"Gary Schafer" wrote

Jack, my apologies. You did not suggest a #6 down conductor on such a
tower as I reread your original post. But you did suggest a down
conductor on the tower, you just didn't mention the size.

O.k. Gary. And I didn't mean to suggest a down conductor on a tower either,
as its true there is little evidence that it carries more than 25-30% of the
total energy with the tower carrying the balance. But it sure is specified
in a LOT of places. I follow specifications to the letter unless there is
clear evidence that in my particular facility it could be harmful or
counterproductive.

DOWN CONDUCTOR ON A BUILDING:
A down conductor on a building is a whole different deal than on a
tower. On a building the "metal stairways, handrails, roof flashing,
etc." are bonded to prevent flashovers as the down conductor runs by.
The down conductor is run because there is no better path to connect
to.

DOWN CONDUCTOR ON A TOWER:
My comments about a down conductor on a tower were based around the
fact that a down conductor on such a tower is useless. Tower joints
should be bonded to negate any resistance in the joints but a down
conductor the length of the tower is a waste of wire. I was trying to
point out that any down conductor, coax cables or anything else
running down the tower, including the tower, would all share the
lightning energy. It is impossible to isolate any part.

The odd system or two specifies isolated tower down conductors. A majority
proscribe down conductors and bonding the entire length, and another small
percentage use an air terminal but bond it directly to to the tower legs and
eliminate the down conductor. Please don't use PolyPhaser's toy lightning
machine antics as evidence there. Big industry and government spend millions
bringing rocket-triggered lightning down towers all the time, and if down
conductors are still being specified they must help. How much, and if it
applies to a typical ham tower, let the user decide I guess.


The fact that a typical tower has so much mass, and as a result much
lower inductance, in comparison to any down conductor that you could
hang on the tower negates it's usefulness.

And yes, I know that some people do install down conductors on towers.
Some people also hang pointy dissipation arrays on the top of their
tower too.

Some engineers and "communications managers" specify them also. Hams
don't have a lock on ignorance in this department.

That's true, but maybe 25-30% of the energy directed on a faster path via
down conductor means something, I have not seen why. The ESE Dissipaters
however - another ball of ceiling wax and snake oil. Throughly disproven yet
to this minute it ties up the Lightning Standards Group of the IEEE while
they fight over the language to discredit it with. There is also at least
one insurance company that discounts for use of the CTS (Charge Transfer
System). There is no reliable science behind this theory but it hangs on...

Please don't take my comments so far out of context. But you're right it
is
reserved, its reserved for those who pay the fees and time to subscribe
to
organizations that license the printing of the codes, and constantly
discuss
and explain changes, applications and plan future requirement for them.

I would guess then that the general library is also "reserved".

A good library might have at least the NEC-70 (Electrical code). You can buy
the NFPA-780 Standard for the Installation of Lightning Protection Systems
for about $40 from NFPA directly. The October 2004 edition is the latest.
The full NEC-70 is a lot more.

Sometimes it is difficult to tell what you are discussing as you want
to throw so many things into the fire.

Roger that, sorry. My brain is awash with this stuff but it has been the
most productive, interesting and generally most fun project I have taken on
in years.

I thought we were discussing lightning strike dissipation and
preventing it from reaching the shack. How can you ignore the AC
components? They are a major part of it.

I think we were trying to answer a posters question about grounding the coax
shield before it meets an arrestor, yes.

BURIED COAX:
The relevance here was explained in my first post on the subject.
Let's try again. I asked if you thought that if you buried the feed
wire from your long wire antenna, it can be insulated if you choose
but a single wire, if you would get as much signal from your antenna
to your receiver as you would if that same feed wire was not buried
but run in the air away from ground. What do you think would happen to
the RF on that feed wire? Do you think that it would go unattenuated
the same as it would if the wire was in the air? Or would you loose
most of the signal if it were buried?

This is relevant to burying the coax lines coming off a tower and
leading into your shack. The coax shield will be carrying large
amounts of lightning energy during a strike that it receives when the
top of the tower is struck. Even though the coax is grounded at the
base of the tower. since there is no perfect ground system you are not
going to be able to dump all the energy to the tower ground.

For the moment forget about possible induced currents into the cable
itself from nearby strikes etc.

Lets come back to that then, because it would require DEEP burying to avoid
(It is commonly avoided by shield grounding, arrestors, grounded steel
conduit, geometry, and distance). Anyway, yes, there would be some
attenuation from a cheap wire insulator that was not designed to prevent RF
from travel in either direction. But 95% or greater coax shielding with its
dialectric inner liner is designed to keep RF from penetrating. While it
allows travel on its inner and outer shield covers during some oddities of
RF behavior, it does not allow attenuation of the RF signal by design. Then
lets agree it's designed to be 50ohm, or 60ohm, if you want to use the anal
British system (which we know is actually correct there Limeys). So if the
point you were making was just that some inductance from earth is felt (even
on coax) - I can't disagree, but neither would this wildly high impedance to
even wet earth impress lightning in the least bit. In dry soil it would be
invisible for all practical purposes. After all we stress the need to ground
(via 8-10' of copper electrode) the coax shield (and the center conductor
via arrestors) so what good is insulated earth? That was my point, and
unless you count on unprotected coax breaking-down and arcing into the earth
along its length, the burying part is just a no-gain event (just like a
tower downcomductor???) Substantially all of the energy from a tower or
antenna strike is already on the coax and it's first arrestor, top-shield
bond, (middle shield bond if you like) and tower bottom shield ground (and
another arrestor if you like) at the same time. Its even on the GROUND at
the same time. They all rise in near unison within about 5 microseconds and
fall in potential within 20-40 micro seconds. Where damage usually occurs
is when unprotected coax has a distant ground only, and the voltage remains
high its entire length as a result. This is where I believe you might tempt
the voltage out anywhere along a buried path, maybe in thousands of little
holes. It could just as easily happen laying on the ground though, so I fail
to see what surrounding it in dirt can do. Apparently so do engineers who
specify controls and installation procedures. That doesn't mean your
argument can't be a good one. I just don't see it as that yet.

73,
Jack Painter
Virginia Beach VA