User's Manual

COBALT UHF-SERIES CHAPTER 9: RFID OVERVIEW
PAGE 132 OF 140
All materials reduce the power of the RF signal to some extent, but d
irect contact on
metals and liquids
can cause particular problems:
 Metals reflect the signal
. A metal object may change the tuning of a tag (or
frequency on which it can receive signals), reflecting the RF waves from a
reader, or block communication from a specific antenna.
 Liquids (including
atmospheric moisture) absorb the signal. All radio waves
are partially absorbed by atmospheric moisture. Atmospheric absorption reduces,
or attenuates, the strength of radio signals over long distances. In addition, the
effects of attenuation increases according to the frequency; thereby, UHF signals
are generally more degraded by moisture than lower bands.
9.2.3 Limiting Interference and UHF Signal Attenuation
The following hints may be useful to get the best performance from your RFID
application.
Reading holes can be reduced in a number of ways:
 By removing metal from the reading area: if the reflections can be reduced so
will the holes
 By movement of the tag past the antenna.
As the tag moves, it will cross the holes and be read at some point. The exact
speed is a compromise between the number of tags and the bandwidth of the
system. Because of Governmental regulation, for instance, readers are capable
of reading more tags in the same time in the USA than is possible in Europe.
Multiple readers operating in the same environment may interfere with one another.
A number of techniques can help limit these unwanted effects:
 Use photo-cell triggering to initiate reading - don’t have the reader
transmitting all the time
 Reduce the power
 Reduce the downlink rate
 Shield between reading systems with absorptive material (metals could make
the problem worse)
Most important, testing should be performed in the actual environment to achieve
more precise range results.