In this light, the cases
presented earlier are reevaluated and the results are presented in Table 4.3.
From the values in Table 4.3, it can be observed that in practice, if the tag chip has an
R??2500 V and a C??500 fF, the theoretically achievable minimum re?¬‚ection coef?¬?cient
becomes even smaller and still presents no practical limit to the maximum power transfer
to an RFID chip across the UHF RFID bands in the regions considered earlier.
4.3.4 Environmental Constraints
Understanding the effects of various environmental factors on a tag antenna is important
so that suitable antennas can be developed to overcome any dif?¬?culties.
Liquids and metals play an important role in the performance with respect to the manner
in which they affect electromagnetic waves. High dielectric and lossy materials such as
liquids absorb or attenuate UHF RF energy and detune tag antennas reducing radiation
ef?¬?ciency, while metals can either absorb or re?¬‚ect RF energy depending on the amount
and shape of the metal. The unwanted result of a tag??™s reduced performance when
attached to materials with high dielectric constants or metallic objects needs to be taken
into consideration during the tag design process.
TABLE 4.1
Regulated Frequency Allocations in the UHF Band for RFID Use
in a Number of Major Geographic Regions around the World
Region Frequency Range (MHz) Bandwidth (MHz)
Europe 865??“868 3
United States 902??“928 26
Japan 952??“954 2
Australia 918??“926 8
TABLE 4.
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