Not to disagree with Bill because he's been a BIG help to a lot of us here
on this reflector (ME included) regarding station protection.
BUT...
I was confused about where to put my SPGP also ..My original design had it
right outside the shack in an enclosure
but I changed the design after reading the Ron and Roger Block QST
"Lightning Protection" series "What Your Mother Never told you!.
In that article there was this excerpt:
"...As we'll detail later on, the primary ground system is represented
by a set of copper-clad ground rods, interconnected below grade, with bare
copper radials. Also fundamental to a good protection scheme is the creation
of a single point ground WITHIN the ham shack. This single point ground will
be used to MOUNT ALL of the I/O protection equipment and to provide a ground
for all of the equipment cabinets at the station's operating position. This
INTERIOR single point ground is CONNECTED to the external ground system
(composed of those radials and ground rods) by the lowest impedance copper
strap that you can manage. The tower ground system outside and the single
point ground system inside must be solidly interconnected with a low
impedance metallic strap, so that your coax cable's shield is not the only
interconnection conductor between these two ground points. Keeping as much
of the lightning's energy off of the coax shield as possible is essential to
minimizing damage from a direct hit. For larger strikes, it's best to
incorporate a grounding kit prior to the protector, to save your expensive
coax connectors from arcing damage. An effective (good quality) coaxial
in-line protector can then be used to handle smaller strike currents that
may be tempted to travel down the cable itself."
After conferring with Ron over lunch (he lives close by) we decided it was
best to move the SPGP inside the shack
to allow easier bonding of all the protection components, including an AC
protector, to one plate. The plate was then
connected to the EXTERIOR ground system via 2 6" copper straps. The reason
for the two straps was to equal the total
cross section of cable(s) entering (11") with the ground leaving (12") to
attach to the exterior ground system.
This keeps all voltages rising and falling together with zero current flow.
My exterior ground system consists of 2" strap cadwelded to 4 legs of
radials off the tower consisting of
4 8' 5/8" ground rods 16' apart on each leg and also a perimeter ground
connected to the AC panel all tied
to the SPGP via the 2 6" straps.
I should mention that my tower is only 10 feet from the shack and
Electromagnetic Pulse protection is near impossible
given the proximity of the tower. Unless I wanted to encase my shack in
copper $$$$.
BTW...This whole system did not come cheap! But the idea is to keep the
energy in the ground field and most of it
outside the house. Also, Ron's SPGP is in his closet next to his station.
Joe - KC2TN
-----Original Message-----
From: towertalk-admin@contesting.com
[mailto:towertalk-admin@contesting.com]On Behalf Of Jon Ogden
Sent: Thursday, September 19, 2002 4:25 PM
To: Bill Hider (N3RR); Tower Talk (mail list)
Subject: Re: [Towertalk] Box forSPG and Lightning protectors
on 9/19/02 2:23 PM, Bill Hider (N3RR) at n3rr@erols.com wrote:
> As I read some of the comments/questions about covering the lightning
> protection boxes, I'm wondering if consideration was taken into account
for
> a Single Point Ground (SPG)??
>
> If the lightning protectors are just laying around on the ground,
presumably
> near the ground rod, and then coax or control cables are just run into the
> house without a SPG, what's wrong with this picture? If that's the
> scenario, you're missing a couple layers of protection.
But if the lightning protectors are all tied into the same ground rod, isn't
that the same as the SPG?
>
> First of all, a SPG box provides the common ground point ensuring no
> difference in potential even in the face of lightning.
> Secondly, a SPG box made of metal, rather than plastic, provides a Faraday
> shield for all wires passing through the metal SPG box. A plastic box
does
> not.
> Third, a steel box, rather than aluminum, provides a magnetic shield
against
> magnetic fields (produced by lightning strikes) inducing currents into
your
> cables then entering the house.
>
Yes, but the box only shields the stuff inside the box. What about all of
the coax that is run inside the house that is not shielded by any metal? I
fail to see how a metal box that encloses other metal boxes is going to do
much.
73,
Jon
NA9D
-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)
Life Member: ARRL, NRA
Member: AMSAT, DXCC
http://www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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