I don't entirely agree with this explanation, although part of what
you're recommending is excellent practice.
First, lightning is an RF event, NOT a DC event. IEEE studies have
shown that the energy in lightning typically has a VERY broad peak
centered around 1 MHz, with most of energy between about 100 kHz and 10
MHz. By virtue of its electrical wavelength, nothing on that tower is at
the potential of the earth at RF.
Second, when we connect a coax shield to the tower at top and bottom, we
are BONDING it to the tower. That's a VERY good thing to do, and we do
that to keep the coax at the same potential as the tower so that a
lightning strike won't cause arcing on the inside of the coax. I've
seen some 7/8-in hard line where this happened. :)
Third, I don't think Mother Nature would buy your concept of a
"boundary," but she is usually looking for a path to earth, so taking a
coax shield to earth sooner rather than later is a good thing. BUT --
ALL grounds in any premises MUST be bonded together for lightning
safety. The intent is that their potential all rise equally with respect
to earth.
Fourth, lighting follows the path of lowest impedance, which at RF is
virtually all due to the reactance of the path, not its resistance.
Fifth, no matter what we do, lighting will induce current on any loops
it finds, so all the grounds we carefully bond together aren't exactly
at the same potential. :)
Sixth, a lightning protector that we add to an antenna feedline where it
enters our shack contains a gas discharge tube that shorts the center of
the coax to the shield, which minimizes the voltage at the input to our
radio. That coax shield must also go to earth, and the point where it
does must be bonded to all other grounds -- shack, power, CATV, telco, etc.
There's a lot more about this in the Power Point for a talk I did at
Pacificon a few years ago.
http://k9yc.com/GroundingAndAudio.pdf
73, Jim K9YC
On Fri,11/4/2016 6:08 AM, Earl Morse wrote:
Think of your grounding points as boundaries. If you cross a boundary then you
have to do something to the cable such as ground the shields or use
Polyphasors. Typical boundaries would be top of tower, base of tower, and
entrance to shack. Typical things that are done would be to ground shields at
top of tower, bottom of tower, and at shack entrance where any lightning
protection (Polyphasor) would also be located.
In your case you may want to a boundary at the coax switch half way up the
tower and ground all the shields there too.
As for your case, I would ground the shield of the OCF at the base of the tower
where it passed through that boundary. You could avoid that by running the
coax straight to the switch halfway up the tower instead of routing it back to
ground and then back up the tower.
The point is that you work hard to set up these "boundaries" to bleed off a
lightning strike but if you bypass any of them you give a sneak path that lightning may
use to bypass them.
Earl
N8SS
----------------------------------------------------------------------
Message: 1
Date: Thu, 3 Nov 2016 22:58:48 -0400 (EDT)
From: "jcjacobsen@q.com" <jcjacobsen@q.com>
To: towertalk@contesting.com
Subject: [TowerTalk] Feed line grounding
Message-ID:
<744847813.2686980.1478228328356.JavaMail.root@md47.quartz.synacor.com>
Content-Type: text/plain; charset=utf-8
I haven't found anything addressing this, so I give to the collective group:
All feed line shields are grounded at top of tower. A feed line switch is
located about half way up.
The main feed line shield is grounded at tower bottom. Now for the question:
Have an OCF dipole
supported a distance away from the tower from a tree. It's feed line drops
down, runs on the
ground to the tower base and then UP the tower to the switch. Would it be wise
to ground the shield
of this coax to the tower base. OR is this unnecessary?
Thanks in advance.
73 K9WN Jake
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