You are describing a configuration that is similar, but not identical,
to commercial tower installations. In these installations, the
coax/waveguide leaves the tower above ground and enters the equipment
shelter above ground via a cable tray and grounding bulkhead. On the
bulkhead are mounted the requisite lightning suppressors along with one or
two wide pieces of copper flashing. The flashing is bonded to the outside
ground ring that (typically) circles the shelter and extends outside to
another ring that encircles the fenced-in area of the installation.
You didn't mention whether you have grounded your cable runs anywhere
before the Polyphaser panel outside your shack. At a minimum you ought to
ensure that the shields of your coax runs are grounded at the top of the
tower and where they jump off to attach to the messinger wire (you called it
a tag line). Then make sure your Polyphaser panel is connected to an
adequate ground system; my tummy tells me that a single ground rod outside
your shack will be inadequate for this purpose.
Your (unshielded?) control wires might be another can of worms. IME
(In my estimation) this is the most vulnerable segment of your cable system
due the exposure the unshielded (?) conductors have to lightning. If a bolt
hits the horizontal segment of the cable, its energy could travel in both
directions, damaging the rotator innards. Hopefully your Polyphaser
suppressor on the control lines at the shack end will minimize passthrough
energy into the shack. You might want to consider using shielded cable
here, or installing a second Polyphaser suppressor on the control lines at
the tower (in a protective enclosure.)
Allowing the cables to leave the tower at that elevation above ground
is not the preferred Amateur method. Polyphaser advises that all such
cables should be run to ground level and bonded to the tower at that point,
thus minimizing the potential difference between the shield(s) and ground.
As it's constructed now, you tower installation could potentially (pun
intended) couple (25'/114') of the lightning-induced voltage of the tower
onto your cable shields.
BOTTOM LINE: Make sure you ground your coax runs at the top and bottom of
the tower before they hop onto the tag line, and rethink the control cable
installation a bit.
Gene Smar AD3F
----- Original Message -----
From: "ab5mm" <firstname.lastname@example.org>
Sent: Wednesday, June 30, 2004 6:43 PM
Subject: [TowerTalk] Am I Asking for Trouble?
> One of my towers is a 114' Rohn 25 with the usual antennas on top. All
> the control wires and coax come from the top down to the 25' level. A
> tag line of 1/4" steel cable departs the tower at 25' elevation and runs
> 55' over to the shack. This cable end is attached to the wooden
> structure via a steel plate bolted to the fascia board with 2x8 backing.
> The coax and control wires are suspended from this cable and enter the
> shack via a PolyPhaser panel. The tower is grounded well, as are the guy
> post and guy wires.
> Have I just invited Mother Nature over to burn my shack down with this
> tag/support line?
> Thanks for any input & 73,
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
> TowerTalk mailing list
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
TowerTalk mailing list