Environmental Engineering Reference
In-Depth Information
evaluation of the liquid dielectric properties and levels. To circumvent this problem,
the optimal strategy should simultaneously involve a suitable calibration procedure
and a good impedance matching of each component of the measurement setup.
In this regard, the most critical component to be designed is the probe-head sec-
tion, which should guarantee a perfectly matched cable-to-probe connection, as well
as the mechanical stability of the probe central conductor. Additionally, the probe-
head structure should be included in the SOL calibration measurements, in order
to de-embed its effect and to accurately derive the response related only to the
probe section filled with the sample under test. As a matter of fact, all these re-
quirements were considered in the design of the the enhanced configuration of the
coaxial probe. Fig. 4.7 shows the 433 mm long coaxial probe, the 160 mm long
probe-head portion as well as the calibration structure that exactly reproduces the
internal probe-head portion: all these components are virtually 50
-matched, as
explained in the following. For this probe, the inner and outer diameters of the probe
conductors (
a
Ω
=
9mmand
b
=
21 mm, respectively) were chosen, once again, to en-
sure a 50
Ω
-matched characteristic impedance when the probe is in air, according
to (3.14).
The chosen dimensions limit the frequency bandwidth of the probe to about
6 GHz, when the dielectric material considered is air. However, when the probe is in-
serted into a different material, this limit decreases proportionally to the square root
of the dielectric permittivity of the material. In particular, for the considered cases,
the liquid with the highest permittivity is water, which limits the useful bandwidth
of the probe to approximately 700 MHz.
Fig. 4.7
Coaxial probe external configuration, details of calibration structure, internal probe
head and inner probe conductor, external probe head shield and external probe shield, cross-
section probe dimensions [22]
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