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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