[TowerTalk] w7ekb & ground rods

[Jim Lux]

On 1/19/15 8:45 AM, Ken wrote:
> It seems to me that the ground above my rock layer (@ 36-40”)  gets really dry during the summer.  Does that dry dirt have enough conductivity to be useful?  I do not know the answer to that question.
>
> Are there different answers depending on why we have the ground rod?  (RF ground, power line ground, or lightning protection)
>

Yes..

ground rods make terrible RF grounds, in general (where RF is HF and 
up): skin effect means that wires and rods have high ac resistance. 
(skin depth in copper at 10 MHz is about 0.8 mils/0.02 mm.)

They also have significant series L (1 microhenry/meter for a wire.. so 
a 30 foot run to the rod is a 10 uH inductor, that's 600 ohms reactive 
impedance.

Rods are really for electrical safety ground and/or lightning ground. 
And they don't work all that well for that, unless deployed in large 
numbers.  The advantage of a rod is that it's easy to install by 
driving, but as an electrical connection to the earth, it's just not 
that wonderful: the surface area is quite small (8 foot rod, 1" in 
diameter is only 300 square inches.  You could probably do better, 
electrically, by burying a 1 foot square plate (288 square inches).


Rods are also used in phone and power line applications.. you drive a 
rod at every pole (or wrap the ground wire around the foot of the pole 
when planting it).  Even if any one rod has crummy characteristics, 
there's lots of other rods in the circuit to help establish the common 
voltage reference and provide a fault current return.  I've had telco 
installers drive a new rod next to the existing rods on the general 
principle that at least they knew the new rod was in good condition: 
faster to just do a new rod than to test the existing one.