| RFID, as an emerging technology for contactless automatic identification, is widely used in retail, transportation logistics, warehouse management, access control land so on in recent years. UHF RFID means passive radio frequency identification technoque at 800/900 MHz band. Because of its merits, like fast reading rate, able to recognize the high-speed moving objects, able to identify large numbers of objectives, far working distance, lowcost,UHF RFID is got attention by the governments and is developing fast. Reader antenna as key components of RFID system, will directly affect the performance of RFID system. Reader antenna is usually required to be circularly polarized. And according to some applications,Reader antenna is also required to be wide band or miniaturized. Phased array antenna has a high gain and scanns beams fast and flexibly. Therefore, phased array antenna technology is applied to the UHF RFID systems, in order to extend reader’s coverage range and improve reading rate in this paper.The fundamental theory of the phased array antenna and UHF RFID technology is expounded firstly. And a simple phased array system which will be used in the RFID system is proposed. The system includes antenna array, the phase-shifting feed network, phase shifter control circuit. The main research content of this paper is to design the circularly polarized antenna and array. Two different types of circularly polarized antenna are designed as elements of phased array, in order to satisfy different applications.In the third chapter, a circularly polarized antenna element with broadband and high-gain is designed, mainly applied for the fixed RFID system. After the analysis and comparison of common types of circular polarization antenna, I design the antenna based on the form of circular microstrip patch antenna. After I analysed the Circular polarization technology and bandwidth widening techniques of microstrip antenna,the method of orthogonal doubly-fed circular patch is choosed to achieve circular polarization. And by the use of a coupling patch and air substrate,the designed antenna gets a wide working band,860 MHz to 960 MHz, covering the operating frequency band of the all standard of UHF RFID in the world.In the fourth chapter, a small circularly polarized antenna element, mainly used for handheld reader has been designed. The antenna adopts the resonant quadrifilar spiral antenna structure. The resonant arms of antenna are replaced by bending inverted-F antenna printing on the PCB, to reduce size and to improve impedance matching.The circular polarization performance of this antenna is strong and stable, and wide-width beam. It’s frequency band coverages 902 MHz to 928 MHz range, which is the most popular operating frequency band for UHF RFID.In the fifth chapter, antenna arrays will be fabricated with the antenna element designed in the former chapters. There are two kinds of array structures, 1x4 linear array and the 2x2 area array. The array’s performance,such as gain,Beam scanning and ow side-lobe have been Analyzed and optimizated. Simulation results show that the designed antenna array with high gain, low sidelobe, beam scanning, are usefull for the phased array’s application to RFID field. |
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