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Re: [TowerTalk] Soil resistivity test done: Some questions

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Subject: Re: [TowerTalk] Soil resistivity test done: Some questions
From: "David Robbins K1TTT" <>
Date: Sat, 9 Sep 2006 16:46:56 -0000
List-post: <>
'Resistivity' or 'Resistance'??  I am confused by the wording of some of
your measurements... and since you don't give units I have to assume certain
things.  Note that 'resistance' is measured in ohms, 'resistivity' is in
ohm-meters, or occasionally ohms-cm.

> In a place where I have 2 ground roads (each 2.5m) I have a resistivity
> of 120.

Is this measured 'resistivity' of the soil using the 4 probe method or the
'resistance' of the 2 rod system??  A resistance of 120 ohms with 2 rods
isn't all that great, a resistivity of 120 ohm-meters is pretty darn good.  

2 rods 2m long in 125 ohm-meter soil would be about 30 ohms which I would
consider reasonable for a tower ground... assuming of course that there
would be more rods, and some kind of perimeter ground all connected together
with it back at the house end.  If that soil is 500 ohm meters than that
would be about 120 ohms for those same 2 rods.  Try organizing your
questions a bit better and include all the units, that may help a bit. 

> I have calculated that you must put 36 ground roads in parallel to
> achive a resitance of 13.8

power companies often aim for 25 ohms or so for tower grounds, where did you
come up with the 13.8 target??

Changes with depth are common, but not very useful unless you can drive very
long rods... or you happen to have a water table that is consistently high.

Also, remember that rods have to be spaced out at least as much as their
length, more is better, otherwise the improvement reduces.  i.e. two 2m rods
spaced 4m apart may have 1/2 the resistance, at only 2m apart it may only be
1/1.5 and at 1m apart you might as well not bother with the 2nd one.  on the
plus side, the buried bare wire connecting rods adds to the effectiveness,
4m of wire between 2 rods is almost as effective as the 2nd rod itself.

If you have very poor soil you may be better running radials.  You not only
get lightning performance improvement, but if there are enough of them you
could also help reduce losses of any verticals you put over them.  For
lightning grounding, 4 radial, 1m down, and 10m long in 500 ohm-meter soil
gives a resistance of about 15 ohms... only .1m down only reduces that to 20
ohms, still a respectible ground and likely much easier than 36 rods.

David Robbins K1TTT
AR-Cluster node: 145.69MHz or telnet://

> -----Original Message-----
> From: [mailto:towertalk-
>] On Behalf Of Beat Meier
> Sent: Friday, September 08, 2006 20:04
> To:
> Subject: [TowerTalk] Soil resistivity test done: Some questions
> Hi all
> I'm still a grounding dumy but I've just bought a GEO test and have done
> some soil resistivity test
> (for just 800US$ I can do 4 pole methods, cheapest I have found and
> still has serial port
> to colect data ...)
> I'm still in putting my new tower of 200ft ...
> Now I have some questions.
> In a place where I have 2 ground roads (each 2.5m) I have a resistivity
> of 120.
> 4m away I have a resistivity of 500.
> The test was done with the 4 pole method.
> I have not measured the ground of the house, only the soil just above
> where the
> roads are with several depth and in the deps of 3 m I have this 120
> Now my questions:
> 1. This low resistivity comes from the 2 ground roads already there
> I have read that 2 ground roads in parallel will half the resistivity
> i..e 500/2 250 but I have  120
> If you use 4 you get Resistividad/4 i.e. 500/4=125
> Is this formula right?
> The other idea is that the fundament of the house is very near (about
> 1m) could this also
> have such influence to lower the resistivity because of concret??
> 2. The resistivity of 500 is not very good, isn't it?
> I have calculated that you must put 36 ground roads in parallel to
> achive a resitance of 13.8
> which is still pretty bad (of course only if the formula above is right)
> BTW: The tests have showed what someone already told me that the soil
> here is not good
> to go in depth ... I have the following:
> a=2m -> 486
> a=3m -> 546
> a=4M ->535
> a=6m -> 429
> so it seems that going in deep does not make any difference (apart from
> the cables you must use
> to connect the ground roads) in contrast to use parallel ground roads.
> And last question:
> How many copper have good ground roads. The ground roads I can buy here
> have only 200um of copper
> (5/8''-2m or 5/8''-3m). I have seen that they peel quickly if driven
> into earth and there is any obstacle ...
> What I can get also is plate of copper 30cmx30cm but this costs about 8
> times more than one ground road
> I don't know how they lower the resistividad of the soil ... I have read
> in an article about
> 0.8*Resitivida/circumference  is this right?
> What about putting using the fundament of the house i.e. drive a whole
> in the concrete and connect a
> copper or what ever cable ot the iron and put again concret?
> Thanks for any ideas or comments
> Beat
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