Now I'm confused.
I (will) have a main panel on the garage wall. The power company ground rod
is right there. It's not much of a ground rod.
There's a sub panel up in the radio room over the garage. That sub panel is
connected to the main panel with four wires, 2 black and one white neutral
for 220, and a separate green ground wire. Neutral is attached to the green
ground wire only at the main panel.
The radio equipment will be connected to one heavy conductor (a copper or
aluminum bar) within the shack.
The tower is a bit more than 100 feet away. It has several ground rods. If
lightning hits, the tower and its antennas are the most likely first
I'm planning to put some PolyPhaser lightning arrestors inline to the
antenna coax, and through that effort the coax shield will be bonded to the
tower's ground system.
Are you advocating a separate wire 100+' heavy copper wire to connect the
tower's ground system to the power company ground rod outside the garage?
Is this "single point ground"? Looks like multiple paths to ground to me.
Is there a reason to run a separate wire from the shack ground plate to the
power company ground rod, or would it suffice to connect it to the green
wire inside the sub panel, which has a green wire to the main panel and then
to the power company ground rod. Isn't that more "single point" than a
separate parallel ground line?
[mailto:email@example.com] On Behalf Of Jim Brown
Sent: Wednesday, August 04, 2010 10:03 AM
To: Tower Talk List
Subject: Re: [TowerTalk] A different question (I think) on SPG
On Wed, 04 Aug 2010 12:19:12 -0400, firstname.lastname@example.org wrote:
>You are confused by the term SPG. It does NOT mean that EVERYTHING is
>tied to ONE POINT!
Unfortunately, you are too. :)
>The ground you have outside your shack is the SPG for your
>equipment/shack, provided all 'boxes' in the shack are tied to it and
>not simply to each other.
WRONG! DEAD WRONG!
All equipment in the shack should be bonded together by very short, beefy
copper. This combination of bonds is what goes to the earth electrode.
Further, ALL earth electrodes must be bonded together by short, fat
conductors. This, in fact, is Gary's problem, it is a common problem, and it
is the same problem I had in my house in Chicago. The fact that it is hard
doesn't mean that we should not try to solve it. If we don't, lightning may
do it for us. :)
>The common practice, many years ago, was the 'daisy-chain' the ground
>from box to box to box, etc ... and then ONE wire went to ground.
>This is a problem waiting to happen.
Again, I strongly disagree. There is nothing wrong with short fat conductors
bonding equipment together and bonding the combination of those bonds to the
earth electrode system. AND all of that MUST be bonded to the power system
>Tie EACH of the 'boxes' in the shack to the SPG outside with SEPARATE
>ground wires. Thus, the SPG you have serves its purpose.
Running a half dozen wires from your operating desk down to a ground rod
with no connection between them in the shack is a recipe for hum, buzz, and
>The grounds at each tower are great for dissipating the initial 'blow'
>if struck, but NOTHING will keep the 'strike current' from travelling
>from tower to shack.
I don't agree. Remember, lightning is NOT A DC EVENT, it is a TRANSIENT
event, which means it is an RF EVENT. The towers are far enough away from
the house that the inductance of the connections between them provides SOME
insolation between them.
The most important thing here is to ground each tower AND ALL THE WIRING as
effectively as possible. That means bonding coax to the tower at top and
bottom. It also means bypassing CONTROL LINES to the tower at top and
bottom. Coaxial ferrite chokes on the lines between the tower and the shack
can also help keep the strike (at least until they explode). :)
>The idea is to MINIMIZE the potential damage.
And to minimize the LIKELIHOOD of damage.
Lightning current will usually find the lowest impedance path to ground
relative to the point(s) where the charge is building. It it's building by
your tower, you want to give it the lowest possible impedance path to earth
at the base of the tower, and you want the impedance back toward the house
to be high. It is the ratio of those two impedances that protects your
house. The impedance back toward the house is the inductance of the coax and
the control lines.
Getting back to Gary's situation. If it's not possible to bond from one side
of the U to the other in a straight line, I would build a perimeter ring of
ground rods around the house at spacings equal to their length, bond them
all together, and then bring bonds to them from the power service entrance,
CATV, telco, and shack. I would NOT bond the towers to the house ground. I
would also bond the power system green wire to that ring at multiple points.
What I've described is almost exactly what was done at an AT&T Long Lines
site on a mountain top where I've had a ham station. There are multiple
ground rods around the perimeter of a 60 x 100 ft building, all tied into
building steel close to each rods, and the power system service is bonded to
that at the point of entry. At that site, the tower is right next to the
building, it's 36 x 36 ft at the base, there are rods near each leg, they
are bonded together, to the tower, and to the building perimeter.
73, Jim Brown K9YC
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