"Rick (W-A-one-R-K-T)" wrote in
news

In the 2007 ARRL Handbook page 3.6, there is a discussion of building
a ground ring around the base of the tower, connected to three ground
rods each connected to each leg of the tower.

In the narrative it says "Space rods at least 6 feet apart" but then
in the caption for the accompanying drawing it says Locate ground rods
on the ring as close as possible to their respective tower legs".

On a Rohn 25G, the rods can't be "as close as possible" to their
respective tower legs and still be anywhere near 6 feet apart.

So, which is it ... close to their respective tower legs, or 6 feet
apart?

What is the reason for the 6-foot separation, anyway? The rods all go
into the ground. Why would it make a difference if they're closer
together than 6 feet?

The electrodes are placed in a quite resistive medium.

Increasing the diameter of an electrode does reduce the resistance of the
earth connection but by very little. Fig 1 at
http://www.vk1od.net/post/earthing.htm shows the effect of different
diameters on electrode resistance.

If you place a second electrode right next to an existing one, you will
obtain almost no reduction in resistance to earth, it is not a lot
different to increasing the diameter of the electrode. At increased
spacing, the improvement is better and at large spacing aproaches halving
the resistance of the single electrode, but offset to some extent by the
inductance and resistance of the connecting conductor.

In homogenous ground (and that is some assumption), it turns out that
increasing the spacing of driven vertical electrods beyond their own
length is of quickly diminishing benefit, so a common ROT is to drive
electrodes in a matrix of cell size about the length of the rods.

Someone else raised the issue of step voltage (the voltage between your
feet when you walk on ground carrying a large current). That is usually
mitigated by a (amongst other things) a conductive earth mat layed on the
surface and bonded to the electrode system. Nevertheless, it is not a
good idea for two people to carry a length of pipe (or a beam) over
ground subject to fault currents or likely lightning discharge.

So back to your specific question, if you place the three electrodes
close to each other (as a consequency of placing them at the tower legs),
the earth resistance is higher than it should be with greater spacing. If
you space them too far, the added inductance of the conductor connecting
them offsets to some extent the reduction in electrode resistance. I
haven't read the article to which you refer, but I wouldn't waste time
driving an electrode less than 2.4m, and I would not bother driving a
2.4m electrode electrode within 2.4m of another.

Electrodes are real cheap, and excluding rock, you can drive them with an
electric hammer and an adapter in minutes. You are trying to shed
discharge current to earth, so don't be shy about it.

Owen