Radar Technology for Level Gauging - Krohne

6.7 Propagation along electric lines (TDR method)
In the TDR method, waves are propagated not through the tank atmosphere but along an
electric line (see chapters 4.4.1 to 4.4.4), shown in Fig. 29 by way of example with a rod or
wire rope.
An interference reflection is generally formed at the junction between measuring system and
rod (see chapter 4.4.8).
The propagation rate along the line is equal to the speed of light, independent of the type
and dimensions of the line (see chapter 6.1.2). Line attenuation occurs with the finite conductivity
of the metallic material, but this only becomes apparent in steel lines of more than
10 m (32 ft) in length 26 .
On the basis of the field patterns for single-wire, two-wire and coaxial cables shown in
Fig. 30, it will be recognized that the possibility of interference from adjacent metal objects
is the greatest with the single-wire system and non-existent with the coaxial system.
26 As the skin effect is heavily dependent on the magnetic property µr, a completely non-magnetic (austenitic) steel is
to be favoured. If in doubt, copper or aluminium should be used, both of which, moreover, have a conductivity that is
approximately 40 and 25 times, resp., higher than steel.
Distance
Signal level
Interference signal
Useful signal
Fig. 29:
TDR arrangement
with a rod
Radar handbook 37
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Fig. 30:
Pattern of electric field
lines in the plane
across the line:
a) single-wire,
b) two-wire,
c) coaxial cable.