Sounds like good, solid advice. It is good to have multiple grounds at the
shack entry point. Of course, they should all be tied together with other
ground rods at the tower, phone and power entry points.
My shack entry point is an aluminum plate in the window with 4 gauge copper
cables at each end running to separate 8 foot ground rods. There are 8 coax
arrestors and 6 rotor/control cable arrestors, all Polyphaser, mounted on
this plate. All equipment inside the shack is connected to a Harger copper
bar which is connected to the plate via 1.5 inch copper braid. There is a
relay coax/power disconnect box in the shack that grounds all coax cables
from the plate and all power cords from the equipment when turned off.
So far, in the past 5 years, there has been no damage after 4 observed
direct strikes to the tower. Well, I have to modify that statement. Four
arrestors have been replaced. :)
There is a wealth of information about lightning protection on Polyphaser's
Look under Tech Info, Engineering Notes. A good place to start is PEN1016:
Ham Radio Station Protection.
I like to think of lightning as a sea of electrons, all looking for a place
to settle on the ground. There are so many in a bolt of lightning that not
all will come down the tower. They are all looking for the path of least
resistance. The idea is to "capture" as many as possible and take them to
ground. The remaining electrons will find other paths such as power lines,
telephone lines, surrounding trees, etc. I can imagine that the potential
reaching the arrestors may well be billions of volts. This means tens of
thousands of volts may end up in the shack, so the coax cables need to be
grounded where they connect to the equipment. If they are disconnected and
laid on the table or floor, there is the possibility that an arc will form
between the cable and the concrete slab. Needless to say, this arc can
easily ignite carpet-wood-plastic, etc.
[mailto:firstname.lastname@example.org] On Behalf Of Phil Camera
Sent: Wednesday, June 30, 2004 8:22 PM
Subject: Re: [TowerTalk] Am I Asking for Trouble?
Based on my understanding, the answer would be most probably yes. Is this
Polyphaser panel above ground? If there is not a very low
resistance/impedance connection from that to your SPG (single point ground),
then yes the surge energy will not all be shunted to ground by the arrestors
which are mounted above grade. My understanding, after much discussions
with Polyphaser and ICE folks, is the the best bet would be to drop the
cables down to the ground at the shack end, to your arrestors mounted right
at the SPG and then route them up to the entrance. My only concern is that
if this up route was long enough, then a nearby surge may be induced into
that length and end up in the equipment. Email or call Polyphaser or ICE
and ask their opinions; they are the experts. Phil KB9CRY
> 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
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