Investigation Of Ultra High Frequency Near-Field Radio Frequency Identification Reader Antenna

Nowadays,with the rapid development of the Internet of Things（IoT）,the ultra high frequency（UHF）near field radio frequency identification（RFID）system based on inductive couple has been widely used in modern intelligent logistics,warehousing management,industrial sensing and other fields due to its fast reading speed,high data transmission rate and environmental adaptable.The reader antenna as the major part of the system has an important impact on the reading performance.At UHF band,the near field RFID reader antenna is required to provide a large interrogation zone.To avoid the interference between systems and the misreading of tags,the reader antenna needs to limit its near field magnetic field distribution and suppress its radiation.In addition,in the design process,the placement of tags is also a consideration.This paper has carried out a series of research on these indicators and problems.The innovations of this paper are as following:1)This paper proposed a UHF near field RFID reader antenna combing the co-line dipole and the modified Yagi-Uda antenna.During the research of the near field performance of co-line dipole,it is found that a large and uniform magnetic field distribution can be generated through controlling the space between the dipoles and the amplitude and phase of the surface current.Meanwhile,utilizing the travelling wave characteristic of Yagi-Uda,the proposed antenna can achieve a large interrogation zone.Moreover,by introducing two extra absorbing load units and two extra reflectors,the proposed antenna can limit its near field magnetic field distribution and suppress its radiation.2)Based on the segmented electrically large dipole,this paper proposed a UHF near field reader antenna.The results of dispersion analysis show that by the ways of segment and capacitive load,the surface current of the dipole with the 1.5λ（where,λis the wavelength of corresponding frequency）can be kept nearly in-phase and present an approximate cosine distribution.Based on this,the proposed antenna is made up of two identical segmented electrically large dipoles,in which one is the feed unit,the other as parasitic unit.Through adjusting the space between the two units to form the opposite directed currents（ODCs）,the proposed antenna can generate a large,strong and uniform near field magnetic field distribution.The experimental results show that under the height of 50 mm,the interrogation zone is about 480 mm′270 mm.And,the band of S₁₁￡-10 dB is 53 MHz from 884.5 MHz to 937.5 MHz.3)This paper proposed a UHF near field RFID reader antenna based on the segmented electrically large dipole array.This antenna consists of a segmented electrically large dipole array and two convex parabolic plates.Based on the 2),the surface currents of the dipole array can be kept in one direction at any time by loading appropriate lumped capacitors,so that the dipole array can generate a uniform near field magnetic field.The two convex parabolic plates as reflectors can enhance the strength of the near field magnetic field between them and lower the strength outside them.Moreover,by adding the extra two plates can improve the impedance characteristics of the proposed antenna and suppress its radiation.The measure results show that the proposed antenna has the band about 117 MHz from 843 MHz to 960 MHz,which nearly covers the global UHF near field RFID standard.Under the height of 50 mm,the interrogation zone of the proposed antenna is about 480 mm′300 mm,and outside the region tags are difficult to be read.4)This paper proposed a cross dipole to be used in UHF near field RFID applications.This antenna is a single feed system with two perpendicular dipoles.Theoretical analysis shows that the proposed antenna can generate a strong and uniform near field magnetic field distribution in the direction parallel to its surface,which is benefit to read the tags placed vertically on its surface.The strength and uniformity of the near field magnetic field can be adjusted by changing the length of dipoles.The measures show that the proposed antenna has a large bandwidth about 210 MHz from 852 MHz to 1062 MHz.Its relative bandwidth is up to about 21.9%.Otherwise,under the condition of the 23dBmW input power and the height less than 104 mm,the proposed antenna can read the tags placed vertically on its surface within the area of 142 mm′142 mm.Through the design and research of the previous four antennas,this paper provides a new solution for some problems existing in the current UHF near-field RFID reader system,enriches the design method of reader antenna,and helps to further promote the development and application of Internet of Things.