WB9UWA Jim Shaffer wrote:
> Hi Pete,
> Why fight the dragon if you can persuade him to take another path?
Why
> is the lightening visiting your rotor contol box anyway? Could it be
> that tastey AC power line ground at the other end of your AC line
> cord?
May be, lightning does very strange things I've noticed.
I would of course use MOVs at the rotators themselves and again at
the base of the tower but at the master ground bar at the service
entrance, I'd rather have something as safe as DC blocked Polyphasers
to keep the overall common mode surge on all cables coming in to the
house to the absolute minimum.
> My tower base is only 15 feet from the underground AC power feed to
> the house. Since I added a heavy ground lead from my tower base to
the
> AC power feed ground, my VHF shack no longer suffers from lightening
> hits. Of course I still make sure the 90 foot tower is grounded with
> local lightening rods as well.
Well, I put a big order into Harger today, shipping by truck soon I
hope. Enough ground rods, copper strap, wire, clamps, and Polyphaser
stuff (including an AC unit) to keep 40KA at bay, at least on the
power and coax lines.
The plan is to tie each leg of the tower (to be installed this
summer) via 1.5" copper strap to a ground rod at the perimeter of the
foundation (Trylon Titan) along with six more in a hex pattern spaced
roughly 16' out from center. Polyphaser strap-to-rod clamps are used
to make the strap connections from these three rods out to the other
six in a three-by-two star pattern with #16 bare solid Cu radial run
out from each of the inner three rods for use with loading the tower
as a vertical. The three rods at the foundation would be connected
to each other as well with strap in a ring. Still debating adding a
Ufer ground to the foundation.
A NEMA 4X box at the house service entrance, with a copper plate
inside, will hold up to six IS-50UX-C0/C1 suppressors as well as six
Harger #216 bonding taps for the #6 pigtails from the Times ground
kits on all incoming coax feeds (DavisRF BuryFlex). The plate will
be taken to earth via six 1.5" copper straps over a two foot drop
down the wall to be clamped via Polyphaser strap-to-strap clamps to
the strap coming from the tower ground field. This buried strap from
the ground field goes a couple of feet more to the ground rod at the
service entrance where it's attached with a Polyphaser strap-to-rod
clamp. This rod is then bonded to the service entrance ground with
#6 wire.
Another #6 wire is connected to this rod and then buried around to
the front of the house where it ties to another ground rod at the
bedroom window where the shack is. This ground rod has a number of
radials (#16 bare solid Cu) attached with a SS hose clamp (to make an
effective RF ground) and another strap-to-rod clamp attaches a piece
of 1.5" strap less than 2.5' long that slips under the window
(aluminum) and clamps to the station's ground window (copper plate).
Coax feeds from the NEMA box to the ground window are through the
attic (passed through grounded 2" EMT conduit up from the NEMA box
through the soffit as an electrostatic shield) and have ferrite
toroids on them as common mode suppressors to inhibit RF ground
loops. They terminate to the common port of Diamond coax switches,
one port of which is terminated with a homemade 47ohm 3W ceramic
resistor in a PL-259, the other port of which is pigtailed (with
ferrite beads) to a rig. The rigs are tied to the ground window with
1/2" braid.
Turning the coax switches to put the termination on the coax feed
disconnects the rigs and shorts their inputs to the ground window
while providing the requisite termination for the Polyphasers to work
correctly. The 20A AC Polyphaser suppressor is on the ground window
as well, BTW.
That about sums it up except that it doesn't address rotator and
control lines. These could only be shunted to earth at the master
ground bar in the NEMA box and again at the ground window but MOVs
fail open. Then the surge can bypass the entire shebang and whack
the shack, as it were.
The alternative is to disconnect the rotator cable and short all
lines to the ground window. This doesn't suppress the surge before
it gets in the house though and the common mode potential from that
could easily get by the ground window to cause at least incipient
damage.
There's a way to add series protection in a homemade suppressor but,
like I said, it's very difficult to test. Too bad there's nothing
out there off the shelf.
> Keep all leads at the same potential. That includes, feedlines, AC
> power ground, and anything else connected to your expensive gear.
Just
> leave the gas pipes alone. Dragons and gas pipes don't mix.
Yep, I'll go with that, no argument there. The problem I see is that
failure of shunt protection allows the surge potential to crawl down
the rotator cable through the attic in parallel with the coax feeds.
Not a good thing with the furnace's gas line up there...
> 73, Jim Shaffer, WB9UWA.
Thanks very much and 73,
Pete, AD5HD
> ----- Original Message -----
> From: Pete Goudreau <goudpj@mac.com>
> To: <towertalk@contesting.com>
> Sent: Wednesday, March 20, 2002 9:31 PM
> Subject: [Towertalk] rotator cable lightning protection
>> After some research, including towertalk archive searches, it looks
>> like the only protection units out there for control and rotator
>> cables are shunt types - Polyphaser and I.C.E. come to mind. While
>> researching the topic, there also seems to be a goodly number of
>> stories of lightning surges getting past exploding MOVs in said
>> protectors and blowing up most of the equipment in the shack.
>> Not being much of a fan of MOVs in the first place, I figured there
>> had to be some series type protectors out there somewhere but I'm
not
>> finding them at all. Design is relatively easy but extremely
>> difficult to test correctly.
>> Anybody have any suggestions as to the safest way to keep the lines
>> connected but well suppressed?
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