Which doesn't mean that 2 closely spaced rods are worse than 1 rod but that
they just aren't as effective as 2 properly spaced rods. This assuming
that the rods are firmly bonded together with a lo-z conductor.
According to most of the lightning and grounding literature, you can
imagine the soil dissipation area of individual rods as half of a ball the
radius of the length of the ground rod starting at the soil surface and
sweeping down to the tip of the rod. Overlapping these "balls" of
dissipation is a waste of rods. IE, rods should be spaced 2x the rod
length for maximum effectiveness. I tend to believe that in most
situations, 3ea properly spaced 8ft rods are more effective than 1ea 24ft
rod.
On the other hand, in some soil types (ie, sand), you supposedly get some
benefit from closely spaced rods in that the multiple rods aren't as prone
to glass coating in huge strikes. I have exactly zero experience with that
type of grounding failure although we have pulled a few glass coated rods
out of the ground.
I own several different types of ground testers, 3 wire, 4 wire, clamp-on,
etc. I don't trust any of them to tell me anything other than DC
resistance to ground. Your eye and knowledge of lo-z materials is a better
judge of proper RF grounding than any tester that I know of.
We build broadcast grounding to _be_ lo z and trust that it is going to
remain so. If I am doing an inspection and see a long piece of #6 wire
being used as an RF or lightning ground, it gets replaced with strap. We
typically use 4in wide copper strap with all brazed or exothermic bonded
connections. In general, Low Z = short as possible, large surface area and
no sharp bends. And yes we sometimes have to use smaller than desired
conductors simply because that is all that will work in that situation.
These small conductors are kept very, very short and as straight as
possible on it's way to the main ground strap.
Regards,
On Sun, Jan 10, 2016 at 12:37 AM, Jim Brown <jim@audiosystemsgroup.com>
wrote:
> The reason for rod spacing is to minimize inductive coupling between them,
> which reduces their effectiveness.
>
> 73, Jim K9YC
>
Kevin C. Kidd, CSRE/AMD
AM Ground Systems Company - WD4RAT
kkidd@kkbc.com -- 866-22-RADIO -- 866-227-2346
www.amgroundsystems.com
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