On Mon, 22 Nov 2004 22:42:20 -0500, "Jack Painter"
wrote:

"Gary Schafer" wrote
What isolates the shield of the coax from carrying current?
As long as it is connected to the tower at one end it is going to have
strike current on it whether you want it there or not. Nothing you can
do about it. Paralleling other conductors will reduce it's total
current but you still have to deal with it on the coax line.

Incorrect. First, a strike termination device is placed higher than other
equipment with its own down conductor. Then a lightning arrestor and shield
bonding are specified at the top of the tower, shield bonding along the path
(up to three times) and at the bottom, then more shield grounding and
another lightning arrestor at the facility entrance.

If you don't want to call the coax shield a grounding conductor that's
ok but that won't stop the current on it.

Current is maintained at a safe level on the coax center conductor and
shielding by the above.

Who told you that you should put lightning protectors at the tower as
well as at the building entrance? What good do you think they do at
the tower other than cost more money?

National telecommunication companies who specify them in white papers and
engineering plans for lightning protection. I have been studying these
systems for 18 months now and find this procedure consistently applied. The
specific information is proprietary but all I had to do was ask for it. I
found the information available via the USAF and other agencies I normally
deal with was somewhat old, so I started asking commercial companies what
they currently use, and could I have copies of their plans. That's where
this information comes from. That and the National Electrical Code and
National Fire Protection Association, October 2004 editions. Studying the
NEC 250 grounding and bonding and the NFPA-780 offers more information to
safely operate communication sequipment, especially during thunderstorms,
than all the amatuer radio operators advice put together. Most of the
amatuers giving this advice have no personal understanding of why or how
this works, they just repeat stories or instructions they heard from someone
else. Probably the biggest collection of dangerous information ever shared
is what hams offer about lightning protection. Even the ARRL which makes an
incredible effort to educate at the issue, has information so old in many
cases it has not been used in best available practice for over ten years.

With buried coax the ground acts like a large choke on the cable also.
Exactly what you want. The ground increases the cables natural
inductance.
/clipped

Your mistaken on this stuff Gary, we either shed or prevent lightning energy
from coax by shield grounding, surge protection devices and sometimes
encasement in grounded conduit. No plan or specification calls for
earth-burying coax to deliver what you promise, and I believe your theory is
electrically impossible, unless as I said over and over, the dialectric
breakdown occurs, which means the installation was improper in the first
place, or overcome by statistically rare events.

73,
Jack Painter
Virginia Beach VA


Jack,

I don't think you really understand all you are reading. It sounds
like you are digging up stuff designed to sell a lot of protection
devices.

As far as the NEC requiring grounding and bonding of structures, They
want to be sure that there is a continuous bond on things they are
concerned with. They don't always consider what they are grounding.

Do you really think that placing a down conductor of #6 wire on a
tower with a 6 or 8 foot face (big tower) is going to make any
difference in the impedance path that the lightning is going to see.
Each leg may be 2 or 3" in diameter itself. The impedance of the tower
will be so much lower than that wire. The lightning won't know the
difference whether that wire is there or not.

Even if the added down conductor did carry a large part of the current
it would get coupled to the tower anyway before it reached the bottom.
That is what happens to the coax lines in reverse. Any energy that the
tower is carrying is coupled to the coax lines whether they are
grounded to the tower or not. You ground them at multiple points to
prevent flashovers between the lines and the tower.

You can not keep the lightning energy off the coax lines or any other
lines coming down the tower. They are all mutual. When those lines
leave the tower at the bottom they are going to have some energy on
them unless you have a perfect ground at the bottom of the tower.

A grounded antenna will keep voltage levels on the center conductor at
a safe level. No need for a protection device at the top.

I agree that there is a lot of mis-information on lightning floating
around. But don't cut all the hams short either. Some of them have
lots of experience in this area.
Think about what you read rather than taking in mounds of propaganda
and repeating it.

Do you think that you can bury the feed wire for your long wire
antenna and have it work very well? What do you think will happen to
the RF on it? Will it make it all the way back to your receiver the
same as it would if it were above ground?

73
Gary K4FMX