Oscillations, Waves, and Interactions - GWDG

342 R. Pottel, J. Haller and U. Kaatze
Figure 7. Principle of quasistatic electrical input impedance measurement system [56]: 1,
network analyzer with S, signal output port, as well as R, reference and M, measurement
signal input port; 2, reflection test set; 3, sample cell; 4, process control computer.
T (z) at small sample thickness z also the sonic wavelength λ within the sample can
be derived.
In dielectric spectrometry at frequencies below 3 GHz quasistatic electric input
impedance measurements can be performed utilizing a suitable network analyzer
with reflection test set (Fig. 7). Sample cells from the cut-off variety [56] have proven
well suited. Such cells basically consist of a coaxial line/circular waveguide transition,
the waveguide diameter being sufficiently small to excite the device, filled with the
sample liquid, below the cutoff frequency of its fundamental TM01 field mode. The
evanescent electromagnetic field in the waveguide section, the field in the feeding
coaxial line, as well as the field lines passing the sealing dielectric window can be
represented by a network of capacitors, the capacitances of which are determined by
calibration measurements [46,56,57].
Because of the frequency dependence of the Bν-term in the total sonic absorption
per wavelength (Eq. 6) the sonic absorption coefficient of aqueous solutions is small at
frequencies below about 10 MHz. To increase the sensitivity in the ultrasonic absorption
spectrometry at low frequencies, resonator methods are employed. The superior
measuring ability of network analyzers is again utilized. It is convenient, however, to
determine the transfer function of the resonator rather than its reflection coefficient
(Fig. 8). The liquid is contained in a circular cylindrical cavity. The end faces of the
cell are normally formed by piezoelectric quartz transducers [58–60], but reflectors
made of glass, with transducers attached to their back faces, are also applied [61].
In order to reduce diffraction of the sonic waves at the low-frequency measurement