Research On Reader Antenna And Key Technology In RFID System

In recent years, radio frequency identification (RFID) is greatly developed. Reader antennas are very important component in RFID systems. Generally speaking, high gain and circular polarization antenna or antenna array is popular in far field RFID system. Strong magnetic field, wide bandwidth and low gain near field antennae also become hot research issue. On the other hand, isolation between input and output of the RFID reader is very important because the isolation greatly influences the sensitivity of whole system. Decreasing the power level of transmitter in the receiver signal channel is very important for the performances of RFID system.The studies of the paper mainly focus on the theory and experiment of the radio frequency identification systems. In order to improve the performances of RFID system, the antenna of backscattered RFID, circular polarization antenna array, circular polarization radome, near field antenna, RFID receiver and isolation between input and output are all researched in detail. The conclusions of this paper are beneficial for the improvement of RFID system. The main inovations are listed below.1. The axial ratios of single circular polarization antennas in any kinds of planar antenna array are deduced. The axial ratio of dipole antennas array, rectangle microstrip antennas array and circular microstrip antennas array are all analyzed by the mathematical method. VSWR, isolation, axial ratio and pattern of rectangle microstrip circular polarization antenna array are analyzed in detail by HFSS. A novel circular polarization antenna radome is proposed. The antenna radome could be located above many kinds of linear polarization antenna and employed as polarizer and director. In the experiments, the radome is located above the single microstrtip linear polarization antenna, and then the antenna could generate circular polarization wave and increase the gain by3dB.2. Double S antenna (DSA) and two layers and open circuit terminal (TLOT) RFID near field antennae are proposed in this paper. The two antennas have advantages of wide bandwidth and strong magnetic field. The current of outer loop could keep in phase because of the folded metal line. One of the proposed antennas could generate strong and in phase magnetic field because weak current in the inner metal line. Another antenna is composed of two PCB layers which are radiation layer and feed network layer. Metal line is printed on the top layer whereas feed network is printed on the bottom layer. The feed network and radiation part are separated in order to avoid interruption. The terminal of the antenna loop is folded, so that the current of outer loop is strong enough because the outer loop is far away from the current node point. The processing price is reduced because no balun is employed.3. Metal and liquid around TLOT antenna is investigated in detail, it is found that the antenna could be greatly changed when the antenna is influenced by the liquid and metal. Three experiments confirm that the antenna could also be influenced by other kinds of medium. In order to reduce the negative effect of the metal on magnetic field orientation, inductance-like terminal is proposed. This novel structure could bring long phase-shift of current. The simulation and experiment results confirm that the novel terminal brings weak influence on magnetic field. The magnetic field empty-hole of the antenna is greatly reduced.4. Directional coupler improvement and carrier cancellation are proposed. In this paper, a novel RFID signal receiver is designed in order to receive the transmitted and received signals of RFID system. The sensitivity of the RFID receiver is greatly restricted by huge power difference of the reader signal and tag signal. High impedance line is connected to the coupling port in order to generate reflected signal which will cancel the signal of isolation port, the directivity of the directional coupler become30dB by using this method. Carrier cancellation is a novel method which is applied to reduce the power of the transmitter in the receiver channel. Logarithmic detector, SCM, vector multiplier, power splitter and directional coupler are employed to generate cancelling signal in the tag receiver. After using the carrier cancellation, the power level of reader signal in tag signal channel is only1/12of the original power level. The carrier cancellation could also cancel the reflection of antenna which is hard to be elimitated by the directional coupler or circulator.