Siemens SIMATIC RF600 Series System Manual page 59

● 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.
①
②
Figure 4-12
SIMATIC RF600
System Manual, 11/2018, J31069-D0171-U001-A21-7618
Identification situation with two transponders in an ideal radio/antenna field
Identification situation with two transponders in a real radio/antenna field with reflections that can
lead to obliteration and overshoots
Propagation of UHF RFID antenna fields
4.6 Planning and installation of UHF read points
RF600 system planning
59