On 9/28/2013 1:27 PM, Hans Hammarquist wrote:
The idea is not to let any current through the aluminum foil. ALL connections
to the foil is in ONE point, the point there everything is entering the shack.
I do not believe tht the foil would protect the shack from a direct lightning
strike. What is the possibility that will happened anyhow with a 85 foot tower
next to it. OK, never say never!
This is where the Single Point Ground (SPG) is important and that every
lead coming into the shack is protected. Power, if multiple circuits
needs to follow the sane routes for their entire lengths. Use a
relatively large, grounded, "bulkhead" through which all leads must
pass. Grounding and bypassing all leads is to insure the induced
voltage from nearby strikes causes the voltage on all lines to rise at
the same rate and to the same potential. Your common grounding sounds good.
My idea is to protect the gears inside the shack when lightning strikes the
tower. I don't believe the current going through the foil will be that high. It
is determined by the capacitance between the shack and the ground, and the rise
time of the voltage, right.
Lightening has punched holes (sometimes large) through Airplanes as they
happened to be "in the way. I don't think foil on a grounded structure
would offer much protection.
A good grounding system and a SPG at the shack entrance are the
important points.
Presently, I have an aluminum plate under my radio equipment to which everything is
grounded. The plate acts like a "half" Faraday shield, and I believe that
arrangement saved me a couple of times. (I have had three (3) know direct strikes in my
house and have learned the hard way how to protect electronics; telephones, stereos,
etc.) I have not lost any ham equipment yet.
Grounding of the foil will be via a conductor connected to the tower. Hope that
is not against any code.
Any connection to the foil would need to cover a relatively large area
such as an Aluminum plate of several square feet with the connector on it.
The first 6 years after installing the tower and ground system, it took
an average of 3 visually verified hits per year for a total of 17 as
reported by neighbors. The tower is a 100' ROHN 45G with a 30' DOM
steel mast holding a TH5, 7L 6-meter C3i with a close to 30' boom and a
14' cross boom on top at 130 feet holding a pair of 11L 440s inboard of
a pair of 12L 144s. That was way too much above the top of the tower.
After seeing them whipping as much as 30-40 degrees in a strong wind,
I'm amazed the top antennas held together.
http://www.rogerhalstead.com/ham_files/Tower29.htm and
http://www.rogerhalstead.com/ham_files/Tower26.htm
Right now all the antennas are down for maint and satellite dishes are gone.
Plus I'm installing an LM470 crank up in the place of the small tower at
the end of the shop. These are all tied together with over 600 feet of
bare #2 with 32 or 32 ground rods CadWelded to it.
73,
Roger (K8RI)
73 de,
Hans - N2JFS
-----Original Message-----
From: Roger (K8RI) on TT <K8RI-on-TowerTalk@tm.net>
To: towertalk <towertalk@contesting.com>
Sent: Sat, Sep 28, 2013 11:56 am
Subject: Re: [TowerTalk] Shielded shack
On 9/28/2013 1:51 AM, Grant Saviers wrote:
If it is lightning protection and not RFI shielding that is desired,
then what is done for explosive magazines ought to be enough!
http://www.seftim.fr/publications/DDESB/17-3-LIGHTNING_PROTECTION_FOR_CRITICAL_EXPLOSIVES_OPERATIONS.pdf
has many references, but there are older documents I've seen in prior
google searches. e.g. http://www.apd.army.mil/pdffiles/p385_64.pdf
Or standards for protection of petroleum storage facilities.
Remember you are building a cube and all 6 sides need to be covered.
IE: It's just a stick built room covered with a grid created out of
rerod for lightening protection and a LOT CHEAPER than using copper
unless you don't have a welder and have to hire that done. That is a
lot of rerod.<:-))
I'd pitch the foil idea. Hard to bond reliably, fragile, and very
little current capacity before vaporizing and setting the "shack" on fire.
I would add, that if the tower or shack ever takes a direct hit, your
ears will likely ring for an hour or so. Course, good insulation is
also good sound proofing.
73
Roger (K8R)
I would also think that foil is a bad choice as a conductor should you
have a strike when #2 or larger is what is commonly used in large
structures frequently hit.
On 9/27/2013 8:05 PM, Hans Hammarquist wrote:
Gentlemen,
I am on the process to build myself a (small) free-standing shack
next to my tower. I am thinking (which usually is dangerous) that I
should "dress" my shack in aluminum foil, making it into a large
Faraday Cage/shielded room.
By connecting the foil to the tower and make sure all wires entering
my shack enters through a (relatively) small window I should be able
to make the ultimate lightning protection.
What can go wrong with this? The foil will be connected through-out,
walls, floor, ceiling. All the feed-through connectors, surge
protectors etc will also the connect to the foil. I will using a wide
aluminum strip to connect the tower to the foil. Yes, the tower has
nine (9) gounding rods, so I believe that is OK.
Any feedback?
73 de,
Hans - N2JFS
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