The Research Of Dual-band Circularly Polarized Antennas For RFID Reader Applications

Radio Frequency Identification(RFID)is a kind of automatic identification technology in the field of wireless communication.It has been widely used in many industries such as purchasing,manufacturing and logistics management.Since the reader antenna is an essential component to RFID system,it has attracted more and more attention in recent years.There are mainly two research directions about the reader antenna.One of them is broadband circularly polarized(CP)technology and the other is dual-band CP technology.The dual-band CP reader antenna for RFID applications is studied in this paper supported by the National Natural Science Foundation of China(Foundation number:61401056).The mainly completed works are as follows:(1)A single-fed coupling structure with a dual-band complex impedance transformer is proposed and the dual-band dual-CP antenna with a small frequency-ratio is studied.Since the Chinese UHF frequencies authorized for RFID applications are 840 to 845 MHz and 920 to 925 MHz,the dual-band reader antenna characterizes a frequency-ratio of 1.095.However,the existing dual-band CP microstrip antenna structure has difficulty in achieving different polarizations in lower and higher frequency bands simultaneously in the case of a small frequency-ratio.In this paper,an 840/920 MHz dual-band dual-CP reader antenna is designed by employing the proposed feeding structure.The proposed antenna not only gets good input impedance matching within two working frequency bands but also achieves different polarizations while right-hand circular polarization in lower frequency band and left-hand circular polarization in higher frequency band.(2)The dual-band CP antenna with a large frequency-ratio is studied by the use of the proposed single-fed coupling structure with a dual-band complex impedance transformer.The reader antenna operating at 915 MHz and 2.45 GHz simultaneously has the merits of reading and writing data rapidly in far fields of antenna than low frequencies and it can also achieve a good compromise between antenna performance and production costs with respect to the reader antenna working at 5.8 GHz.However,it characterizes a frequency-ratio of 2.678.After carrying out circular polarization,the difficulties in the design of single-fed dual-band antenna is how to get good dual-band input impedance matching in the case of a large frequency-ratio.Then a 0.915/2.45 GHz dual-band CP reader antenna is designed by employing the single-fed coupling structure to realize dual-band circular polarization and the dual-band complex impedance transformer to achieve dual-band impedance matching.(3)A wideband power distribution and phase shift feeding network operating at 0.8 to 2.6 GHz is designed and the bandwidth enhancement technology of the 0.915/2.45 GHz dual-band CP antenna is studied.The results show that both the impedance bandwidth and AR bandwidth of the double-fed 0.915/2.45 GHz CP antenna fed by the wideband power distribution and phase shift feeding network are enhanced compared with the single-fed 0.915/2.45 GHz CP antenna with a dual-band complex impedance transformer.