Agriculture Reference
In-Depth Information
80
60
water temperature = 25° C
40
20
0
0.1
1
10
100
1000
frequency in GHz
300
30
3
0,3
0.03
wavelength in cm
Fig. 5.14
Permittivities of water with a temperature of 25 °C as depending on frequencies or
lengths of microwaves (Compiled from data by Komarov et al.
2005
)
For sensing of soil moisture, it is the real part of the relative permittivity or the
dielectric constant that is used. Because regarding this physical criterion, water
has outstanding properties. Within a wide range of frequencies, the real part of the
relative permittivity or the dielectric constant is
•
around 80 for free water
•
between 3 and 7 for dry soil minerals (sand, clay
etc
.)
•
between 2 and 5 for dry organic constituents of soils
•
around 1 for air or for a vacuum.
Hence in principle, both the
dielectric constant
and the
capacitance
- that
depends on it - offer good prerequisites for differentiating between water and other
soil constituents. Yet there are still some important details to consider.
Both the real part and the imaginary part of the permittivity of moist materials
depend strongly on the frequency of an electromagnetic field. This can be explained
by interactions with water molecules. Because of its dipole structure, the water mol-
ecules get polarized in an electromagnetic field. And the alternating electromag-
netic field causes the polarized water molecules to vibrate. As a result of inertial
forces, these vibrations can get out of line with the respective frequency of the
electromagnetic field.
The courses of the permittivity curves - as shown in Fig.
5.14
over frequencies
and corresponding wavelengths - probably are mainly due to such effects on molec-
ular vibrations. Regarding signals from the real part of the permittivity or the dielec-
tric constants, the frequencies used for water sensing usually are below 10 GHz.
Hence the respective wavelengths are above 3 cm.
