12.7.6 Near Field??”Electric Field
In RFID systems, the coupling can be via the magnetic ?¬?eld or the electric ?¬?eld. While in
the near ?¬?eld the coupling is almost always chosen to be by way of the magnetic ?¬?eld, it
is possible with an appropriate electric ?¬?eld antenna to couple to the electric ?¬?eld, and
that can be done whether the tag is in the far ?¬?eld or the near ?¬?eld. The energy transfer is
then provided by the electric ?¬‚ux terminating on the antenna surface and inducing a
charge on the antenna. The induced charge will oscillate as the ?¬?eld oscillates to produce
a current.
The issues of understanding and optimizing coupling to the electric ?¬?eld are important.
The coupling volume theory developed here for the electric ?¬?eld case assists in that effort
and allows comparisons between different antenna structures for their effectiveness and
ef?¬?ciency in terms of their actual physical volume and the coupling volume. It is also of
interest in making comparisons with the magnetic ?¬?eld case.
When we can focus on the energy storage measure of exciting ?¬?eld, which is the reactive
power per unit volume in the ?¬?eld created by the interrogator at the label position, we can
de?¬?ne a ?¬?gure of merit of a label antenna as the ratio
Vc ??
Reactive power flowing in the untuned label capacitor when it is open-circuited Volume density of reactive power created by the interrogator at the label position
,
which clearly has the dimensions of volume, and is for this reason also called the coupling
volume of the label antenna.
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