To: <topband@contesting.com>
> 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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