If you have a rod that is 5/8 diameter, why is making it 2 feet deeper going to change the horizontal dimensions of the "sphere of influence"? It will change the vertical dimension but horizontally why wouldn't it remain the same?
In order to remain a sphere, as any one dimension changes, so do all the others.
The deeper the rod, the larger the diameter of the sphere of influence. Just as the shallower, the smaller.
It's more geometry than electrical theory
I think there has been a few misconceptions brought on by the guides of twice the length of the rod apart.
The term of twice the length of the rod was just a guideline but really has very little to do as to how big the sphere of influence is, using this, one could say that the SOI of a ten foot rod would be 20' when it would be much closer to 26'-28', a longer rod will for the most part extend the amount of soil in contact with the rod to 2' deeper, as well as the SOI would also extend 2' deeper, so the SOI would just look like a longer tube with a round bottom under the bottom of the rod, it does not increase the diameter of the SOI but maybe by a fraction of the 2' increase in depth.
If we go back and look at Gary's experiment we can see that even though he only drove these rods 5 feet into the ground the diameters of the SOI's he got are very close to what one would expect from a 8'-10' rod, as a matter fact because of his high conductive soil his SOI were smaller as a result of the conductivity of the soil, not the shallow depth of his rods as one might be lead to believe, if he had driven the rods deeper it might have increased the diameter a few inches but that would have been it.
In the afore posted PDF on the second page in the left paragraph he came close to the real life diameter of a common SOI:
Earth electrode resistance is the number of ohms of
resistance measured between the ground rod and a
distant point on the earth called remote earth. At this
point, the earth electrode resistance
no longer increases
appreciably when the distance
measured from the grounding
electrode increases. This distance
is typically about 25 ft
for a 10-ft ground rod.
In real life test it is more like 26' to 28' for about 100Ω per cm of soil, I have never seen much of a change in this distance per the length of the rod as he stated, the bigger difference is more dependent upon the resistance of the soil, as this plays a much larger role in what the size of the SOI will be.
But I can see where being that it is common to install an 8'-10 rod, it would be easy as a directive to just use the 2 times the length of the rod as a good reference?
Now since I know Mark is into radio as I was on the commercial side, in doing many radio towers grounding grids under the tower it is easy to get crossed up on what is required for a ground plane (AM broadcast) then as for lightning protection, although the two work together and function as both, we used to install as common 40 to 50 foot sectional rods that were cad welded together as they were driven, these were driven (in most specs) every 20 foot in a circular pattern around the base of the tower in rings every 20 foot out to a point that depended upon the frequency of the transmitter (325' was common for stations around 1250kz), these rods would be cad welded together with copper conductors in a circle pattern and in a radial pattern to form a grid ground plane that would mirror the tower in hight, as at these frequencies the whole tower was the transmitting antenna, I have also seen where 40' spacing was spec as since the rods were out of the SOI it really didn't change anything.
so we can see two very different requirements but with the one for radio towers can have a very much longer distance between the rods or closer as because of the long wave length it really doesn't effect the function of the ground plane because once you reached the outside distance of the SOI it doesn't change the resistance between the rods.
Oh Mark I have also helped a couple hams design a grounding grid for a 160m ground mounted ground plane using this same system of rods and grid. oh are they fun to do, or should I say allot of work if you don't have a trencher and rod driver.
