Study On Passive UHF RFID Tag Antenna

Internet of things (IOT) is an emerging strategic industry and has gotten great attention globally. As a popular technology of IOT, the Radio Frequency Identification (RFID) technology is also experiencing rapid development and widespread application. In compared with other RFID technologies, the ultra high frequency (UHF) RFID technology appeal more concern with further reading distance, larger capacity and lower cost. A complete UHF RFID system comprises tag, reader and antenna (tag antenna and reader antenna) three parts. The tag is a key factor of RFID system and should be fixed on the object with information. As all known the metallic surface has great influence on performance of tag antenna so it’s necessary to research design method of tag antenna for metallic surface. What’s more, the best case is that tag antenna can maintain stable performance sticked on different object including metallic and nonmetallic surface.This paper gives a detailed study of UHF RFID system on tag antenna design and performance measure methods based on antenna design knowledge and electromagnetic filed theory.First, in consideration of characteristics of UHF RFID system, the thesis explores different types of antennas used for passive tags and finally concluded that microstrip antenna (patch antenna) is a best choice. At the same time, this part also introduces the simulation methods, performance indexes and measuring methods for tag antenna.Second, the thesis design a UHF RFID tag antenna working in850-960MHz frequency for a steel plant project. This tag antenna has simple patch structure with short edge on the sidewall. The tag chip Monza4is used for information storage and transmission.In theory the transmission coefficient of the tag antenna can reach0.95and the practical max read range is also longer than3meter. The tag antenna has a good performance which satisfies the design requirements. In order to explore disadvantages and advantages for passive tags in2.45GHz frequency band, a tag antenna working in this frequency band is designed. The antenna’s radiation body is made by three dipole-like structures based on by glass and FR4substrate which can effectively reduce antenna size and at the same time ensure the impedance stability. It can work on the metallic surface and non-metallic surface with the transmission coefficient (ｔ) achieving0.98and the reader range is respective3.12m and4.31m which shows a good stability. The read range is shorter than that of active tag but the cost is lower. In conclusion, it’s better to choose the860～960MHz frequency band for passive UHF RFID tag to achieve long read range and low cost.Third, by reasearching the patch cavity theory, the thesis proposes an effective method to regulate the impedance and gain of tag patch antenna.This method will change the size of patch antenna’s ground at the radiation gaps and the nonradiation gaps to achieve matched impedance and increase antenna gain. At the same time, the thesis also puts forward simple and general design methods for tag patch antenna to make it working on different surfaces with stable performance.Finally, by using two single antennas and an isolation structure, the thesis proposes a compact tri-band MIMO antenna, which shows the probability of MIMO antenna with small size and good performance.And the thesis also does some research on the application area of MIMO antenna in ultra-frequency RFID system.