| Summary: | 碩士 === 國立宜蘭大學 === 電子工程學系碩士班 === 102 === This thesis aims at designing circularly polarized reader and tag antennas for UHF RFID applications. A RFID system has two types of antenna. One is reader antenna and the other is tag antenna. Traditionally reader antennas are usually very large due to gain and polarization considerations. They are not suitable for the handheld RFID reader so that we proposed a compact printed quadrifilar helix antenna (PQHA). The circularly polarized PQHA is fed by lumped-element feeding system with quadrature-phase and equal output signals. The miniaturized antenna can preserve antenna gain but the bandwidth of the impedance and axial ratio still cover the global UHF RFID band (860-960 MHz) for the portable RFID reader.
The quadrature-phase feeding system was composed of an 180∘power splitter and two wideband quadrature hybrid couplers. All the power splitters were designed by using lumped elements since the sizes of the transmission-line types were too big at the UHF band. The even-odd mode decomposition theory is introduced to analyze the quadrature hybrid and ring hybrid in the beginning. Then, two 90∘quadrature hybrid couplers and one 180∘power splitter were integrated to realize the quadrature-phase feeding system for the PQHA. The integrated feeding system is very small and the size of the constructed structure is only 80 80 1.6 mm3. Next, the miniaturized resonant-type PQHA operated in quadrifilar axial mode was designed and fed by the quadrature-phase feeding signals. The PQHA was optimized by using HFSS simulation tool to miniaturize the antenna size. After finishing optimum design and compact size, the proposed PQHA was fabricated by flexible printed circuit (FPC) and integrated with the feeding system to perform experiment and verification. The measured impedance bandwidth is about 34.1% and the measured maximum gain is 5.8 dBic.
In addition to design the reader antenna, circularly polarized tag design is also important for achieving longer reading range. In this thesis, a circularly polarized cross-dipole tag antenna printed on FR4 is proposed to avoid polarization loss and increase reading range. Two asymmetrically orthogonal cross-dipoles are combined to generate degenerate modes for circularly polarization. A symmetric T-matching transformer is employed to conjugate match with a microchip. The body effect is very significant as the tag is designed for on-body applications. A Jerusalem cross frequency selective surface (JC-FSS) artificial magnetic conductor (AMC) placed on the back plane of the cross-dipole tag is studied in order to insulate the influence from the human body. The study finds that the tag with the JC-FSS AMC can increase antenna gain and reading range. Considering body-proximity effects on the tag, we finally finish a CP tag design with longer reading distances, which is about 13 m.
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