OBID
®
Installation
ID ISC.ANT300/300-A
FEIG ELECTRONIC GmbH
Page 34 of 43
M01001-2de-ID-B.DOC
E N G L I S H
5.5 How to measure the voltage standing wave ratio (VSWR)
If an antenna has been tuned, the question is: how good is the adjustment between reader and
antenna? In this case, the VSWR –meter is a very useful aid. This device measures the ratio be-
tween supplied and reflected energy. A VSWR of up to 1.3 :1 is considered to be sufficient. Very
often, a wattmeter is integrated into these devices.
Figure 19:Inserting a VSWR meter into the antenna cable
The cable between the Reader and the SWR meter should either be very short (< 20 cm) or 7.20
m (RG 58=Lambda/2) long. If the VSWR is greater than 1.3:1 after tuning, use trim capacitors C1
and C2 on the board of the base antenna to perform a slight adjustment.
Furthermore the VSWR meter can be used at any time to check the tuning of the base antennas. If
changes in local conditions result in detuning of the antennas, this can be verified whenever
desired.
In addition to the losses indicated by the SWR due to mismatching between the cable and the
antenna, it happens that the Reader drives different output currents depending on the antenna
impedance, resulting in power variance. This means that at 50
Ω
a current of approx. 0.3 A flows.
No current flows when an output is open, and when there is a short circuit the current is limited to
approx. 1.0 A. Matching the antenna also has a slight effect on the noise levels.
base antenna
ID ISC.
ANT300/300
SWR / Power Meter
Reader
ID ISC.LR200
