11-05-2012, 12:09 PM
An overview of dielectric properties measuring techniques
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INTRODUCTION
Dielectric properties of various agri-foods and biological
materials are finding increasing application, as fast and new
technology is adapted for use in their respective industries and
research laboratories. Earlier reports on polar dielectrics and
modelling studies date back more than 70 years (Debye 1929).
The earlier concept of permittivity measurements was based on
dc electrical resistance to determine grain moisture content. A
non-linear increase in resistance of the grain as temperature
decreased gave useful observations (Nelson 1991). However, no
quantitative data were reported (von Hippel 1954). Later on, ac
measurements were commonly employed to measure the change
in capacitance and suitable sample holding capacitors were
developed (Nelson 1998). Grain moisture measurement based
on dielectric properties data became the most prominent
agricultural application.
BASIC MICROWAVE-MATERIAL INTERACTION ASPECTS
When microwaves are directed towards a material, part of the
energy is reflected, part is transmitted through the surface, and
of this latter quantity, part of it is absorbed. The proportions of
energy, which fall into these three categories, have been defined
in terms of the dielectric properties. The fundamental electrical
property through which the interactions are described is the
complex relative permittivity of the material, e*. It is
mathematically expressed as:
PERMITTIVITY MEASUREMENT PRINCIPLES and TECHNIQUES
The measurement methods relevant for any desired application
depend on the nature of the dielectric material to be measured,
both physically and electrically, the frequency of interest, and
the degree of accuracy required. Despite the fact that different
kinds of instruments can be used, measuring instruments that
provide reliable determinations of the required electrical
properties involving the unknown material in the frequency
range of interest can be considered (Nelson 1998). The
challenge in making accurate permittivity or dielectric property
measurements is in designing of the material sample holder for
those measurements (RF and MW frequency ranges) and
adequately modeling the circuit for reliable calculation of the
permittivity from the electrical measurements. If one can
estimate the RF circuit parameters appropriately, the impedance
or admittance for example, the dielectric properties of that
material at that particular frequency can be determined from
equations that properly relate the way in which the permittivity
of the material affects those circuit parameters.
Transmission line technique
This technique is cumbersome because the sample must be
made into a slab or annular geometry. At 2450 MHz, the sample
size is somewhat large, particularly for fats and oils. Commonly
available waveguide test equipment for 2450 MHz is designated
WR-284. For measurements at 915 MHz, only the coaxial line
technique is practical due to the large size of the waveguide
required. Liquids and viscous-fluid type foods can be measured
with this method by using a sample holder at the end of a
vertical transmission line. The dielectric properties can be easily
and inexpensively obtained by the transmission line technique,
particularly if one utilizes a slotted line and standing-wave
indicator (Nelson et al. 1974). A more sophisticated
implementation of the technique utilizes a swept-frequency
network analyzer, where the impedance is measured
automatically as a function of frequency. Figure 5 represents a
typical transmission measuring system using a vector network
analyzer.