Thus, the antenna of Figure 4.2 is about
18 times more sensitive than that of Figure 4.1. Unfortunately, this increased sensitivity
does not translate to a corresponding increase in near-?¬?eld range, as small coil interrogator
antennas have an inverse sixth power decrease in energy density per unit volume as
distance from the interrogator increases.
Clearly, both of the designs illustrated earlier are unsuitable for being placed ?¬‚at against
metal, as the boundary conditions will not allow a normal component of magnetic ?¬‚ux
density at the metal surface. For this situation, the label antenna employing a solenoid with
a magnetic core design shown in Figure 4.3 can be employed.
Eshraghian et al. (1982), Ranasinghe (2007), and Cole et al. (2003) show that without the
magnetic core the coupling volume of a long solenoid is just the physical volume, but when
a magnetic core is inserted, the coupling volume increases by a factor equal to the effective
permeability of the magnetic core.
FIGURE 4.2
A large-loop antenna for an HF 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.)
FIGURE 4.3
An antenna for HF operation against metal. (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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