Hello Tom,
My comments are on your message (below).
>I know we all want to know conductivity. This first because popular when Sevik
>pounded a few rods in the ground and measured on 60 Hz, which became a
>commonly shared >but totally useless measurement.
>
I measured the impedance from 1 to 180MHz continuously with miniVNA. The
presented values (27 to 5mS/m) are for 1833kHz so, are not useless.
>Of course it is much better when we use the actual operating frequency.
>
I don't mention this but in parallel I had measured the VSWR of my 160m INV.L
and 80m INV.V. I can say that the most affected of the ground conductivity
variation is the INV.L. This is very intuitive because it is much bounded with
the ground then the INV.V (dipole).
Since the conductivity were 27mS/m, the central frequency (resonance) were
moved down with the lowest limit now (5mS/m).
I should mention that this is not very clearly correlated as with the
conductivity. This is due to the different medium changes. The antenna is
influenced also by the surrounding air which is very changeable (even during
one day!). I measure the air humidity, temperature, pressure and wind with
5minutes intervals 24h/day all year long at 10m AGL and at ground level. The
most influence factor on antenna impedance was air humidity!
Here I'm talking about facts.
>It seems to me using two rods in the ground and measuring impedance, in an
>attempt to estimate ground conductivity, might not be very reliable. We are
>not doing anything >more than measuring a small area around the rods and
>(especially) rod contact. Rod contact on a freshly pounded rod from soil
>wetting varies greatly, especially with a smaller >diameter rod, because the
>soil is not fully settled against the rod.
>
You are right. We are measure the small amount of ground from the surrounding
area of the antenna but, if we do this in several places then the area can be
statistically appreciated.
Regarding the contact between probe and soil...I use two aluminum 6mm diameter
cylindrical pipes and 40cm length. Those two pipes were leaved permanently in
the ground (it was never removed since May!).
The aluminum resistivity is 2.82 * 10^-8 ohm*m or 35*10^6 S conductivity.
Comparing this value with the measured one (0.027S) we can say that the error
of contact resistance is neglected. Assuming we have 1 ohm overall contact
resistance (which is very very conservative!). This will introduce a 3%
measurement error which is insignificant.
The soil is a live matter. Its components are affected by temperature, humidity
and pressure and are moving! Because of that, when is drought (like it is on my
side) the ground form cleaves. The same thing is happens around the probe rods
but at the microscopic level. This is a natural process which reflect the
reality. To be sure the rods are in firm contact with the ground, several times
I gently move the rods without no measurement modifications observed! This give
me very much confidence in the measurements.
>Have these methods ever been verified with impedance changes in a low dipole,
>or field strength readings, or are they just a proposal?
>
As I mentioned above, yes there are verified with impedance changes of an INV.V
(low dipole but on 80m band) and the INV.L (160m).
The INV.L was optimized based on field strength measurements but since then (3
years ago), no such measurements were conducted.
In this way, you gave me a good suggestion. Next time I'll measure the field
strength. This will count for the lowest ground conductivity. Another few will
be done toward a higher conductivity as soon as it will "become available" (God
knows when).
One more comment.
Conductivity were measured for zero to 40cm deep in one ground volume. That
means the conductivity of the ground between 0 to 40cm is integrated and the
final result were measured. The ground humidity increase with depth being lower
at its surface then at 1m deep (for example).
At least, this is the case in a droughty season. It will be interesting to
measure the ground conductivity gradient deep to 5-7m. Why?
Due to skin effect, the physical ground level is not the same with the
electrical ground level. A wave is going deep into the ground with skin effect
depth.
For 1.8MHz wave and 27mS/m ground conductivity, the wave will travel down
to 2.3m and for 5mS/m, 5.3m. So, we can consider the ground being lower then it
is or the antenna higher above the ground with 2 to 5m (in this case)! That is
way will be good to measure the ground gradient down to 5-7m.
I can't do this. Because of that, I assume the ground is a uniform isotropic
material with the measured conductivity which is not true.
This is a limitation of the method. I have no idea what is the error introduced
by this fact.
One solution is to measure the conductivity with 5m rods. This way, the
measurements result will be an integration of the whole volume which will be
much better.
Unfortunately, this is not a practically approach.
Any ideas?
73 de YO3FFF
Cristi
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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