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The permitivity e of sandy soil
is strongly dependent on the water content due to the high dielectric
constant of free water.
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The right graph
shows the variation of the electric reflection coefficient at center
frequency of the sensing electrodes, including the matching circuitry which
are put into a sandy soil, with respect to a change of the water content of
this soil.
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The blue line
gives a simulation and the triangles represent measurements points.
Essentially, the sensing electrodes can be simulated by an open ended line.
The electrical length of the line varies with the changing water content of
the soil due to the changing permitivity. Additionally, the losses increase
as the water content increases.
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Also shown are,
again, the lines which result in a constant acoustic reflection of reflector
#2, when this acoustic reflector is loaded with an external element with this
electrical reflection coefficient.
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You can see that
we chose the matching circuit in such a way that the variation of the
impedance of the sensor range from minimal acoustic reflection up to the
point of maximum reflection of the split finger IDT.
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The theoretical
dynamics of the reflection with this load impedance range from -5db to -30db,
which gives a dynamic range of 25dB.
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