It is almost invariably true that
unless the antenna is very small, it does have this property. Proper analysis requires that it
is 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
12.13, in which there is no obvious effort to couple to either electric or magnetic ?¬?eld alone.
12.7 Coupling Relations
12.7.1 Near and Far Fields
In the analysis of the performance of RFID systems, it is important to consider whether the
labels are placed in the far (propagating) or near (energy storage) ?¬?elds of the interrogator
FIGURE 12.9
An 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. With permission.)
FIGURE 12.10
A parallel plate 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. With permission.)
240 RFID Handbook: Applications, Technology, Security, and Privacy
antenna. When that antenna is of small gain, the distance which divides the near and far
?¬?elds is the size or the radian sphere of radius r??l=(2p), where l is the free space
electromagnetic wavelength at the operating frequency.
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