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Verfügbare Sprachen
BIS U Basic Manual
11
Measures for Improving the Operational Reliability of UHF Systems
11.1 Field Reserve and
Working Distance
26
In a real environment, the primary wave emitted by the antenna reflects against large objects
such as walls, floors, deposited transport containers etc. and causes independent,
uncontrollable propagation in the form of a stray secondary wave.
In the worst case scenario, interference between the primary wave and the secondary waves
can cause field attenuation. In a multi-reflective environment, it is virtually impossible to predict
the field intensity at a specific location. It should also be noted that a change in the surrounding
area caused by moving transportation equipment, for example, may cause the field intensity to
change over time.
Local or temporary decreases in the field intensity have the same effect as a deliberate reduction
in the transmitting power during reading operations. If the transmitting power of the antenna is
then reduced to a point where the data carrier can just about be detected, a decrease in the field
intensity within the multi-reflective environment is followed by an interruption in communication.
Figures 12, 13 and 14 clearly show that the fluctuations increase in line with the distance to the
transmitting antenna. In order to ensure that the electric field never falls below the excitation field
intensity of the data carrier in a multi-reflective environment, even when the field intensity
fluctuates, field reserves within range of the fluctuation amplitude must be taken into
consideration. This results in a calculative increase in the response field intensity and definition of
the so-called working distance at the intersection point with the power curve.
8,00
2 watts
7,00
6,00
5,00
4,00
0.5 watts
3,00
2,00
1,00
0,00
0
100
200
Figure 15:
Graphic derivation of the working distance for a data carrier
When designing a UHF system, it is recommended that the following rules be observed:
–
The values for the working distance of a data carrier specified by the manufacturer or system
supplier should not be exceeded.
–
For a data carrier positioned at the point of operation, the response field intensity must be
calculated by reducing the transmitting power. The transmitting power must then be
increased by the field reserve prior to operation. A field reserve of 50%...100% is considered
sufficient for most applications.
Theoretical reading range
Working distance
Field reserve
Field reserve
300
400
500
600
Distance to the antenna in cm
700 cm
350 cm
6 dB
700
800
900
1000
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