Research On Antenna Of Far - Distance RFID Reader In UHF Band

Radio Frequency Identification (RFID), is a tempting technology for identifying tags wirelessly using the far-field radiation property or near-field coupling property of electromagnetic wave generated by an reader antenna. The efficiencies of many systems such as medical products, pharmaceutical logistics, bio-sensing applications will be greatly enlarge. As one of the critical components, reader antenna has been widely concerned. The antenna employed in RFID system can be divided into low frequency (LF), high frequency (HF), ultrahigh frequency (UHF) and microwave frequency (MW) antennas.Recently, near-field UHF band RFID received a lot of attention since its way of identifying tags by near-field coupling. The biggest challenge in near-field UHF band RFID system is to achieve a relatively strong and uniform magnetic field distribution. In low-frequency and high-frequency near-field RFID system, the strong and uniform magnetic field distribution is easily achieved since antennas are electrically small. But in UHF band, the dimension of the antenna is comparable with one wavelength, the current distribution along the loop reverses at every half-wavelength which would make it complicated in achieving uniform magnetic field.Besides, in order to be applicable in real environment, near-field antennas are usually backed with shield plate made of metal. With the presence of a metal plate near the antenna, the uniformity and strength of the near-field will be influenced as well as far field radiation property. In this thesis, the influences of the shield plate on near-field properties are studied, as well as its influences on antenna input impedance, band width, and field distribution.We studied two strategies to achieve far/near field antenna. One is the combination of a microstrip antenna and an opposite directed currents antenna; another is the combination of a folded dipole antenna and an opposite directed currents antenna. Both of them can used to read near field tags as well as far field tags. Besides, the near-field of the far-field antenna can compensate the dead zone of the opposite directed near-field antenna, the near-field antenna will obviously influence the H plane radiation pattern of far-field antenna.At last, a far/near field antenna based on folded dipole and opposite directed currents is fabricated and a test platform is constructed to evaluate the properties of the antenna. Results are given, and a good consistency is achieved with the simulated results.