[TowerTalk] ground rod depth problem - and understanding ground rods

[K4SAV]

My understanding of ground rod performance characteristics during a 
strike leaves a lot to be desired, and I can't find any information to 
answer those questions either.  We have rules that specify distance 
between rods because of' ground saturation and the need to spread the 
charge over a larger area.  I don't understand exactly what happens with 
the underground plasma that takes place around a rod during a strike, 
and what that does to the ground rod impedance, and  how that affects 
ground saturation.  I would guess that the impedance of that ground rod 
during a strike is a huge non-linear function, not even close to what 
you might measure with any instruments under normal conditions.  
Besides, if I had that information I could do an accurate model of a 
ground system instead of having to ballpark and conservatively estimate 
everything.

Then if you encase the ground rod in concrete, how does that effect the 
underground plasma and the rod impedance during a strike.  Also what 
happens to the concrete.  I would guess that it might explode if there 
were insufficient ground rods in the system.  I wonder how many would be 
sufficient.  If the impedance of the ground rod is much lower when 
encased in concrete, why don't the commercial cell tower companies use 
concrete around the rods?  I wonder if they have tried it.  Would 
concrete be better than packing the hole with bentonite?  I know there 
is some information on Ufer grounds but those are just guidelines and 
really don't answer the details of how things work.

Lots of questions and nowhere to go for answers.

Jerry, K4SAV


On 11/14/2012 7:00 AM, Jim Lux wrote:
> volume isn't the important metric.. surface area is.. a bar 20 feet 
> long and 1x1 foot cross section is 82 square feet in cross section.
>
> I guess, though, the top of the footing isn't usually buried, so 
> probably 60 or so square feet..
>
> Concrete is almost always higher conductivity than the soil 
> surrounding it (unless you're using some exotic low conductivity 
> concrete) because it's hygroscopic.
>
> So instead of a contact area between conductor (rod) and soil 
> (probably not even a square foot), you have a fairly good contact that 
> can't be disturbed between wire and concrete, and then a very large 
> contact area between soil and concrete, along with the "current 
> spreading" from the concrete, so the current density at the 
> concrete/soil interface is low.
>
> In fact, for RF and transients, the *capacitive* coupling from the 
> concrete to the soil is pretty good.
>
>