RFID阅读距离.doc

Performance of any wireless system depends on several antenna characteristics and propagation channel properties which include： Antennas: Operating frequency band; Gain characteristics maximum gain, radiation pattern, beam-width ; Matching VSWR or return loss ; Polarization axial ratio ; Sensitivity to nearby objects with different properties. Propagation channel: Path loss; Spatial and temporal fading statistics Ricean/ Rayleigh parameters, delay spread, coherence bandwidth . The power chip absorbed by the RFID tag chip can be expressed as： is the output power of the reader, is the impedance matching coefficient between the reader and its antenna, is the coupling coefficient the power transmission loss between the two arbitrarily oriented reader and tag antennas, is the impedance matching coefficient between the tag chip and its antenna （能量传输系数） is the reader output impedance typically 50 Ohm , is the transmitting antenna impedance, is the tag antenna impedance, is the chip impedance Main factors which affect coupling Coefficient are: Reader and tag antenna geometries; Relative position of antennas distance and orientation ; Environment, including any objects near antennas The signal strength at the tag location can be obtained from the transmitted EIRP and path loss as: is the output power of the RFID reader transmitter and t is the gain of the reader antenna is the transmitted EIRP . Passive UHF RFID tag can be viewed as an RF source emitting a modulated signal with differential EIRP: S is the power density of an EM wave incident on the RFID tag. E is the electric field of an incoming wave The power of the modulated tag signal received by the mono-static reader antenna in an arbitrary propagation environment can be written in terms of differential EIRP as: In free space, equation can be rewritten in the form of a classical radar equation: The power collected by a perfectly matched antenna load chip can be expressed either via incide