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[TowerTalk] RE: Tower Grounding Query!

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Subject: [TowerTalk] RE: Tower Grounding Query!
From: Dick Green" < (Dick Green)
Date: Thu, 12 Aug 1999 10:56:01 -0400
I have some additions questions on this subject.

John N5CQ wrote:

> I use a rule of thumb that
> if the towers are more than 80' apart, they are essentially two separate
> entities as far as lightning strikes are concerned, and they need two
> separate ground systems which are not tied together.

I definitely agree with that, although it seems to me that as long as each
tower has its own, adequate ground system there would be no harm in tying
them together (aside from a possible waste of wire.) True?

If there's a question about the adequacy of the tower ground systems
(something it's hard to be sure about) could it possibly help to tie the
tower grounds together? If the ground system of one tower became saturated
with current from a lightning strike, wouldn't the excess current attempt to
follow the connecting wire to the other tower's ground system? If the
connecting wire is buried, wouldn't it act as a long, horizontal ground rod
which would dissipate some of the current?

The alternative path would be to follow the coax to the shack. Hopefully the
coax will be grounded at the entry panel to the single-point ground, as
recommended in all the texts. It seems to me that if the single-point ground
is more than 80 feet from the tower ground, it is a separate entity, just
like the second tower. If we are admonished to make sure that the coax is
grounded to the single-point ground, that implies that excess current over
what is absorbed by the tower ground can in fact travel along the coax to
the shack. If that's true, it should be able to travel along a connecting
wire to the other tower's ground system, too. That sounds like a good safety
bet to me.

> I also think each
> tower should be 80' or more away from the point where the feed lines enter
> the shack, and at that point you should have another ground system, which
> the common point for all grounds (equipment chassis, utility ground tie
> telephone, etc.) inside the shack.

I have the same question about tying the tower ground system to a
single-point ground that is more than 80 feet away. If the tower ground
system becomes saturated, wouldn't a connection to the single-point ground
system be helpful?

Before I knew about the seperate entity theory, I laid 250' of 1/0 in the
bottom of the 4'-deep conduit trench and connected it between the first
tower's ground system (12 rods in a radial pattern) and the single-point
ground outside the shack. I've been hoping all that wire wasn't wasted, and
that either it's a good idea to connect the ground systems together or that
the wire will act as a gigantic horizontal ground rod. Any comments (other
than to tell me I'm an idiot?)

I've recently installed a second tower with its own ground system 175 feet
from the shack. This time the cables are buried just under the sod, and I
haven't tied the tower ground to the single-point ground outside the shack.
There's a satellite dish about 40 feet from the tower with it's own ground
rod (poorly installed, I might add.) I figure I should repair and upgrade
the dish ground and tie it to the tower ground. Then the question remains as
to whether I should connect these systems to the single-point ground (a
little over 100 feet from the dish.) Comments?

Finally, I have a 2M Ringo on the roof that's not grounded to anything. The
coax runs directly into the attic and down to the shack. Seems to me that
the lightning path for this antenna is down the coax to the shack, through
the 2M mobile (chassis not grounded) to the 12VDC power supply to the
single-point ground! I've probably been just lucky that there hasn't been a
bad hit, although there's a huge tree right over the antenna that might have
saved me. My plan is to connect a heavy ground wire to the antenna mast and
run that down the side of the house (about 35 feet) to a ground rod, which
would in turn be connected to the single-point ground (about another 20 feet
away.) The antenna is DC-grounded, so I've been wondering whether a
lightning suppressor on the coax at the antenna end is necessary in this
case. Comments?

> I use 4 ground rods along a line with 8'
spacing, tied together with copper strap along one side of the shack.

Why 8' spacing? The Polyphaser book says the rods should be seperated by
twice their length (16') because the effective absorbtion area around each
rod is a "sphere of influence" with a radius equal to the length of the rod.
Overlapping the spheres causes the soil to saturate faster. They do point
out that if soil conductivity is very poor, the ground in the sphere around
a given rod won't absorb enough and the inductance of the 16'
interconnecting cable will choke off the remaining current (i.e., it won't
reach the second rod.) That left me puzzled about how much separation I
should use. The soil around here is rocky, so I settled on 12' as a
compromise. How did you decide on 8'?

73, Dick WC1M

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