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