TopBand: soil conductivity

[Tom Rauch]

> Date:          Wed, 25 Mar 1998 22:59:32 -0500 (EST)
> From:          Tom Frenaye <frenaye@pcnet.com>
> Subject:       TopBand: soil conductivity
> To:            topband@contesting.com

Hi All,

 
> KM1H said> Very Poor Soil, I would estimate 1 mS/M.
> K0HA said he might see 1000 mS/M when real wet

That may be true for ground rods only driven near the surface, at dc, 
or at VHF, but certainly not for what everyone has been talking 
about. The skin depth is at least 20 feet in typical good soil and as 
deep as 50 feet or more in poor soil. It would have to be some VERY 
dry soil and a real "soaker" to change the soil that affects 160 
meter systems (unless you only use a ground rod ground) a noticable 
amount.
 
> I'm planning to measure things here.  Are there better ways to measure 
> soil conductivity than the info in the ARRL Antenna Book (by W2FMI and 
> W2FNQ originally in 4/78 and 3/81 QST)?  Looks like a simple device.

The method in the Handbook is useless, since it only measures low 
frequency conductivity (at 60 Hz) and conductivity near the surface. 
You'd be closer looking at BC engineering charts and  using the 
average value they give for your area.

The only correct way to measure conductivity is to measure 
attenuation or absorption of an RF signal at the operating 
frequency, and the signal has to be a far field source. 

You could install a low dipole, and measure the impedance of the 
dipole as yopu raise and lower it and calculate conductivity that 
way. That's what I did when I measured my elevated radial system. My 
actual soil conductivity was 4 mS/m, while the ARRL Handbook method 
indicated 0.5 mS/m. That's because the ground surface was mixed with 
rock and dry, while five feet down and lower there was wet clay and 
rock.

 
73, Tom W8JI
w8ji.tom@MCIONE.com

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