A passive tag contains an
antenna, which is used for two purposes: (1) to harvest energy from the reader signal,
4 RFID Handbook: Applications, Technology, Security, and Privacy
command, and carrier wave (CW) and (2) to communicate with the reader. The amount of
energy that the tag receives depends on many factors, but the distance between the reader
and tag, the reader transmitter power, and the ef?¬?ciency of the RFID tag antennas are the
keys. The following section elaborates on all the factors governing the received power.
The impedance matching between the antenna and the tag circuitry determines the
amount of energy that can be transferred between the antenna and tag circuitry. When
matched, the maximum amount of energy is transferred between the antenna and the tag
circuitry and this occurs when the imaginary parts of the complex tag circuitry impedance
and antenna impedance completely cancel each other resulting only in a real resistance.
The target input impedance value is not 50 V as in a typical RF system, which provides a
degree of freedom and ?¬‚exibility in an antenna design and matching networks. When the
matching is not optimal, a parasitic imaginary part is present in the impedance quantity.
This can be either capacitive or inductive, and results in less power being transferred from
the antenna to the tag circuitry.
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