3.5). Hence considering
the requirements of practicability and maximum power transfer, the ??????best??™??™ antenna for an
RFID IC is a planar inductive antenna with a reactance that is able to tune out the
capacitance of the label IC and also provide an adequate match for the real impedance of
the label IC. The following section will consider modeling the input impedance of a typical
label antenna by formulating a three-parameter circuit model.
RFID IC
Thin substrate
(polyethylene
terephthalate, PET)
Antenna terminals
(etched out copper
or aluminum)
Protective coating
FIGURE 4.9
A direct chip attachment of an RFID IC.
RFID IC
Mounting pad
FIGURE 4.10
An RFID strap.
Far-Field Tag Antenna Design Methodology 71
4.3.2 Label Antenna Equivalent Circuits
Equivalent circuits for small magnetic ?¬?eld-sensitive antennas and electric ?¬?eld-sensitive
antennas are shown in Figures 4.11 and 4.12. The range of validity of these equivalent
circuits is where the reactance properties of the antenna may be described by a single
parameter, L or C. When the antenna is larger, the reactance properties can be described by
an appropriate mixture of L and C.
In Figure 4.11, the source voltage is the voltage induced in the ?¬‚ux-collecting area of the
coil by magnetic ?¬?elds other than those ?¬?elds which resulted from current ?¬‚owing within
the coil itself.
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