7.6.4 RF Opaque or RF Absorbing Materials
Another effective, but more expensive, way to reduce reader interference and collision is to
use RF opaque or RF absorbing materials to contain the interrogating signal within the
designated zone of interrogation. For the case study presented in Section 7.5, the use of
such materials is shown in Figure 7.10. Although there will still be some signal leakage
through the door openings, it would not have caused much interference. This is due to the
fact that the signal strengths at the sides of the antenna are relatively weak as compared
with the front of the antenna. According to Leong et al. (2006b), the gain at the side of a
typical RFID antenna is 20 dB less than the gain at the front of the antenna.
7.6.5 Frequent Rearrangement of Channels
Interrogating Channels can be switched around every cycle of ??????Listen Before Talk.??™??™ This is
to prevent the jamming of the interrogation signal by any external noise. Figure 7.11 shows
a simple example on how the switching is done. There are other more complex switching
FIGURE 7.9
Using sensors in an RFID system. Both the antennas at
dock door 3 are switched offwhen the absence of truck
3 is detected. (From Leong, K.S., M.L. Ng, and P.H.
Cole. 2006a. Synchronization ofRFIDreaders fordense
RFID reader environments. International Symposiumon
Applications and the Internet.
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