Environmental Engineering Reference
In-Depth Information
b
1
=
S
11
a
1
+
S
12
a
2
,
(2.27)
b
2
=
S
21
a
1
+
S
22
a
2
.
(2.28)
The scattering parameters are therefore measured as
b
1
a
1
|
a
2
=
0
S
11
=
,
(2.29)
b
2
a
1
|
a
2
=
0
,
S
21
=
(2.30)
b
1
a
2
|
a
1
=
0
S
12
=
,
(2.31)
b
2
a
2
|
a
1
=
0
.
S
22
=
(2.32)
The potential of
S-parameters
for monitoring and diagnostics purposes will be clearer
in the following chapters. In fact, it will be shown that, when the
system to be moni-
tored
can be considered as an electrical network, the
S-parameters
of the system can
disclose useful information on the system itself. In particular, the scattering parame-
ters can be used to extrapolate the dielectric characteristics of the SUT. The retrieved
information, although related to the dielectric behavior, may be appropriately asso-
ciated with several quantitative and/or qualitative properties of the sample.
2.4
Reflectometry Measurements for Dielectric Spectroscopy
Dielectric spectroscopy, also called impedance spectroscopy, measures the dielectric
properties of a medium as a function of frequency. It is based on the interaction of
an external field with the electric dipole moment of the sample, often expressed by
permittivity. This technique measures the impedance of a system over a range of fre-
quencies, and, therefore, the frequency response of the system, including the energy
storage and dissipation properties. Almost any physico-chemical system possesses
energy storage and dissipation properties that can be analyzed through dielectric
spectroscopy.
There are several approaches that can be used to measure the dielectric proper-
ties of materials, such as open-ended probe technique, free-space transmission tech-
nique, cavity resonant systems, time domain reflectometry, and so on. An overview
of some of the most common methods for measuring dielectric properties of mate-
rials can be found in [27].
In particular, dielectric spectroscopy performed through microwave reflectom-
etry, has gained enormous popularity over the past few years and is now being
widely employed in a wide variety of scientific fields that go from food analysis
[20] to biomedical applications [24, 19, 12]. As will be detailed in the following
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