHz. The makers are put at 953 MHz. Z in and Z IC are 17.7+j72.5 Ω and 30.7 j78.5 Ω, respectively, both at 953 MHz. The Smith chart is normalized by 50 Ω. The measurement setup is illustrated in Fig. 3 (a). The prototype tag is placed in front of a circularly polarized reader patch antenna (NEC TOKIN, ICT-5050) and a feasible reading range is measured in various directions by rotating the sample to obtain a practical directivity patterns. Since the tag is asymmetrical, the measurements are done for the two orthogonal directions of rotations as shown in the figure. In the evaluation, a 953 MHz reader (NEC TOKIN, ICT-5055, EPC Class1 Gen.2) is used with the output power of 30 dbm. The measured power fed to the circularly polarized patch antenna is 26.9 dbm including cable losses. As a comparison, the reading range measurements are also carried out for other two commercial tags available; a ready-made RFID tag inlet and a conventional card sized on-metal tag using a metal-backed spacer. The ready-made RFID tag inlet is optimized to work on a corrugated card board considering a practical use. The size of the tag inlet is 11 93 mm 2. The conventional card sized on-metal tag has the footprint of 54 86 mm 2 (the card size) with much larger thickness of 1.8 mm than the prototype (0.94 mm). Figure 3 (b) shows the measured reading ranges in the broadside direction (θ = 0 deg). When using a ready-made RFID tag inlet (11 93 mm 2 ), the broadside reading range is 4.1 m in free space, however, it is drastically degraded down to 0.22 m on the metal plate. On the other hand, when using the proposed prototype tag, the measured reading range is 2.7 m in free space and is as long as 2.6 m even on the metal plate. Incidentally, the reading range is zero simply with the small loop pattern alone without the mushroom structure. This leads to the fact that the loop does not radiate at all and works just as a coupling element. The radiation is obviously brought by the mushroom structure itself. When using a conventional card sized onmetal tag, the measured reading range is 2.5 m on the metal plate, whereas 280
Fig. 3. Measured reading range performance. (a) Measurement configuration. (b) Broadside reading ranges. (c) Measured directivity of the proposed tag. it is degraded to 1.1 m in free space. Therefore, we can conclude that the proposed prototype tag is tolerant and has a longer reading range regardless of the fact that the thickness is nearly a half than the conventional card sized on-metal RFID tag. Figure 3 (c) shows measured directivities of the proposed tag in the two different circulation directions with the vertical and horizontal settings shown in Fig. 3 (a). It is found that the measured directivities exhibit a slightly directed pattern in the broadside with no significant nulls or abnormal directivity. Incidentally, there is a slight difference in the reading rage between the two settings in the broadside direction. This is considered to be due to the imperfect axial ratio characteristics of the circularly polarized reader patch antenna used in the measurements. 5 Conclusions We have proposed a novel thin card-sized on-metal 953 MHz UHF RFID tag using a radiative mushroom structure. The size of the prototype tag is 54 86 0.94 mm 3. It has been shown that the proposed tag has a tolerant performance with the reading rage as long as 2.6 m even on a large metal plate. Note that the reading range is longer than that with a conventional card size on-metal tag regardless of the fact that the thickness is nearly a 281
half. Acknowledgments The authors wish to acknowledge Mr. Yukihiro Numata of NFS Business Development & Promotion Division for his assistance in experiments, Mr. Masashi Ikeda and Mr. Katsuaki Tamashiro of EMC Division for their technical supports, Mr. Shoichi Sato of Advanced Materials Research and Development Division, Mr. Masayuki Morimoto of New Business Promotion Division, Mr. Fumihiro Katakura of Associate Senior Vice President, Dr. Shigeyoshi Yoshida of Associate Senior Vice President and Member of the Board, and Mr. Yoshihiko Saiki of Corporate Auditor in NEC TOKIN Co. for their valuable discussions and supports. The authors also wish to acknowledge the blind reviewer for instructive suggestions. 282