On 20 Nov 2004 17:59:12 -0800, (Brian Kelly) wrote:

Gary Schafer wrote in message . ..
On Fri, 19 Nov 2004 07:44:50 -0500, "Floyd Sense"


As Jack mentioned grounding the cable "at the bottom of the tower like
is used nation wide in tower designs" is ideal. But unfortunately that
is not how it usually gets done. Often the lines come off the tower at
6 to 10 feet above the ground to go to the building in a cable tray.
But it would indeed be best if they were taken all the way to ground
level before exiting the tower.

The reason being that during a strike the tower and associated lines
on it develop considerable voltage drop due to the high current being
conducted. Coming off the tower above ground is like taping a resistor
part way up from the ground end. Allowing more voltage to exit on the
lines rather than the potential at the base of the tower where it is
closer to ground. The tower usually has considerable inductance for
voltage to develop across.

I'm one of those who pulls the coax off the tower at around eight feet
and hangs it on a carrier wire from the tower to the outside wall near
the shack.

Theref are many installations like yours in existance. It was the
"common way" to do it some years ago. Not the best though.

In the past I've had end insulators at both ends of the
carrier wire. Your point about grounding the coax at the base of the
tower is well taken but is obviously not possible in these situations.
It occurs to me that the same effect can be accomplished by connecting
a #6 or #8 solid wire between the the coax shields where they bend
away from the tower and the base of the tower. Yes?

No that won't do much good. If you ground the coax shield to the tower
where it bends away from the tower you will have a much better (lower
inductance) to ground with the tower than what the wire would provide.
The wire would do almost no good at all when compared to the much
larger tower in parallel.


Taking it a bit further it also occurs to me that the carrier wire
could be connected to the base of the tower at the point where the
tower connects to the ground rods there, then up the tower and
connected to both the coax shields at the eight foot level and the
tower again.

Same as above. Grounding the carrier to the tower will do much more
than a wire to the ground rods at the tower. The carrier wire should
not be insulated from the tower. It and the coax should both be
grounded to the tower at the exit point. Otherwise you can have
flashover's to the carrier.

Then horizontally to the house wall with the coax, then
down to the ground rods just outside the shack to which the equipment
is also grounded. I'd also connect the coax shields to the carrier
wire again at the point where they turn away from the wire and go
through the wall. One hefty continuous, unbroken length of copper
wire. There would still be voltage differentials involved because
there is no escape from the inductances BUT . . . is my thinking in
the right direction here?

Connecting the carrier wire to the coax again at the house is a good
idea for the same reason you should connect it at the tower. to
prevent flashovers to the cables. The same situation exist on the
tower itself with lines running down. That is why they should be
grounded to the tower at several points. Especially on a tall tower.

The tower has inductance just like any piece of wire has. Although the
tower inductance is less than just a length of wire it still has
inductance. When lightning strikes the top, the tower and lines all
share the current to ground. The farther up from ground you are the
higher the voltage will be with respect to ground. It can be
significant. Especially on a smaller tower. Leaving the tower only a
few feet above ground with your coax line is putting that line at some
point above ground that can have high voltage.

The best way is to run the lines all the way to the bottom of the
tower, ground them there, and then run underground to the house to
your ground rods. Don't forget to also run a ground lead from your
house ground to your tower ground system too. Bury it along with the
cables. That will give you more contact with the earth as well as
tying the grounds together.

73
Gary K4FMX


73
Gary K4FMX

w3rv