Reading of UHF band passive RFID tags using a reflector # Yuki Oowashi 1 , Shigeki Takeda 1 Kenichi Kagoshima 1 1 Ibaraki University 4-12-1 Nakanarusawa, Hitachi, Ibaraki, 316-8511 Japan, Email:[email protected]Abstract Due to the high cost of tags which are designed for metallic objects, they have not been commonly used compared to the conventional ones. On the other hand, although reading distance becomes short, a recognition technique using a dipole tag is very effective to reduce introduction cost of RFID. In this paper, we have proposed the technique for improving the recognition performance of UHF RFID which is used in close proximity of metallic objects. We have employed a waveguide like square shaped reflector in order to be able to read tags attached any position including the reverse side of a metallic objects which is assumed to be a cylinder. It is clarified that optimum electromagnetic distribution is similar to TM 01 mode of a coaxial waveguide and that the validity of this property is evaluated by method of moment based electromagnetic simulation and experiment. Keywords : UHF RFID tag, Reflector, Reverse side 1. Introduction Radio frequency identification (RFID) is noncontact recognition technology to send and receive information by wireless. Recently, the RFID technology becomes to be widely used. However, RFID tags have weakness that their performance is easily affected by metallic objects. We will present a performance of RFID tags which are used in the close proximity of a metallic objects. Fig.1 shows the introduction example. In general, when the tag is got close to metal, the recognizing performance is greatly deteriorated because the impedance of tag is changed. Especially, receiving power of tags attached to reverse side of metallic object is lower than those which are attached to front or lateral side. Thus, it is difficult to recognize tags. In this paper, we have proposed a technique for improving the recognition performance of UHF RFID which is used in close proximity of metallic objects. We have employed a waveguide like square shaped reflector in order to be able to read tags attached to reverse side of metallic objects and assumed a shape of a metallic object to be cylinder. It will be clarified that optimal electromagnetic distribution is similar to TM 01 mode of coaxial waveguide and validity of this property is evaluated by method of moment based electromagnetic simulation and experiment. Fig.1 Example of practical use. 2. Problem of recognizing tags attached to reverse side of metallic objects at free space Reading characteristics of RFID tags which is attached to front, lateral and reverse side of a metallic cylinder are evaluated in free space. Geometry of a experimental set up is shown in Fig.2. Since radius of a metallic cylinder is 34.45 mm, distance between the reader and tag is started from 50 mm, and it is varied up to 200 mm with intervals of 50 mm in each three tags position of front, lateral, and reverse side. Furthermore, measurement has been carried out for different z of 0 mm and 100 mm. z of 100 mm corresponds to the edge of the reader antenna. The reader is RF-ATLP002 by Mitsubishi Electric Corporation [1]. Width, height and thickness are 200 mm, 200 mm and 25 mm, respectively. Transmitting power is 30 dBm. A UHF band RFID tag is RF-TGP005-W (Fig.3) [2]. metallic object direction of movement metallic plate reader UHF RFID tag belt conveyer waveguide like reflector metallic object direction of movement metallic plate reader UHF RFID tag belt conveyer waveguide like reflector
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Reading of UHF band passive RFID tags using a reflectorap-s.ei.tuat.ac.jp/isapx/2011/pdf/[FrD4-5] A14_1005.pdfReading of UHF band passive RFID tags using a reflector #Yuki Oowashi
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Fig.5 Electric field distribution (Ez component) in free space.
3. Reading technique of RFID tags attached to metallic objects 3.1. Method of moment analysis As shown in Fig.4 and Fig.5, it is very hard to recognize a tag in the reverse side. Therefore,
we propose a square shaped reflector to improve reading characteristics of tags attached to a reverse
side of a metallic cylinder. Fig.6 shows the analytical model. In this way, the technique for
improving the recognition rate of tag on reverse side was examined. The input impedance of the
patch antenna is 47.0-j8.02 ohm in free space at 953MHz and source impedance is set to be
complex conjugate of this input impedance. Transmitting power of the reader is set to be 30 dBm.