Design And Research Of RFID Reader Antenna Based On Branch Line Coupler

In recent years,with the rise of the Internet of things,RFID technology as an important component of the Internet of things,has attracted much attention,and its application market has expanded rapidly.It is widely used in the fields of electronic toll collection system,logistics and location tracking.In the RFID system,antenna is one of the most critical components,and its performance directly affects the identification distance,efficiency and sensitivity of the system.The main research object of this paper is the reader antenna in the RFID system.And the main target is to realize the broadband,miniaturization and dualfrequency characteristics of the antenna.In the RFID system,the tag antenna mostly uses linear polarization antenna.In order to reduce the loss of recognition caused by polarization mismatch and increase the recognition range,the circular polarization antenna is mostly used in the reader antenna.There are many ways to realize circular polarization.This paper mainly studies the realization of circular polarization antenna based on branch line coupler.Due to the output ports of branch-line couplers have the inherent characteristics of equal amplitude and phase difference of 90 degrees,which satisfies the basic conditions of circular polarization radiation,thus the circular polarization radiation can be well realized by using branch-line couplers.When the two input ports of the branch line coupler are exchanged,circular polarization in different rotational directions can be realized.For the RFID system,it can be used as both transmitting antenna and receiving antenna,which increases the compatibility of the system.For traditional branch line couplers,the length of each branch line is a quarter of the wavelength at resonant frequency.So the size of branch line couplers at lower frequencies will be very large.Firstly,according to the UHF RFID band of the different countries,a miniaturized branch-line coupler with capacitive loading method is simulated.The resonant frequency of this branch-line coupler is 900 MHz and the size is reduced by 64% compared with the traditional one.Then,based on the miniaturized branch-line coupler,two RFID reader antennas working in UHF band are designed and simulated.One of the antennas uses a stacked structure and couples the rectangular radiation patches by a pair of orthogonal slots.The air layer is helpful to improve the antenna gain.The maximum gain of the antenna is 6.6dBi,and the gain is greater than 2dBi in the whole working frequency band.The impedance bandwidth and axis ratio bandwidth of the antenna are 0.76-0.96 GHz and 0.85-0.96 GHz,respectively,which basically cover the application frequency band of UHF RFID in the world.Due to the stack structure and rectangular radiation patch,the overall size of the antenna is still too large.Finally,in order to further decrease the size of the antenna,a compact circularly polarized antenna with low profile is designed based on the circular polarization theory of the loop antenna.This antenna use single-layer substrate and feed by a slot.The impedance bandwidth and axis ratio bandwidth of the antenna are 0.83-1.0 GHz and 0.81-0.99 GHz,respectively,which cover the global UHF RFID application band well.The overall size of the antenna is 100*100*1.6mm3,the gain is stable at about 2dBi,and the circular polarization beam width reaches 120 degrees,which can realize tag recognition in a wide range.Next,in order to improve the compatibility of the system,it is often necessary to install different bands of antennas in a system.So,the research of dual-band and multi-band antennas has become a research hotspot for antenna researchers.In this paper,the dual-band branch line,which is realized by loading step impedance branches,is used as the feeding network.And two loops of different sizes are fed by the network at two orthogonal positions and then dualband resonant antenna is realized.The dual-band antenna operates at the microwave band of RFID(2.45/5.8GHz).The impedance bandwidths of the two frequencies are 2.28-2.8GHz and 5.32-5.92 GHz,respectively.And the axial bandwidths are 2.24-2.7GHz and 5.67-5.91 GHz,respectively.Both bandwidths cover the commonly used international RFID bands.The gain at both frequencies is about 6.5dBi,and the beam width of 2.45 GHz reaches 163.5 degrees.So this antenna can achieve recognition of wide angle range and long distance.