6).
By this time, it should be clear to the reader that there are many scenarios in which the
interrogator=tag combination will not work. Figure 1.7 provides a most favorable orientation
for the two dipole antennas.
1.2.2 Polarization
Polarization is the term used to describe the motion of the tip of the electric ?¬?eld vector
when electromagnetic (EM) energy is transmitted from an antenna, in this particular case,
radio frequency waves for RFID. Consider the two sinusoidal waveforms in Figure 1.8,
where the two waves are 908 out of phase. Each of these waveforms can be represented as a
Thetaz
x
Phi
y
FIGURE 1.5
Most unfavorable orientation for the tag.
Thetaz
x
y
Phi
FIGURE 1.6
Another highly unfavorable orientation.
8 RFID Handbook: Applications, Technology, Security, and Privacy
vector with magnitude, jEj and jHj, and angles, ?¬?Q and ?¬?F, respectively, where the two
vectors will always be 908 out of phase.
Consider two dipole antennas A and B, aligned on a center line as shown in Figure 1.9,
where we wish to view the E ?¬?eld between the two antennas from point a looking to
point b.
The ?¬?eld pattern generated by an ideal dipole is shown in Figure 1.10 (left). The ?¬?eld
patterns of the two aligned dipoles are shown in Figure 1.10 (right). While the full
explanation of how the pattern is calculated is not necessary here, there is an intuitive
understanding that can be derived by the color scheme where red indicates the strongest
signal level, that is, the largest magnitude of jEj.
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