"Simple, but wrong."
Nope simple and it works. The arrestor manufacturers have done lot of testing
and calculations and real world testing of their products. The proof? Do any
of the commercial radio stations or your fire and police departments disconnect
and stop operating when there's a storm?
They don't, right? Know why? They are using the same arrestors and design as
we're talking about here. They're not replacing equipment all the time so a
properly designed and installed grounding system does indeed work.
"There is no such thing as "no voltage difference" when you are talking
a million amps or so."
As stated, it's more like kilo amps but it does sure seem like a million!!
And remember we are more concerned with nearby strikes than a direct strike. A
strike a mile away is said to be able to induce enough energy in your system to
possibly zap your electronics. I know from personal experience that a direct
strike that blew up my neighbor's tree, 300 feet from my barn, 400 feet from my
little tower and yagi, and 700 feet from my VHF/UHF antennas, induced more than
30 amps of current in the barn electrical. None of my equipment was affected
and the VHF/UHF is on 24/7.
"Let's say your tower takes a direct hit. Lightning wants to go to
ground. It doesn't want to go to your house, but it will if there is a
voltage difference and a path. You may think the base of your tower
has a zero impedance path to ground, but at a million amps, I
guarantee the impedance is small, but not zero. Even a perfectly
straight tower leg or piece of ground wire has some inductance and
resistance. Because your coax and rotator lines come off the tower at
some point above earth ground, even if only a few inches, they are
going to see a potential difference, and it can be quite significant
depending on your particular configuration. This potential difference
is going to seek its own path to ground, and that path is through the
coax/rotor wires running off to your house. At the point where those
wires enter the house, you need to provide a second path to ground
lest the voltage pulse get into your house and equipment and find its
own path to ground."
You are totally correct and at the house entrance is what is considered the
SPG, which may be more than one rod. It's just a term, SPG, and it can be
confusing. That little bit of energy heading down the coaxes/cables will be
shunted to ground via the arrestors located on those lines at the SPG.
"And this is why the concept of a "Single Point Ground" makes no sense
to me. In the case above, there must be at least two grounds or there
will be trouble."
A properly designed grounding system, to me, consists of 5 elements:
1) Tower/Antenna/Mast Ground (Tower legs each should have 75 feet of
grounding radials with rods every 2X height.)
2) SPG (Low impedance/resistance ground where all the cables collect before
entering the house.)
3) Arrestors (On all cables and tied via low impedance/resistance to the SPG.
Also, don't forget arrestors on phone/TV/internet lines and those may not be at
the SPG but you may want to establish their own SPG and then bond that, via an
outdoor path, to the radio SPG.)
4) Shack Ground (All internal equipment grounds are tied together and routed,
via low impedance/resistance ground back outside to the SPG.)
5) Service Entrance Ground (This is bonded via an outdoor path, to the radio
SPG.)
There you have it, 5 easy pieces, to a properly designed grounding system.
It really works.
--
Phil - KB9CRY
Lockport, IL
http://nidxa.org/memberWWW/kb9cry_home.htm
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