Research On Antennas And Propagation Characteristics In Passive Backscatter RFID Systems

Radio Frequency Identification (RFID) is one kind of the contact-less automatic identification technology via radio frequency signal, which is one of the most developed rapidly technologies in automatic identification technology field. It uses radio frequency to identify the target and acquire data via non-contact radio communication. In recent years, with the growing application requirements for RFID, RFID technology, in particular, working at UHF and microwave bands develops very quickly. Meanwhile, with the operating frequency of RFID system increasing, for passive RFID systems working at the UHF and microwave bands and on the basis of backscattering modulation principle to work, antennas design and performance optimization of tags become very important. Based on this, according to the relevant RFID technologies and standards at home and abroad, combined with the existing research situations of passive backscatter RFID antennas and needs of the practical applications of engineering, applied electromagnetic, microwave and antenna theories, and other related technologies, this paper researched antenna technology, propagation, radar cross section and effects on the identification distance for the major parameters of antenna in the passive backscatter RFID system, the main content and work are as follows:Firstly, this paper describes the basic theory of antenna design and the working principle of passive backscatter RFID system. The research results show that the antenna performance is affected by many factors. Therefore, comprehensive consideration of these factors is required in the design of antennas.Secondly, from the reader antennas generally considered to proceed, reader configuration types and antenna selections are researched according to the practical application for passive backscatter RFID systems. Existing research situations of reader antenna is detailed presented. Based on this, this paper designs a novel microstrip patch antenna structure for backscatter RFID readers, focusing on analysis of effects on the proposed antennas main properties for its various geometric parameters, and deriving an optimization of all parameters for the proposed antennas. Furthermore, the proposed antennas are simulated and measured, and experimental and simulation results are basically consistent. The results show that reader antenna is based on the specific application requirements to select and design, different application categories and the application environment have corresponding reader configuration. Therefore, the corresponding reader antenna is different. Reader antenna design is moving towards a low profile, small size, low cost, multi-band coverage, etc. To fully meeting the application requirements, reader antenna design is a challenging task. Some compact, low cost, wide bandwidth, high gain, circular polarized reader antennas design can be achieved by integrating the single-port-fed, square microstrip patch cutting angles, patch center digging holes, embedded slits and other methods organically in this paper. Designed by this method, the dimensions of the proposed UHF reader antennas are around 69×69mm2, and the dimensions of the proposed microwave reader antennas are around 29×29mm2. Compared to the existing literature, the dimentions of the proposed antennas are reduced by approximately 50%, and the proposed reader antennas have better impedance bandwidth characteristics.Thirdly, antenna design technologies for the passive backscatter RFID tags are studied systematically, and aimed at the existing situations of its researches and practical application, two new type microstrip antenna structures are designed, which can be well meet applications of backscatter UHF RFID tag antennas and microwave tag antennas respectively. Taking into account identification distance of passive backscatter RFID system and tag identificating performance optimization, the results show that impedance matching between an antenna and a chip is very important for tag antenna design in passive backscatter RFID systems. At present, the matches between UHF tag antennas and chips are mainly implemented in two ways of inductive coupling and capacitive coupling, and the matching implementation way is determined according to the quality factor of tag chip. For UHF tag antennas design, using inverted-F and bent line-type structures can reduce the antenna size, the latter of which has become a mainstream design structure. In this paper, impedance properties of the proposed UHF tag antennas with a deformed bent line-type structure are in line with EPC Class 1 Gen2 standard, the dimensions of the proposed antennas are all around 100×15 mm2, and the proposed antennas have some advantages of compact, wide bandwidth, high gain and quasi-isotropic radiation performance, etc. For microwave antennas design for tags, to reduce the size and broaden the bandwidth, a variety of slots are usually embedded in a microstrip patch. In this paper, to reduce antenna size and broaden antenna bandwidth, U and T-shaped slots are embedded in a square patch for microwave tag antennas design. Designed by this method, the dimensions of the proposed microwave tag antennas are around 35x35mm2. Compared with the existing literature, the proposed antenna structure has some advantages of compact, low cost, wide bandwidth, high gain and good radiation characteristics, etc.Fourthly, propagation characteristics are studied for passive backscatter RFID systems. The research results show that path loss models of backscatter RFID systems are differ from traditional wireless communication systems. It is pointed out theoretically that the existing classical 2-ray ground reflection model and some indoor propagation models for RFID systems can be used to help predict propagation characteristics of passive backscatter RFID systems in practical application. This paper gives emphasis on the existing indoor propagation models for RFID systems, whether it is 2-ray model, short-range model or multi-ray model, which is not a good description of the actual RFID system. It is a very urgent and difficult task to build an accurate propagation model for passive backscatter RFID systems.Finally, based on the theory of passive backscatter RFID technology and antenna scattering, some calculating methods for radar cross-section of tag antenna with different loads are presented. Integrated radar cross-section and other major parameters with electromagnetic propagation environment, the identification distance of passive backscatter RFID system is analyzed. Furthermore, by measuring backscattered power in the case of different loads, and then combined with these calculating methods, radar cross-section of the tag is found in this way. Theoretical analysis results are found to be in good agreement with experimental data. The results researched are useful for optimizing tag performance in passive RFID systems. In addition, this paper presents a comprehensive analysis of various parameters of antenna on the identification distance, which is rarely seen at home and abroad.