A circularly polarized antenna does have a cost in terms of the distance at which the tag
can be read. Typically, circularly polarized antennas do not have as great a range as
linearly polarized antennas. One use of a circularly polarized antenna is when reader=tag
orientations vary. In practice, linearly polarized antennas are used when maximum distance
is required and reader=tag orientation is ?¬?xed. This is often the case in a manufacturing
plant. Further, portals, which employ multiple antennas, typically four, can position
antennas to cover all three, or at least two of the three coordinate axes. Here, the extra
FIGURE 1.15
Planes of the interrogator antenna (a) and rotated tag
antenna (b) and (c). (a) (b)
a = 0
a
(c)
Phi
ThetaZ
ThetaZ
Y
X
Y
Phi
X
FIGURE 1.16
Patch=dipole orientations favorable (left) and unfavorable (right).
12 RFID Handbook: Applications, Technology, Security, and Privacy
range, or power capabilities, of the linearly polarized antenna can help in reading a tag
through material that is RF-unfriendly.
1.2.3 Re?¬‚ection
Re?¬‚ection is a property of the matching of the impedance of the antenna to the impedance
of the load (chip) on the RFID tag. Consider the connection of the two components as
shown in Figure 1.17.
The re?¬‚ection coef?¬?cient is de?¬?ned as:
G ?? (ZL ZA)=(ZL ?? ZA): (1:2)
If the load impedance is shorted (0), Gshort??1.
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