Appendix
A.1 Planning and installation of UHF read points
SIMATIC RF650R/RF680R/RF685R
308 Configuration Manual, 03/2018, C79000-G8976-C386-06
The signal strength of the transponder response is known as the RSSI value (Received
Signal Strength Indicator). The RSSI value is a one byte value (0 to 255), the higher the
value the better the signal strength (according to the IEEE 802.11 standard).
The actual RSSI value depends on numerous parameters:
● transponder type used,
● chip used in the transponder,
● connected antenna,
● transmit power,
● distance between antenna and transponder,
● reflections,
● noise level in the channel used and in neighboring channels
The RSSI value is important for the automatic evaluation of the read point and for filtering. A
simple comparison of the RSSI values of two transponders is nevertheless not possible
because the values are influenced by the transponder tolerances and the non-homogeneous
antenna field. This means that it is possible that a transponder positioned closer to the RFID
antenna has a lower RSSI value than a transponder much further away.
Propagation of the antenna field
The waves do not propagate as a homogeneous field, there is superposition of the waves
that can cause the following effects:
● Overshoots and field gaps due to obliteration of two waves
These are caused by reflection and the resulting propagation on different paths
(comparable with fading effects on the car radio, e.g. noise when the vehicle is standing)
● Generation of overshoots due to reflecting objects and surfaces
This can be illustrated by comparing it with a "hall of mirrors". The signal transmitted by the
reader is reflected (several times) by metallic objects such as housings, steel supports or
grilles and this can lead to unwanted effects and read errors. Is also possible that a
transponder is not identified although it is located in the assumed direct identification range
of the reader. It can also happen that a transponder moving outside the antenna field is read
out due to overshoots.
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