Radar Technology for Level Gauging - Krohne

● Cyclic compounds because of their high symmetry of faces have lower εr values.
3. Radar-Füllstandsmesssysteme
● The dielectric characteristic of nitrogen or halogen derivatives varies considerably,
and depends essentially on the symmetry in the molecule. Dichlorobenzene is a case
in point, with the 3 positional isomers having ε r values of between 2.5 and 10.
A table with ε r values of different products can be found in Annex A.
7.3.2 Frequency dependence
The permittivity ε r decreases in step with increasing frequency f. Due to dielectric relaxation
(i.e. the electrically polarized molecules cannot align themselves fast enough in the highfrequency
field), there is a transitional frequency range 30 in which ε r drops. For most liquids,
these frequencies lie between a few 100 kHz and a few 100 GHz. Above this range, ε r remains
constant 31 .With a few media, this range is however located just inside the 5-100 GHz
microwave range. A good example is water, which has at different temperatures the complex
dielectric constant ε r’+ jε r’’, shown in Fig. 33 32 .
30 In this transitional range the imaginary component of εr is relatively high, i.e. wave attenuation would also occur
in the medium. For surface reflection, however, this has no negative influence.
31 A further drop in the εr due to ion or electron resonance occurs only far above the microwave range, e.g. in the
infrared or visible light range.
32 The imaginary part jεr” describes the dielectric losses in the dielectric.
Radar handbook 41
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Fig. 33:
Dielectric constant of
water as a function of
frequency [Meinke]