Research And Application Of Environmental Tolerance Of UHF RFID Antenna

Ultra High Frequency(UHF)antenna is susceptible to surrounding environment,especially metal environment,which results in changing of antenna input impedance,gain,or directivity,even results in antenna malfunction.This drawback of UHF antenna limits the use of UHF RFID technology,especially item-level or part-level applications.In order to overcome environmental effect on UHF antenna,this thesis has conduct in-depth studies including: mechanism of the environmental influence on UHF antenna,designing of environment tolerant antenna,and a set of solutions have been proposed for most common antennas used in UHF RFID.Some theoretical and engineering achievements have been made.Major research works and contributions are summarized as follow:Mechanism of the environmental influence on UHF antenna.Radiation model has been proposed to investigate antenna malfunction when dipole antenna is near metal.Analytical fomulas of dipole antenna near metal have ben derived,and it was found that the reason for malfunction of dipole is serious impedance mismatch which is caused by changing antenna input impedance.Simulation models have been proposed based on transmission model to investigate metal edge effect on patch antenna,and it was found that metal edge effect mainly causes impedance mismatch due to changing radiation conductance and resonant frequency of radiation patch,and the edge effect along non-radiation edges of the patch antenna has much stronger influence on patch antenna than that of radiation edges.Dipole antenna based on transmission mode current has been proposed and investigated to solve impedance mismatching when dipole antenna is brought near metal.In order to introduce transmission mode current into dipole antenna,an offset fed folded dipole antenna with one open end has been proposed.Transmission models for the offset fed folded dopole in freespace or on metal have been established,analytical formulas for input impdance have been derived,and it is proved that increasing the ratio of transmission mode current can be utilized to stabilize input impdance of dipole antenna.By introducing transmission mode current,dipole is capable of maintaining good performance even when brought near metal.Patch antenna based on capacitive feed has been studied to counter impdance mismatch caused by metal edge effect.By comparing transmission models of direct feed and capacitive feed,it was found that capavitive feed is capable of reducing the impact of environment on antenna input impedance.By utilizing virtual ground in the middle of radiation patch,via to ground can be elimilated for patch antenna,which makes the capacitive fed patch antenna both environmental tolerant and compatible with low cost manufacturing process.Broadband impdance optimization based on real code genetic algorithm.Miniaturized UHF antenna has very high Q factor,and needs broadband matching to improve its environmental adaptability and reliability.Against drawbacks of existing broadband impdance optimization methods,new network topology and chromosome representation method have been introduced to the real code genetic algorithm,and network topology and components values can be optimized in a single run.By utilizing the real code genetic algorithm in an electrically small loop antenna,the feasibility of the algoritm has been verified,and both bandwidth and environmental adaptability of the electrically small loop antenna has been largely improved.Methods proposed in this thesis has been used for the designing of high performance UHF RFID tags according to the requirement of high-speed railway catenary inspection and airplane seat management.Long operational range environmental tolerant UHF RFID tag and miniaturized metal-mountable tag with bandwidth over 100 MHz have been developed respectively.The detailed tag design and manufacturing process has been revealed.Performance of the tags have been verified by field tests,the long range environmental tolerant tag satisfied the requirement of catenary inspection and reading range differences is within 15% in all field test conditions;performance of the broadband miniaturized metal-mountable tag reaches level C of AS5678 standard,and meets requirements of aviation industry even embeded in metal cavity.