Hi Jurgen,
> I just discovered that my ground rod isn't ideal by having the assumed 0 ohm
> to
> earth. Measuring with 10VDC the current from neutral/waterpipe to the rod
> gives
> about 140 ohms.
For cave radio, where we use earth connections, we typically get a few hundred
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 several
> 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
> 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 shallow 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, circles, ...) but it
doesn't seem to make much difference.
We normally use tent pegs. You can get metal ones that are L-shaped in cross
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 holes 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
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