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Re: [TowerTalk] A different question (I think) on SPG

To: "Gary E. Jones" <>
Subject: Re: [TowerTalk] A different question (I think) on SPG
From: Kipton Moravec <>
Date: Wed, 04 Aug 2010 11:21:03 -0500
List-post: <">>
Let me see if I can take a whack at this.

On Wed, 2010-08-04 at 10:51 -0500, Gary E. Jones wrote:
> I have read the threads on SPG over the years, but I think I don't really
> understand a couple of concepts that are fundamental. Maybe someone can
> help. 
> I live in a 4000 square foot ranch house which is in the shape of a "U" with
> sleeping areas on one end of one leg of the U, and Garage/laundry/den on the
> other leg of the U, and the traditional living areas in the center of the U.
> The house has two 200A 240 volt distribution panels as the house was all
> electric initially. The phone and power services all come into the house on
> the garage end of one leg of the U. The shack is one of the bedrooms on the
> other leg of the U. 

Make sure phone and power grounds are tied together.

> Further, the house is 500' from the two 90' crank up towers, each with two
> yagis stacked on them. The towers are grounded through three ground rods per
> tower. Further, the feedlines for the yagis (buryflex) enter into
> weatherproof boxes mounted on 4x4 weather resistant posts. The boxes have
> remote coax switches in them to switch the various yagis on each tower. The
> remote switches are grounded and I will be separately grounding each piece
> of buryflex at the base of the towers. There is Heliax from the remote
> switches (500' for one tower and 600' for the other tower) to the house. 

When you get a lightning strike where do you want the power to go? Down
your switch wires and coax or down a ground wire? (Ground Wire is the
correct answer.) 
> Now, I assume there is no logic of trying to put the house and the two
> towers at the same common ground point and that the towers have to be
> separately grounded at their base (the way that I have it now). That means
> there is a 500 and 600 foot separation between the towers and the house.
> Further, the shack is on one side of the house and the common utility
> grounds are in the center of the house. I have a separate ground rod right
> outside the shack window, so it is not a SPG even for the house. 

O.K. where is the lightning going to go to find the lowest point ground?
>From the tower to the coax. Some will be dumped at ground rod at the
shack window. The rest will try to get to the ground at the house power
service panel. Unfortunately your transceiver is in that path.  

> Now, is the conventional wisdom that I have a problem?

You have a problem.

> I can't see an easy or even possible solution. It seems to me that the
> towers have to be grounded separately, and short of running a ground strap a
> hundred feet and boring either under my slab or around my swimming pool,
> there is no easy way to even have a SPG for the house. 

It sounds like that may be what you have to do. You have to tie all the
grounds together. You want the house ground to move with the antenna
ground and your entrance ground, so the power from the lightning goes
through that ground wire and not through the house wires taking
everything out with it.

> One more thing, it has always seemed to me that at the exact instant that a
> tower or antenna gets hit with lightening, the tower ground is going to be
> many volts higher than a second ground 500' away as the charge dissipates
> into the tower "ground". That is going to put a very large voltage
> difference between the two and is my understanding of the whole logic of
> SPG. Am I correct or wrong in my assumptions?  However, if they are all tied
> together, for that same fraction of a second, isn't everything "hot" (even
> the "ground") relative to any other ground that is separated from the common
> ground?

Precisely. That is the idea of what you are trying to do. As all grounds
go up to a higher potential together, there is no potential across your
equipment, and your equipment does not smoke.

Think of a bird sitting on a power line. Even though there are hundreds
of volts in the line the bird does not die, because the the power does
not go through him, it is easier for the power to go through the wire he
is sitting on.  
> Straighten out my thinking. 
> Thanks in advance
>      73
>                  Gary      W5FI 
Kipton Moravec AE5IB .- . ..... .. -...

"Always do right; this will gratify some people and astonish the rest."
--Mark Twain


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