7.3. At HF, we will derive power transfer from
coupling volume concepts to be de?¬?ned in Section 12.7.4. In those calculations, we can
recognize both electric ?¬?eld-sensitive and magnetic ?¬?eld-sensitive designs of label
antennas.
12.6.1 Magnetic Field-Sensitive Antennas
A common example of a magnetic ?¬?eld-sensitive HF label is shown in Figure 12.7. The
label is 42 mm wide by 47 mm high. The label is designed to have suf?¬?ciently many turns
to provide the resonating inductance for the microcircuit input capacitance, as well as a
?¬‚ux-collecting area in the interior which is as large as practicable consistent with the size
requirement for the label.
Advantages of working in the near ?¬?eld at HF rather than at LF are that the number of
turns required to resonate the microcircuit capacitance is small enough for low-resolution
lithography to be used in antenna construction, and no additional external resonating
capacitance is required.
Clearly the design illustrated in Figure 12.7 is unsuitable for being placed ?¬‚at against
metal, as the boundary conditions shown in Figure 12.2 will not allow a normal component
FIGURE 12.5
A small loop antenna.
FIGURE 12.6
A large loop antenna. (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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