Study On Circularly Polarized RFID Antenna

Owing to the exponential growth in the applications of radio frequency identification (RFID), the design of RFID antenna with perfect characteristic is presently one of the most challenging topics. The use of commercial RFID system in practical environment usually requires circularly polarized (CP) reader antenna for orientation diversity. At present, commercially available RFID tags are commonly designed as dipole or microstrip-type with linear polarization (LP), thus, the polarization mismatch between the reader and tag will results in only half of the transmitted power received by the LP tag. Theoretically, the maximum reading range of a CP tag can be improved greatly when the reader is also a CP type (exhibiting the same rotation). The design for circular polarization RFID antenna has a great significance.Based on antenna and microwave circuit theories, circularly polarized RFID antennas are studied in this dissertation. The main academic contributions are summarized as follows.1) Based on the radiation characteristics of dipole antenna, a circular polarized RFID tag antenna is proposed. By using grooving technology, two dipole antennas can achieve same lengths and unequal embed slots. One of dipoles has equal real and imaginary parts of impedance, while another dipole has the complex-conjugated impedance. Thus the difference impedance angle of the dipoles is 90°. Then, the circularly polarized radiation is attained. A modified inductively coupled loop is applied for the impedance match between the antenna and tag. The equivlent circuits of inductively and capacitively coupled fed are proposed. This coupling feed can isolate the real part and imaginary part of the impedance, which made the antenna design simpler. Finally, the circular polarized radiation and impedance matching have been well explained and the design procedure of the antennas has been summarized.2) Based on microstrip antenna, a compact circularly polarized tag antenna in UHF band for metallic object application is proposed. The process for the formation of circularly polarized wave and impedance matching is explained in detail. The compact CP operation is achieved by only adjusting two pairs of unequal crossd-slit lengths. The complex impedance matching of the proposed tag antenna can be easily obtained by selecting the proper dimensions of a quarter-wave microstrip line and short-circuited stub line. This method isolates the radiation structure and feed structure. Finally, in order to ensure its application in the metal surface, the simulation results of the antenna are given.