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Re: [TowerTalk] Fwd: Shielded shack

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Fwd: Shielded shack
From: Jim Lux <jimlux@earthlink.net>
Date: Sat, 28 Sep 2013 11:33:26 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 9/28/13 10:27 AM, 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!


There's no particular advantage to a solid sheet in this application. You're not really making a shield for radiated fields, are you?

One vulnerability is transients coming in on the inside of the coax, although presumably you can clamp those with a gas tube or similar type over voltage device.

common point referencing will take care of the common mode voltage issues (e.g. if the case of the box goes up to 10kV, but so does the case of the box next to it, there's no problem)




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.


Then what you want is making sure everything is referenced to the same point.

If you are concerned about the radiated field, then shielding might make a difference. As a practical matter, most of lightning energy is 1MHz and down, so you're in the "near field" of the radiator. I think most of the energy will be in the magnetic field, not the electric field, so you want good magnetic shielding. Even more important is reducing the victim loop area. No plugging in box A on one side of the shack and Box B on the other side, making a big receiving loop for the transient.

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.


Actually, your plate isn't shielding, but it is providing a good low impedance common reference point for chassis and circuit potentials; you won't have chassis ground of box A being significantly different from chassis ground of box B. And that's a big part of the game.



Grounding of the foil will be via a conductor connected to the tower. Hope that 
is not against any code.



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