Fundamentals of Electromagnetics - RF/Microwave Interaction with Biological Tissues - page 45

known properties are necessary for calibration purpose, such as a short circuit,
an open circuit, and transmission lines with different lengths.
A poor calibration may degrade the measurements without making the
experimenter aware of this. Because there are a variety of devices to be meas-
ured, there are also a number of calibration procedures. They become more
complicated when the number of imperfections to be corrected increases. For
instance, the number of terms needed for calibrating the measurement setup
depends upon the fact that the physical quantity to be measured is a reflec-
tion or a transmission, on a device with small or high losses, reciprocal or not,
and so on.
Scattering matrix parameters are most often used for calculating or esti-
mating the errors. They describe a two-port by its reflection and transmission
characteristics. When measuring the electric properties of a dielectric, the con-
tainer in which the specimen is placed is considered as the electrical two-port.
A number of precautions have to be taken, especially at the higher fre-
quencies, above 20 GHz [17]. In particular, connectors can be a source of
inaccuracy if not adequately chosen, especially the miniature connectors
introduced around 1960. Better connectors are necessary when operating at
higher frequencies.
Measuring the complex permittivity of biological tissues fortunately does
not offer the same variety as can be found with electronic circuits, passive
and active. However, one needs to be careful sometimes: Anisotropy may be
present because of the inhomogeneity of the structure. In such a case, cali-
bration has to take the possibility of anisotropy into account.

1.7.3 Liquids

Measuring the dielectric properties of liquids raises specific difficulties. In par-
ticular, possibly the container must be reusable while being tight. The usual
practice for liquids is to measure the reflection coefficient of an open-ended
coaxial probe [25]. Two difficulties, however, relate to this practice, especially
when applied in the high range of microwave frequencies: The structure radi-
ates and the size of the coaxial probe is very small; hence the mechanical accu-
racy may not be very good.
A new procedure for measuring the complex permittivity of liquids was pre-
sented in 1997 [26]. It is based on the measurement of the scattering para-
meters of waveguide two-ports and on an original calibration method
developed by the authors, called line-line (LL) [27]. The complex permittiv-
ity is obtained explicitly. Hence the method avoids the difficulties related to
the inversion of implicit expressions through tedious numerical iterations.
A waveguide transmission method was used on a vector analyzer, yielding
the four complex scattering parameters: reflection at both ends and transmis-
sion in both ways. A waveguide spacer is placed between the two waveguide
ends of the vector analyzer. The connection between the waveguides and both
ends of the analyzer is made by coax-to-waveguide transitions specific to each