From: Chris Trayner <C.Trayner@leeds.ac.uk>
To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
Date: Thu, 10 Feb 2011 09:00:56 +0000
Thread-Topic: LF: how to get ground rod ressitance low
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Subject: Re: LF: how to get ground rod ressitance low
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Hi Jurgen,

> I just discovered that my ground rod isn't ideal by having the assumed=
 0 ohm to=20
> earth. Measuring with 10VDC the current from neutral/waterpipe to the ro=
d gives=20
> about 140 ohms.=20

For cave radio, where we use earth connections, we typically get a few hun=
dred ohms. We are normally in shallow soil over rock, though, so maybe you=
 could do better.

> I use a 8mm rod 1m long in wet soil. I put in another one and tried seve=
ral=20
> places nearby and got it a bit lower, but far away from ideal.
> So I assume the surface size is too small, one the other hand I want a=
 small=20
> rod that can be pulled out with ease in spring when the season is over.

The surface area is certainly important - the current is concentrated as=
 it gets nearer the electrode, like a traffic jam. The resistance close in=
 to the electrode dominates the total resistance.

For cave radio we tend to use several short electrodes, maybe half a dozen=
 or so. We do this because it is impossible to drive long rods into shallo=
w stony soil, but you might find it a useful trick. The rule of thumb is=
 to space them apart by comfortably more than their length: twice should=
 be enough. We've played with different layouts (straight line of pegs, ci=
rcles, ...) but it doesn't seem to make much difference.

We normally use tent pegs. You can get metal ones that are L-shaped in cro=
ss section. This gives you a large surface area but a small cross-section=
 area (easy to drive in). They normally have a bent-over top, so you can=
 put a bar under it to lever them out.

When I used longer tubes for earths, I drilled a horizontal hole through=
 the top from side to side. Then I could put a screwdriver through the hol=
es to make a handle to pull them up by.


73,
Chris G4OKW

-----------------------
Dr Chris Trayner
School of Electronic & Electrical Engineering,
The University of Leeds,
Leeds LS2 9JT, United Kingdom
Tel: +44 113 34 32053
Fax: +44 113 34 32032