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Author Topic: MARS Exercise the Week of 8 November 2015  (Read 45967 times)
W6EM
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Posts: 1706




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« Reply #30 on: December 13, 2015, 06:53:23 PM »

Electromagnetic shielding derives from Maxwell's equations which proves that the electrical field over a closed surface is a function of the charge and that the magnetic field over a closed surface sums to zero.
Not sure where you imagined that a magnetice field sums to zero over a closed conductive surface.  Definitely NOT true according to Maxwell.

If, however, a high-permiability magnetic material is thick enough that with a given magnetic field intensity it will not saturate, then most of the lines of flux will be enclosed inside the material.  However, most of the time for very long duration pulses, as CMEs are, not so.  Especially with aluminum foil or copper screen.  Fairly easy to achieve attenuation of the B field with very short duration nano second pulses.

I can cite a couple of simple experimental examples if necessary.  One of the simplest would be a permanent magnet held on one side of a piece of copper screen or aluminum foil and a nail on the other side.......  Did the field stop or penetrate?  Or, a "shielded" isolation transformer where a metal foil shield is placed between windings....
Quote
Here a sample manual for all those who want to know details.
http://www.wbdg.org/ccb/FEDMIL/std188_125_1.pdf

Especially see 4.3.1 following.

73
That is a HEMP guide, not a CME guide.  Not the same animal.  Useful treatise on POE protection, though.  MOVs don't work all that well on coax, though.  Too much capacitance.

Here are a couple of very concerning HEMP documents: http://tinyurl.com/okneocg  

http://tinyurl.com/76eqmyq    Both are worthwhile reading.  About the only thing I can think of that will protect from both HEMP and CME would be a large safe.......with the door closed.

« Last Edit: December 14, 2015, 07:40:39 AM by W6EM » Logged
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