Research On UHF RFID Anti-metal Tag

Radio frequency identification (RFID) in the ultra-high-frequency (UHF) band has gained great interest in supply chain management and traffic management because of its long read range. In many applications, RFID tags need to be attached on the surface of metallic objects. However, it is a challenge for label-type passive UHF RFID tags with dipole-like antennas to be mounted on the surface of metal. The metallic surface in the vicinity of an UHF RFID tag antenna changes its impedance matching, radiation efficiency and radiation pattern, which makes the performance of the tag worse or even not readable. To solve this problem, this dissertation first analyzed the cause of the performance deterioration of the tag placed near metallic objects. Based on the analysis, two kinds of anti-metal UHF RFID tag were designed for practical application. Finally, the anti-metal tags were tested for car entering-leaving management. The main contents and innovations of the research are summarized as follows:(1) This dissertation presented the characteristics of dipole-like antennas of UHF RFID tags placed close to metallic surfaces through analyzing antenna parameters. Finite Element Method (FEM) and Method of Moment (MoM) were used to simulate the changes of the antenna parameters near the metallic surface. And, experiments were carried out to verify the simulation. It suggested that a distance of 0.05λ～0.1λ(16mm～32mm) from the metallic surface could restore the dipole-like UHF RFID tag performance to that in free space.(2) In order to meet the low profile requirement, two kinds of anti-metal UHF RFID tag antennas based on the short-stub-feed and open-stub-feed microstrip antennas respectively were designed. The open-stub-feed microstrip antenna had a completely planar structure which was suitable for the mass production using the printing techniques and could decrease the manufacturing cost of the anti-metal tags. Moreover, the open-stub-feed technique made the impedance of the patch antenna be tuned in a large scale for conjugate impedance matching. Finally, a propositional resolution based on the combination of both short-stub-feed and open-stub-feed microstrip antenna was suggested for anti-metal UHF RFID tag antenna design.(3) In order to cover the whole bandwidth of global UHF RFID bands, a wideband anti-metal UHF RFID antenna was designed based on deeply inserted feed microstrip patch antenna with a pair of U slots. By embedding a pair of U slots on the microstrip patch, a new resonant mode adjacent to the original mode was excited and a wideband characteristic was achieved. With this method, two sample anti-metal wideband UHF RFID tag antennas, one covers North America and Europe UHF RFID bands, another covers the two separated UHF RFID bands of China, were designed. Measurement results showed that the proposed antennas had stable performance on metal plates of different sizes in the bands of the two modes.(4) In order to make sure these UHF anti-metal RFID tags could be used for the realistic application, the anti-metal UHF RFID tag was tested for car entering-leaving management. Through theoretical analysis and experiment tests, the reader antennas and the anti-metal RFID tag were installed properly. The tag could be well identified by the reader when the car passed through the measurement door with at different speeds. Tested results were in good agreement with the theoretical analysis. It shows that the UHF RFID is a promising technology for vehicle monitoring and management.