Folks,
Having invested in a SteppIR, a roof-top tower, a
rotor, and several wire antennas, I find I have a lot
of copper running out to the sky from my shack. Too
much to easily disconnect when I hear thunder. Time
to think about a better protective ground system.
Inside the shack, I am thinking of a single-point
ground plane near the feedline entrances. All
connections including AC power, telephone, computer,
SteppIR/rotor control, etc. pass through surge devices
on that plane, and all equipment is hopefully
connected by low inductance strapping to the SPG.
My problem is how to get a good "earth ground" for the
SPG. My house is on a granite ledge with only 1-2 ft
of soil nearby. We are about 80 ft from salt water,
but that doesn't help. The house does have lightning
rods with 3 or 4 downleads (more on this in a moment).
The 20-ft rooftop tower is connected to a lightning
rod ground, but feedlines are not.
I was wondering how the lightning rod grounds were set
up, so I partly excavated one. The results are
visible on my web site:
http://www.aa6e.net/aa6e/grounding .
Is this approach - dual buried horizontal ground rods
- good enough even for home protection? Should I do
something like this for my station? (The shack is not
near any of the existing ground connections.)
It's clear that not even an "excellent" ground
connection can prevent kilovolt surges from a near
hit, because that is what happens to the earth itself.
And full protection from a direct hit to the house is
hard to achieve (IMO) no matter how you try in a
typical home retrofit. The whole shack should be
inside a grounded metal box...
My philosophy now is to concentrate on the SPG system
and not to obsess on the low-Z ground, which must be
pretty hard to realize over rock. (I would put in
something like what I see on the existing lightning
rod system.) If the SPG is really good, and the
equipment is all referred to that carefully, then it
shouldn't matter if the whole system floats up and
down by kilovolts. (Hopefully the op is elsewhere!)
The "kilovolt" scenario doesn't quite catch the full
impact of a direct hit to the antenna. 10,000 amps
into a one ohm ground (?) is 10 kV. No one expects
the Spanish Inquisition! (i.e., worst-case event)
I would appreciate suggestions, especially from anyone
who has faced a similar problem -- and survived the
worst.
73, Martin, AA6E
p.s. Fortunately here in CT, we have fairly low
lightning incidence. Even so, we had a close strike a
couple of years ago. A BIG crack, and both my close
neighbors lost their telephones and other electronics.
We had no troubles here. My tower was down then;
maybe our lightning rods actually caught one. Thank
you, Ben Franklin.
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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