On Sun, 21 Nov 2004 23:06:45 -0500, "Jack Painter"
wrote:


"Brian Kelly" wrote
Gary Schafer wrote
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.

This is correct, and why I mentioned even 6" was "too much".

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.

The last was not a correct assumption. The distance across a conductor (and
in this case it is also the distance to ground) is what allows inductance to
create deadly voltage potentials. Any conductor in series with a lightning
strike will exhibit the same characteristics. 6" above ground near the base
of a tower can translate to as much as 9800v above ground, with just modest
assumptions of a very average return stroke current of 25Ka with a rise time
of 40Ka/usec. It has no bearing whatsoever how tall or short the tower is.
It's not long (or high above ground) before you could see over 100,000v
potential develop where coax leaves any tower too soon.

A little clarification here. When I said "smaller tower" I was not
necessarily referring to a shorter tower but one that has less surface
area. (smaller face)

The main consideration is the distance up from ground that the cables
leave the tower.

A lightning strike is a constant current source. If it is a 20ka
strike the voltage across whatever it hits is going to raise high
enough to conduct 20ka. If you have a low impedance conductor (tower)
the voltage developed across it will be less than it would be on a
high impedance tower (smaller face tower).

That is why large communication towers have less problems with lines
coming off at higher points on the tower. More of the strike current
makes it to ground via the tower with the larger surface it has.

Leaving the tower at some height above ground with the cables is still
a division of the voltage like a voltage divider. The higher up you
are the higher the voltage you will see with respect to ground. But
what determines what that actual voltage goes to is the amount of
strike current and the amount of inductance between the cable exit
point and ground. Of course the cables leaving the tower will also
carry part of the current too.


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.

Burying a grounding electrode conductor is normally a code requirement. But
that is not what you have in connecting the tower ground system to the
station ground, AC mains ground, etc. Those are bonding conductors, and they
are in many cases required to be insulated. Not in this case, but I want you
to understand the difference between grounding, voltage division from many
grounds, and a bonding conductor between your station and the tower. The
latter is to maintain equipotential, and will not carry more than just
equalizing currents. It will be well within the capability of a #6 insulated
wire, should you choose to use that. Personally I would go a little larger
but #6 is as largest that NEC or NFPA recommend for a bond in *most* cases.
So burying the bonding conductor is not a requirement, although to protect
it that is exactly what most facilities do. Neither will burying coax
feedlines help in lightning protection, unless you are counting on them by
design to be grounding electrode conductors! Pretty foolish but heh, if
someone tosses feedlines out a window, they may as well short them to a
ground rod and "bring it on". In that case any more than about 5,000v will
breakdown the dialectric both inside and outside the coax, and anything
nearby may be the next target before it ever reaches the ground rod.


It doesn't matter what you want to call a ground conductor. The point
here is if it can carry any lightning current you are much better off
with it buried in the ground. A bare ground conductor making contact
with the soil acts like additional ground rods. Why would you not want
that?

Burying coax feed lines will help with lightning protection. It
greatly increases the inductance of the lines to lightning. It also
helps to dissipate the energy to ground by the coupling provided. (ie
you get less at the other end)

You can't help but view them as "grounding electrode conductors" as
you may want to call them. After all they are connected to the tower.
They are going to carry lightning current if you want them to or not.
Might as well let them dissipate part of the energy to earth.

A large part of the lightning is RF. You have to treat it as such.

A good lightning ground also makes a very good antenna ground system.
( buried radial system) Think in those terms.

73
Gary K4FMX


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

Gary K4FMX

That is very good news, and it makes your job easier. But good soil or poor
soil, understanding what bonding provides is equally if not more important
than having a ground rod at all. To rest on the laurels of highly conductive
soil and ignore bonding, would be inviting disaster. Yes commercial tower
design does require many shield "bonds" along the height of towers, but as I
said, I applied a reasonable approach which the average short tower or
mast-only owner could and would be likely to comply with - bonding at the
top, bottom and station entrance. I suspect few go even that far.

You may or may not be interested in all the surge protection diatribe in my
website, but it's there because so many unfortunate souls were mislead in
this area. I do think you might benefit from it's coverage of what bonding
does to protect both you and your station, and it is a lot harder for most
to get a hold of then simple mast or tower grounding. It doesn't have to be.

http://members.cox.net/pc-usa/station/ground0.htm

73,
Jack Painter
Virginia Beach, VA