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Old November 22nd 04, 03:36 AM
Brian Kelly
 
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Gary Schafer wrote in message . ..
On 20 Nov 2004 17:59:12 -0800, (Brian Kelly) wrote:

.. . .


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.


Got it.

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.


OK, cancel useless wire from base of tower.

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.


OK again.

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.


I got that from your prior post.

It can be
significant. Especially on a smaller tower.


It took a few seconds to get your point but yes, it's a matter of how
far up the tower the coax departs the tower as a percentage of the
tower height. Since I'm planning a short (35-40 foot tubular crankup)
tower I'll have both a "high inductance tower" and a high pulloff
level in terms of percentage. Not good no matter how one looks at it.

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.


That's a given.

Bury it along with the
cables. That will give you more contact with the earth as well as
tying the grounds together.


The wire will be there but I doubt that I'll be able to bury it.

The whole (small) property is part of a forest of huge old hardwoods
several of which are crowded close to the house particularly along the
wall thru which I need to feed the coax. You'd have to see it to
believe it and it's only six miles from City Hall Philadelphia.
Digging a trench is not possible thru the tangle of roots on any
approach from the tower to the wall. I'm not looking forward to
driving ground rods thru this maze of underground lumber but I'll do
it even if it takes some serious power drilling to accomplish.

What I could do is run all the cables to the bottom of the tower with
shield bonds at the top of the moving section, another one at the top
of the fixed section, another bond halfway down fixed section and the
last one at the bottom of the tower. Which will also be surrounded by
trees. There's a hole below the canopy big enough to accomodate a
Hexbeam or some similar very compact HF antenna if I spot the tower
correctly. Some contractor is going to have a really bad time digging
the hole for tower base. From the base of the tower I'll run the
cables and the carrier wire horizontally on the surface for a few feet
then back up to the eight foot level to a tree trunk. Six feet would
also work and the rest of the run would be per previous.

The good news is that the soil is eternally damp highly conductive
dark loam . .

Gary K4FMX


Thanks Gary.