[TowerTalk] 80M Delta Loop

[Brian Beezley]

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