I'm sure someone here has posited an explanation for the top and bottom
ground connections of coax cable to towers. I don't recall that explanation,
so I took out my copy of Polyphaser's Lightning Protections and Grounding
Solutions for Communications Sites, written by Ken Rand, to look for an answer.
I used this reference heavily when I was designing my tower and grounding
system in 2001.
On page 3 of the booklet, Rand writes that tall towers (>150 feet) ought
to have their coax cable shields grounded to the tower at intervals of not less
thatn 100 feet, to minimize the likelihood of a sideflash from the shield to
the tower during a strike along the coax's length. This side flash, according
to Rand, could result in pinholes in the jacket allowing water to infiltrate
the coax system. Hence the conclusion that we Hams ought to connect our coaxes
to the tower top at a minimum. (Some hand-waving here, I'll admit.)
Also according to Rand, the connection at the bottom of the tower is made
to minimize the magnitude of voltage that is taken away from the tower towards
the equipment shelter/ham shack. That is, if you don't connect the shield to
the bottom of the tower, close to earth ground and zero potential, you could
bring significant lightning energy into the shack on the coax shield. (The
booklet tries to explain the physics involved here.)
I have my Polyphasers connected at the bottom of the tower, rather than at
the shack entrance, because I have an antenna switch located at the tower base.
If I had the Polyphasers back at the shack, I would expose the switch innards
to any lightning energy that might find its way along the center conductor of
the coax coming down from my antennas. So I chose to shunt this energy to
ground/earth outside the switch by mounting the Poly's in the side of the
grounded metal cabinet in which my switch is located. I also wanted to take
advantage of the better (IMHO) ground connection that I have at the tower base
(three buried radial wires and ground rods) rather than at the shack
entrance/SPG, where there is only one ground wire and rod.
BOTTOM LINE: Adverse consequences of not so grounding the coax shields are
possible coax jacket damage and high voltages brought into the shack.
Gene Smar AD3F
-------------- Original message ----------------------
From: "Jerry Keller" <firstname.lastname@example.org>
> In his well-stated list of grounding advice, Gene AD3F includes: "3. You
> ought to ground each coax cable shield to the tower at the top and at the
> bottom of the tower."
> I've seen this advice before, but have never understood the reason for it,
> or what the adverse consequences might be if everything else Gene recommends
> is done, and this is not (as is the case with my system). Can anyone
> explain this for me, please?
> 73, Jerry K3BZ
> ----- Original Message -----
> From: <email@example.com>
> To: "Richard W. Solomon" <firstname.lastname@example.org>; <email@example.com>
> Sent: Wednesday, January 03, 2007 11:52 PM
> Subject: Re: [TowerTalk] Antenna/Tower Grounding (Lightning Protection)
> > Dick:
> > Below are some nuggets of wisdom I've gathered from reading the
> > postings on this reflector over the years.
> > 1. You cannot protect the tower, antennas and rigs from a direct
> > lightning strike. You can only attempt to minimize the damage if that
> > happens.
> > 2. Your tower ought to have at least one ground wire, minimum of #8
> > gauge wire, to one ground rod within two feet of each tower leg. The
> > latest version of the National Electrical Code mandates two ground rods
> > and two wires at each leg.
> > 3. You ought to ground each coax cable shield to the tower at the top
> > and at the bottom of the tower.
> > 4. You ought to install lightning suppressors on all wires and cables
> > coming into your shack. Polyphaser makes these as do ICE and Delta. <All
> > cables> includes the coax runs, your control lines to your rotator and
> > antenna switch(es), and any DC or AC supply lines going up the tower.
> > 5. You ought to install these suppressors immediately outside your
> > shack entrance onto a single-point ground (SPG) panel. This panel ought
> > to be connected to another ground rod via another #8 minimum ground wire.
> > The ground connections from all the electrical systems within your house
> > (electrical supply panel, cable TV, telephone, etc.) should also be
> > connected to this SPG panel.
> > 6. You ought to search the TowerTalk archives for details on how to do
> > all of these things.
> > 7. When you have finished reading all you can stand, come back to this
> > reflector again and ask some more questions. We're only too glad to help.
> > What did I miss mentioning, gents?
> > 73 de
> > Gene Smar AD3F
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