| Antenna is one of the key components of radio frequency identification（RFID）system,it can be divided into near-field antenna and far-field antenna according to the different operating distances.In the near-field system,the tag and the reader are coupled to work through a magnetic field or an electric field.The magnetic coupling has higher stability in liquid and metal environments,and it is more popular in practical applications.Far-field antenna are widely used in fixed and handheld readers such as logistics and warehousing due to their strong radiation ability and large working area.Based on the application of ultra-high frequency（UHF）RFID system,this dissertation has conducted research from three aspects: magnetic coupling near-field uniform antenna,handheld high directional gain far-field antenna and near-far-field composite antenna.The main research work and contributions of the dissertation are as following:1.In order to meet the demand of the near field antenna of UHF RFID reader for the uniform field distribution in the broadband application,combining the two technologies of in-phase current loop and reverse current pair,a center-fed near-field uniform multi-loop antenna is proposed.Four curved four-element printed log-periodic dipole antennas fed with double-sided parallel-strip lines form into a ring array,which is easy to obtain wide-band impedance matching and effectively improve the uniform magnetic field distribution in the circular region.The antenna has been fabricated and measured.The results show that |S11| is less than-10 dB within the frequency range of 799～971 MHz,which covers the universal UHF RFID band.When the input power is 30 dBm,a 100% recognition rate is achieved within 20 cm×20 cm×15 cm above the antenna,and the maximum reading range is 22 cm.The identification area can be increased by increasing the number of log-periodic antennas,while the uniform field distribution of the identified region can be improved by adjusting the number of curved dipole elements.2.Based on the advantages of high front-to-back ratio（FBR）and miniaturization of Moxon antenna,and combining the broadband operating characteristics of the butterfly dipole antenna and the high directivity of the quasi-yagi antenna,two far-field composite antennas for RFID handheld readers were designed.Combining the Moxon antenna with the butterfly dipole antenna to design a miniaturized broadband highiy directional antenna,the measured results show that the reflection coefficient of the antenna is less than-10 dB in 865～1054MHz;in 902～928 MHz,the front-to-back ratio is greater than 15 dB,and the gain is higher than 5 dBi.The antenna is small in size,low in cost,easy to tune,easy to manufacture and process,and has broad application prospects.Combining plane Moxon and quasi-yagi,a compact planar composite antenna with dual-band performance is proposed.The Moxon structure is designed to operate in the lower band,while the quasi-Yagi structure is designed for the higher band.Measured results show that the fabricated antenna has a moderate gain of better than 5 dBi and a large front-to-back ratio of more than 15 dB in both UHF 902～928MHz and industrial scientific and medical（ISM）2.4～2.4835 GHz bands with low cross-polarization levels.The antenna has the advantages of simple structure,easy processing and excellent performance in terms of impedance matching and radiation patterns.It can be easily adjusted for other wireless applications,where both end-fire radiation and dual-band operation are required.3.A near-far-field composite antenna for radio frequency identification applications is proposed.The antenna is based on the Archimedes spiral antenna structure.At 2.45 GHz,the reflection phase of the Aritificial Magnetic Conductor（AMC）reflection structure is 0,thereby reducing the height of the antenna and achieving far-field radiation.Load a 100 Ωgrounding resistance at the end of the spiral arm,and the antenna current presents a traveling wave distribution when working in a low frequency band,so the near field has a good magnetic field distribution characteristic.The antenna has been fabricated and measured.It is a near-field operating mode in 840～960 MHz,and the gain in the frequency band is less than-2.5 dBic;When the input power is 30 dBm,a 100% recognition rate is achieved within 20cm×20 cm×5 cm above the antenna,and the maximum reading range is 11 cm.It is a far-field operating mode of 2.4～2.4835 GHz,the |S11| of the entire frequency band is less than-10 dB,the axial ratio is less than 3 dB,and the gain is greater than 7 dBic.The antenna has a simple principle and excellent performance,and can be well applied to a far-field system. |
