Radar Technology for Level Gauging - Krohne

3. Radar-Füllstandsmesssysteme
7.2 Reflection at interfaces
When microwaves strike an interface between 2 media with relative permittivities of ε r, 1 and
ε r, 2, the reflection factor is:
If the power received from that point in relation to the transmitted power is to be calculated,
it must be borne in mind that the waves still have to travel twice through the transition zone
between the atmosphere and the upper layer with in each case a transmission factor of
(1-R). Therefore, the effective reflection factor R 2 at the interface is:
provided that the waves are propagated loss-free through the upper layer.
The following diagram shows the reflection factor R 1 at the upper liquid layer as a function
of ε r, 1, and also the reflection factor R 2 at the interface for various parameters ε r, 2.
It will be seen that the interface reflection becomes stronger the greater is the difference in
relative permittivity. Even where ε r, 1 >> ε r, 2 the interface can in theory be readily detected.
In practice, however, problems often arise because some products with a high ε r
(e.g. water) absorb microwaves.
Reflection factors with 2 liquid layers
Fig. 32:
Reflection factors
at a liquid-liquid
interface
Radar handbook 39
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