That's the way my lightning protection is configured.
The tower has it's own ground system consisting of a Ufer that bonds
together the rebar, tower legs and 50' radials from each leg. 8' ground rods
are spaced along each radial at 16' intervals, cadwelded to the radial wire.
There's Hoffman metal utility box at the base of the tower with an aluminum
ground plate in the back on which are mounted K5FD double MOV and fused
lightning suppressors (enough for about 100 leads.) Coax from the tower
enters via Polyphaser coaxial suppressors bulkhead mounted in the base of
the metal box. The box, of course, is bonded to the tower ground system.
Cables exit the box and go into the ground about two feet deep (direct bury)
and run about 225 feet to the shack. There, they emerge into an identical
Hoffman metal utility box with a set of MOV and coaxial suppressors,
pretty-much the same as what's at the tower. That utility box is bonded to
the SPG, which is in turn bonded to the main power and telco panels,
satellite TV, etc. Cables enter the shack through the back of the metal box,
where they connect to a large patch panel that makes it possible to
disconnect the shack equipment from the outside lines (I would have put the
patch panel outside, but it wasn't practical.) There's also a
quick-disconnect panel to remove AC power from the shack.
The suppressors at the shack end are critical for protecting the equipment.
The suppressors at the tower are there to protect the cables and some of the
switching equipment mounted there. All coaxial cables running up the tower
are grounded at the top and bottom (well, almost all of them -- still have a
couple left to do :-)
With so many suppressors, why the patch panel? Well, some of the equipment I
use, like SteppIR controllers and Green Heron rotor controllers, have
delicate semiconductors attached directly to leads running to the tower. I
found out the hard way that suppressors really can't protect those chips (a
long story I've told here.)
73, Dick WC1M
-----Original Message-----
From: EZ Rhino [mailto:ezrhino@fastmovers.biz]
Sent: Saturday, September 11, 2010 1:19 PM
To: Jim Brown
Cc: towertalk@contesting.com
Subject: Re: [TowerTalk] Lightning Protection Question
What if you have arrestors at the tower AND the SPG at the shack?
Chris
KF7P
On Sep 11, 2010, at 9:53 , Jim Brown wrote:
On Sat, 11 Sep 2010 10:57:06 -0400, Gene Smar wrote:
> It depends. I have my Polyphasers at the NEMA box at the tower base,
> but I also have a remote coax switch there, too.
I disagree (and agree with K1TTT) -- lightning protection devices are
protecting you radios by short the voltage on the center conductor to
the shield. They cannot do that at the tower -- voltage can be induced
on the coax between the tower and the shack, and a protector at the
tower is too far away to offer much protection at RF. The best place for
them is at the entry panel to the shack.
The SHIELDS of coax SHOULD be bonded at top and bottom of the tower.
73, Jim K9YC
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