A strip line transmission line
(not visible in the picture) on the underside of the right-hand half conveys the driving
signals from the connecting point at the center of the bottom to the driving point terminals
at the center of the top. When the loop antenna is made large such as shown for a 1.2 m
square loop in Figure 12.6, the capacitive tuning elements are best distributed around the
circumference. The balanced drive of Figure 12.5 cannot be implemented in Figure 12.6, but
other balanced drive arrangements can be made.
12.5.2 Far-Field Creation
The almost universal choice for the creation of far ?¬?elds, particularly in the UHF region, is
the patch antenna which should be placed against a ground plane of suf?¬?cient size to
achieve a good front to back ratio.
hW
Vertical plate
Horizontal ground plane
Angle
W
FIGURE 12.4
A monopole plate antenna above a ground plane.
(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.)
Electromagnetic Coupling in RFID 237
12.6 Measures of Label Excitation
An appropriate measure of tag excitation in the near ?¬?eld is the oscillating reactive power
density per unit volume, and in the far ?¬?eld is power propagating per unit area. At UHF,
for calculation of power transfer in the far ?¬?eld, we may use the concepts of gain and
effective area as de?¬?ned in Section 12.
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