At 02:34 PM 2/28/2005, Jerry Keller - K3BZ wrote:
>Is there any way I can take meaningful soil conductivity readings with
>simple equipment?
>
>73, Jerry K3BZ
Depends on what you mean by simple, and what sort of spatial resolution you
want.
A straightforward 2 or 4 probe resistance measurement at 60Hz will get you
in the ballpark (within a factor of 10, and probably closer). Don't put the
probes too far apart, because it basically measures the resistance at a
depth of half the spacing of the probes, and skin depth at HF ranges around
1-15 meters (based on Hagn's ACES paper in 1988, the plots of which are in
the ARRL antenna book)
The ARRL antenna book describes a technique developed by George Hagn at SRI
where you basically create a parallel conductor open wire line inserted
into the soil and measure the transmission line's impedance. You'll need
some sort of impedance bridge for this measurement. Typial measurements
from his paper for 6 inch long probes 3" apart were a few hundred ohms
magnitude and -40 degrees phase (like 230-190j ohms). Perhaps a suitable
6:1 transformer/balun and a MFJ/Autek analyzer might work?
This technique has seen quite a bit of rigorous validation.
Another approach is to use a dipole close to the ground and measure it's
feedpoint impedance. The tradeoff here is the closer to the ground, the
more sensitive it is to being exactly the same distance everywhere, the
physical size, etc. The original NEC validation testing used this kind of
approach.
Both of these approaches should give you values good to perhaps 25%.
In theory, one should be able to measure the Q of a small resonant loop
close to the surface, but calibration might prove to be tricky, as would
accurately de-embedding the other loss sources in a relatively high Q system.
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