Siemens SIMATIC RF600 System Manual page 158

Antennas
6.2 Guidelines for selecting RFID UHF antennas
Impedance
Definition
Impedance is understood as the frequency-dependent resistance. The impedances of the
antenna, reader and antenna cables should always be the same. Differences in the
impedance result in mismatching which in turn means that part of the applied signal is
reflected again and that the antenna is not fed with the optimum power.
Specifications
● Only antennas can be used whose connection has a characteristic impedance of Z =
● The mechanical design of the coaxial antenna connection is of secondary importance; N,
Return loss / VSWR
Definition
Since the impedance at the antenna connection is frequency-dependent, mismatching
automatically occurs with broadband use. This mismatching can be reflected by two
parameters:
● The voltage standing wave ratio VSWR
● The return loss
Voltage standing wave ratio VSWR
The power sent by the transmitter cannot flow unhindered to the antenna and be radiated as
a result of the mismatching described by the VSWR. Part of the power is reflected at the
antenna and returns to the transmitter. The powers in the forward and reverse directions
produce a standing wave which has a voltage maximum and a voltage minimum. The ratio
between these two values is the VSWR (voltage standing wave ratio).
Return loss
The return loss parameter is based on the reflection factor which describes the voltage ratio
between the forward and reverse waves.
Specifications
So that the smallest possible transmitted and received powers are reflected by the antenna
under ideal conditions, you should observe the following data for the VSWR and the return
loss |S
● VSWR < 1.24:1 or
● |S
158
50 Ohm.
TNC and SMA plug connectors are usual.
|/ dB in the respective frequency band (865-870 MHz or 902-928 MHz):
11
|/ dB ≥ 20 dB
11
System Manual, 10/2009, J31069-D0171-U001-A8-7618
SIMATIC RF600