It is almost invariably true
that unless the antenna is very small, it does have this property. Proper analysis requires
that it should be analyzed using the full set of Maxwell??™s equations rather than the subset
or simpli?¬?ed versions that pertain to electrostatic or magnetostatic problems. A good
example of this phenomenon is provided by the electromagnetic ?¬?eld-sensitive antenna
shown in Figure 4.6, in which there is no obvious effort to couple to either electric or
magnetic ?¬?eld alone.
Such electromagnetic antennas are generally useful for operation in the far ?¬?eld because
far-?¬?eld interrogation systems have shorter wavelengths, and antennas of acceptable size
can no longer be considered to be electrically very small but are merely small.
Despite the earlier distinction, there are electromagnetic label reading environments in
the UHF region in which, through re?¬‚ections, either the electric or magnetic ?¬?eld is
emphasized at the expense of the other. For such situations it is normally useful to take
into account the nature of the driving ?¬?elds in antenna design, and shape the design so that
it is recognizably attuned to one or other of those ?¬?elds.
FIGURE 4.4
Electric ?¬?eld-sensitive label. (From Cole, P.H., Jamali, B., and
Ranasinghe, D., Coupling relations in RFID systems, 2003
Auto-ID Center White Paper Series, 2003 by Auto-ID Center.
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