Jim Lux's post on this cuts to the heart of the issue -- it's impedance
that matters most, and that's dominated by inductance, not resistance.
And yes, the contribution of those rods farther from the tower decreases
with the length of wire connecting them to the tower. It really is a
matter of when does your back give out. :)
My tower is 250 ft of coax from the shack, I have lousy (rocky) soil,
the tower is surrounded by hundreds of trees that are at least 50 ft
taller than the tower, and the lightning activity map shows my QTH to
have close to the lowest activity in the US. I stopped at one rod on
each leg. Perhaps I should have done more.
Last week, I installed the Rohn grounding clamps on the legs of my 120
ft Rohn 25 tower, replacing the ones that DX Engineering sells. Here are
photos of two of the clamps, taken from different angles to show how
they attach. The wire is #4, I cleaned it around the clamps with vinegar
before installing. Some of he individual strands above the clamp are
used to attach radials (the tower acts as a passive reflector for two
quarter-wave wires sloping away from the tower that I load on 160M from
the base of those wires).
k9yc.com/Photo/TowerGroundClamp1.jpg
k9yc.com/Photo/TowerGroundClamp2.jpg
The clamps were recommended here by N0AX, and I really like them. Very
robust, very easy to install. The bottom 4 ft of my tower is buried in
the pour, and I painted the tower sections (on the ground) to minimize
their visibility. The paint and bits of concrete around the clamp was
scraped to provide the greatest practical contact. Three copper straps
from the pour are clamped to the wires near where they emerge.
I posted links to a source for these clamps last week.
73, Jim K9YC
On 11/3/2017 9:58 AM, Jeff wrote:
In one of the Polyphaser documents they talk about the rise time and
length of a strike duration. Forget the details but the conclusion
was that rods further out than about 50' would not be effective
because the strike would be over before the outlying rods came into play.
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