Design Guide

Variables Affecting Performance
A DIVISION OF TRIMBLE
Appendix C: Environmental Considerations 123
Tag Orientation: Most tags have folded dipole antennas. They read well when
facing the antenna and when their long edge is oriented toward the antenna, but
very poorly when their short edge is oriented toward the antenna.
Tag Model: Many tag models are available. Each model has its own
performance characteristics.
Antenna Considerations
Use a circularly polarized antenna. Linear antennas can only be used if the tag
orientation to the antenna is consistent, or if not always in the ideal orientation, the
antenna or tag can be rotated for best reading.
Use an antenna whose design naturally presents a short to DC. This will help
eliminate ESD issues.
Use an antenna with a return loss of 17 dB or greater (1.33 VSWR) in the
transmission band of the region the module is using.
Use an outdoor-rated antenna if there is a chance that water or dust could get into the
antenna and change its RF characteristics.
Ensure that the antenna is mounted such that personnel do not stand in the radiation
beam of the antenna unless they are more than 21 cm away from the face of the
antenna (to adhere to FCC limits for long term exposure). If the application calls for
personnel to work in the antenna beam and they will be less than 21 cm from the face
of the antenna, the Nano power should be reduced or a lower gain antenna must be
used (21 cm assumes a 27 dBm power level into an 8.15 dBi antenna).
Multiple Readers
The Reader adversely affect performance of 900 MHz devices. These devices also may
degrade performance of the Reader.
Antennas on other Readers operating in close proximity may interfere with one
another, thus degrading performance of the Readers.
Interference from other antennas may be eliminated or reduced by using either
one or both of the following strategies:
w Affected antennas may be synchronized by a separate user application using
a time-multiplexing strategy.
w Antenna power can be reduced by reconfiguring the RF Transmit Power
setting for the Reader.