)
Far-Field Tag Antenna Design Methodology 67
4.2.2 Electric Field-Sensitive Antennas
Two varieties of electric ?¬?eld-sensitive antennas are shown in Figures 4.4 and 4.5. Figure 4.4
shows a small bow-tie antenna that is intended to be sensitive to electric ?¬?elds in the
horizontal direction.
Figure 4.5 shows an electric ?¬?eld-sensitive antenna that is suitable for placement against
a horizontal metal plate.
Analysis of the structure in Figure 4.4 is provided later in this chapter. The ?¬?gure of
merit for these antennas, when placed in the energy storage electric ?¬?eld is a coupling
volume (Ranasinghe, 2007); for Figure 4.5 it is equal to the physical volume of the structure
and for Figure 4.4 it is derived from the label dimensions, even though the antenna itself
has no physical volume.
Both of the antennas will also have an effective electric ?¬‚ux collecting area but this area
should not be confused with the effective area concept of a radiating antenna or of a far-
?¬?eld antenna. The effective area for a near-?¬?eld electric ?¬?eld-sensitive antenna describes
the extent to which the antenna can extract current from the displacement current density
of the driving electric ?¬?eld.
4.2.3 Electromagnetic Field Antennas
An antenna can be considered as an electromagnetic ?¬?eld antenna on a couple of different
bases. Firstly, if the antenna is capable of responding to both electric and magnetic ?¬?elds,
we would consider it to be an electromagnetic ?¬?eld antenna.
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