超高频天线设计基础知识

UHF antenna design basicsOutline–Energy transmission–Antenna design tuning–FF9510 tuning–PminMeasurement setup–Broadband design in RFID–Antenna design for electronicsPEIRP vs. PERP PowerPERP : Effective radiated power The amount of power that would be necessary at the input terminals of a reference half-wave dipole antenna in order to produce the same maximum field intensity.? PEIRP :Equivalent isotropically radiated power The amount of power that a theoretical isotropic antenna would need to emit to produce the peak power density observed in the direction of maximum antenna gain.
Energy Transmission (Reader to TAG)Transferred power from a reader antenna to the chipRead Range of an UHF ChipPmin -Anechoic chamberPmin -Anechoic chamberPmin –Measurement setupEPC global document “Tag Performance Parameters and Test Methods Version 1.1.1”.Pmin –What can be measured?The information gained from this measurement method is the minimal required power level at the label for powering the IC.Gained information:?Label sensitivity?Label resonance frequency?Label bandwidth?Detuning on material?Production stabilityPminvs. Read-RangeRead-Range is the inverse of PminReference materialNXP established reference materials with dielectric constant (εR) ranging from 2.12 to 12.5FF9510 G2iL 120 μm reference materials―Note the sensitivity of performance with different materials―Thicker the material higher the parasitic losses and poorer the performance―A broadband design is less impacted by such effectsBroadband Design Basics? Bandwidth is dependent on the Q factor? For an antenna tuned at a frequency wo, Q factor is defined by theratio of reactive energy (W) to accepted power (PA)? For electrically small antennas, Q factor is dependent on the slope of impedance curves as given by the approximate expressionwhere R and X are the feedpoint dependent resistance and reactance of the antenna, R’ and X’ are the first derivativesBroadband Design in RFID?To be a global tag, the tag should have a 120 Mhz bandwidth extending from 840-960MhzFor more information on UHF regulations, please see reference [2]Challenges?Inherent challenge in matching to a capacitive load-means antenna is inductive?As tag size reduces the antenna Q increases which makes the impedance matching narrow band?The environment tagged is mostly capacitive which further increases antenna Q and makes broadband matching difficult?A passive low power circuit-means little or no compromise on matching efficiencyStagger Tuning?Idea-Two resonant circuits coupled optimally to achieve a broadband response?Most commonly implemented as shown below –a loop coupled to a dipoleRFID tag circuit -Superposition of Loop and Antenna resonances请输入正文Effect of coupling on stagger tuning|S11|vs. FrequencyKey points on stagger tuning?There exists an optimum coupling for stagger tuning depending on frequency of separation of the two resonances. If the frequency of separation is too large, stagger tuning is not possible?When optimally coupled the resonance with lower Q dominates the response?When tightly coupled the resonance is pulled towards the lower resonanceReference antenna design FF9510FF9510 – Change loop sizeFF9510 – Change dipole lengthFF9510 – Change connection dipole loopFF9510 –Total Gain 868 MHz

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