A huge issue with using a coarse tool like a Soil Conductivity map is
that the worst and best terrain for Soil Conductivity are but a tiny %
areawise and may not even be noticeable on a large scale map. And like
with extremes in either direction, the likelihood of those conditions
persisting over a large area at that same level is extremely small.
At my qth which is ledge and virtually no soil except for minimal dirt
devoid of nutrients, you can go less than a mile away in almost any
direction and you will see an absence of ledge and far darker, nutrient
rich soil. Travel another mile and you have farmland and loam and many
small freshwater ponds and lakes. 20 miles SE of me you have Long
Island Sound which is of course all salt water.
I have never attempted to measure soil conductivity at any of these
locations but I would have to believe that it is far superior to what I
have on my hilltop, at least in the near field. And much of W1 is like
this.
I leave the extremely varied conditions to those with the expertise to
debate it. :-)
73
Bob, KQ2M
On 2024-07-26 09:42, Brian Beezley wrote:
There's a problem with using the FCC conductivity map to rank HF
conductivities that I overlooked: skin depth is much smaller at HF.
Soil with different conductivity may come into play at broadcast
frequencies. This can invalidate a conductivity ranking applied to HF.
The following table extrapolates 1 MHz permittivity and conductivity
for ARRL average ground to HF using the Messier soil model.
Conductivity is in mS/m and SD is skin depth in feet.
MHz Perm Cond SD
1.0 13.0 5.0 25.1
1.8 9.9 5.2 18.8
3.7 7.2 5.6 13.1
7.1 5.5 6.1 9.4
14.2 4.1 6.9 6.6
21.2 3.6 7.4 5.5
28.5 3.2 7.9 4.7
For most bands there's quite an opportunity for soil variation to
change HF conductivity by making deeper soil invisible to RF.
I think it's best to ignore the FCC map for HF questions.
Examples of ground constant variation with depth:
http://ham-radio.com/k6sti/gcvar.htm
Ground constant extrapolator:
http://ham-radio.com/k6sti/extrap.zip
Skin depth calculator:
http://ham-radio.com/k6sti/sd.exe
Brian
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