Siemens SIMATIC RF600 Series System Manual page 59

4.6.1 Technical basics
General
In contrast to inductively coupled HF systems, in UHF technology, there is full propagation of
the radio waves just as in other wireless systems (radio, TV etc). There are both magnetic
and electrical field components present. The following graphic shows the structure of a UHF
system. One characteristic is the design of the transponder that differs greatly from the
structure used in HF systems, e.g. the use of a dipole or helix antenna.
Figure 4-5
RSSI value
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.
SIMATIC RF600
System Manual, 10/2016, J31069-D0171-U001-A19-7618
Structure of a UHF RFID system
4.6 Planning and installation of UHF read points
RF600 system planning
59