As shown in Figure 1.10, the Y axis is the vertical axis of Figure 1.9. Now consider the
view from point a to point b (Figure 1.9 looking left to right) looking along the line
connecting the two points as shown in Figure 1.11.
The magnitude of the E ?¬?eld, jEj, will appear as a vertical line. The antenna A is said to be
linearly polarized. Based on the orientations shown in Figure 1.9, the two antennas are
aligned such that the E ?¬?eld transmitted by A will impinge on antenna B in the same
orientation as viewing the vector in Figure 1.11. Under certain conditions, it is possible to
cause the E ?¬?eld of a particular type of antenna to rotate in angle such that the tip of the
vector of Figure 1.11 will trace a circle as shown in Figure 1.12. In this case, the antenna is
said to be circularly polarized. Any mathematical discussion of how this is done is beyond
FIGURE 1.7
Most favorable polarization??”reader (left) and tag (right).
Time
H Field
E Field
FIGURE 1.8
The E and H ?¬?elds showing phase and time.
Physics and Geometry of RFID 9
FIGURE 1.9
Two dipole antennas aligned vertically
and horizontally as shown.
Feed Receive
a b
A B
Y Y Y Phi
Theta
Z
Theta
Z
Theta
Z
X
Phi
X
Phi
X
FIGURE 1.10
Radiation patterns ideal dipole (left) and aligned dipoles (right).
FIGURE 1.11
The E ?¬?eld in time and angle.
E a
A
FIGURE 1.
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