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

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.

On 21 Nov 2004 18:36:02 -0800, (Brian Kelly) wrote:

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.


Hi Brian,

It always seems to be a compromise when it comes to lightning
protection. It is very difficult to get everything just right.

I have a small crank up tower too. I use a heavy flexible wire that is
clamped to the top of the top section and the other end is clamped to
the top of the bottom section. When the tower is all the way up the
wire is fully extended. This provides some bonding of the tower
sections.
When the tower is cranked down this wire is of little use because it
has a large loop that hangs there. In the summer time when the tower
is cranked down I place a clamp at the bottom to clamp one leg of the
fixed section to a leg of the telescoping section. Not the best setup
but that is about all you can do with this type of tower.
You have it worse with a telescoping tube. No way to bond it when it
is down. All you can hope for is somewhat of a friction connection
when down.

By the way, I don't have near the ground rods installed that should be
either. But my lines all run under ground about 150 feet.

73
Gary K4FMX

"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.

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.

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

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

"Gary Schafer" wrote
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

Hi Gary, the coax feedlines are definitely NOT grounding electrode
conductors. Not only are they incapable of such by design and accordingly
not authorized as grounding conductors, but they could never remain
connected to sensitive equipment if it were so. Neither is the shielding on
coax sufficient to provide equipotential bonding so they are not allowable
bonding conductors either. If anyone wants to sacrifice their coax by not
properly shield-grounding and installing the appropriate number of coax
lightning arrestors (this means on the tower also) then they will turn them
into very ineffective grounding conductors. Burying might help then, but
only because you could guarantee a breakdown in the dialectric and where
safer to have that happen than underground. I understand many operators
allow this and simply toss them out the window, or ground them before a
storm, but there is no good reason for it. Proper installation can allow
them to remain connected to the equipment without sacrificing the coax or
the equipment. Burying coax does not prevent induction by either capacitive
or magnetic induction onto the shields of the coax from a nearby strike. If
coax were enclosed in metal conduit that was grounded at each end, there
would be protection from this. But proper installation of shield grounding
and surge suppression at both ends maintains safe levels of energy on the
feedline and allows its connection to sensitive equipment.Of course in rare
cases there is sufficient energy (such as a 200Ka+ return stroke current) to
overcome any level of protection. But protected stations will certainly fare
a lot better in those rare events than the unprotected ones.

73,
Jack Painter
Virginia Beach VA

On Mon, 22 Nov 2004 19:26:16 -0500, "Jack Painter"
wrote:

"Gary Schafer" wrote
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

Hi Gary, the coax feedlines are definitely NOT grounding electrode
conductors. Not only are they incapable of such by design and accordingly
not authorized as grounding conductors, but they could never remain
connected to sensitive equipment if it were so. Neither is the shielding on
coax sufficient to provide equipotential bonding so they are not allowable
bonding conductors either. If anyone wants to sacrifice their coax by not
properly shield-grounding and installing the appropriate number of coax
lightning arrestors (this means on the tower also) then they will turn them
into very ineffective grounding conductors. Burying might help then, but
only because you could guarantee a breakdown in the dialectric and where
safer to have that happen than underground. I understand many operators
allow this and simply toss them out the window, or ground them before a
storm, but there is no good reason for it. Proper installation can allow
them to remain connected to the equipment without sacrificing the coax or
the equipment. Burying coax does not prevent induction by either capacitive
or magnetic induction onto the shields of the coax from a nearby strike. If
coax were enclosed in metal conduit that was grounded at each end, there
would be protection from this. But proper installation of shield grounding
and surge suppression at both ends maintains safe levels of energy on the
feedline and allows its connection to sensitive equipment.Of course in rare
cases there is sufficient energy (such as a 200Ka+ return stroke current) to
overcome any level of protection. But protected stations will certainly fare
a lot better in those rare events than the unprotected ones.

73,
Jack Painter
Virginia Beach VA



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.

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.

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?


If you use a radial system for a ground at the tower or several ground
rods, the coax run under ground can do the same thing as a radial as
far as dissipating part of the energy. Having a buried radial rather
than one run in the air lets the ground soak up a lot more energy if
it is buried. There will be much less energy at the far end of a
buried radial than one run in the air. A radial run in the air will
dissipate little energy to the ground.

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.

This is the same reason that long radials are not as effective as more
shorter ones in dissipating lightning energy. The inductance of the
long wire gets too high and becomes less effective as a conductor.

If you don't think that buried cables helps reduce lightning energy at
the other end try running a single insulated feed wire for your long
wire antenna underground. See how much attenuation it provides to the
signals.
Burying the coax does the same thing for part of the lightning energy.

73
Gary K4FMX

"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

Just a few thoughts on the subject.

Lightning conductors are most accurately modelled as DC to HF transmission
lines - which indeed is what they are.

In addition to resistance they posses inductance, capacitance, a Zo and
propagation constants depending on length and diameter. It takes time for a
stroke to propagate down and along a set of conductors. It arrives at
different times at different places in the system.

The generator is a high impedance, pulsed current source of so many
thousands of amps. The voltage developed between a conductor, another
conductor, and what's in its environment is Zo times the stroke current.
Volts can leap across gaps.

Once in the ground current travels at a much slower velocity than along a
wire. Voltages developed depend on arrival times at different places. A
ground rod is a short length of line.

Frequencies of 100's of kilohertz are involved. Even reflected volts and
currents occur. Ground conductivity can be allowed for.

Very crude approximations are involved. Nevertheless, any information about
behaviour DURING a strike is better than none. It may be a matter of life
and death.

It would be interesting to calculate, for a given strike current, the
difference in voltage between the front and rear legs of a cow standing near
to and facing a grounded antenna mast.

Radio hams, presumably endowed with more common sense, can always wear
rubber boots while walking around their backyards carrying a field-strength
meter in thunder storms.
----
Reg, G4FGQ