[HCARC] Faraday Cage

Tue Oct 2 08:17:54 EDT 2012

Is anyone else awed by the knowledge in this club? Good answer Kerry.

I remember my Elmer, Herman, W5FES, SK, being excited about a steel shed shack as another layer of tvi/rfi protection. Like Kerry said, everything would have to be filtered.
Sent from my BlackBerry® Smartphone by WCW

-----Original Message-----
From: "Gary and Arlene Johnson" <qltfnish at omniglobal.net>
Sender: hcarc-bounces at mailman.qth.net
Date: Mon, 1 Oct 2012 20:09:42 
To: Kerry Sandstrom<kerryk5ks at hughes.net>
Cc: <hcarc at mailman.qth.net>
Subject: Re: [HCARC] Faraday Cage

When I said Pulse weapon it was because that was all my poor meager mind 
could think of at the time.  I meant EMP burst above the US.  Just thinking 
since I was building the room anyway, I might just ask about it.  I do find 
it interesting that the news talks about it every once in a while and no one 
seems to ever do anything about it.  It's almost like someone is daring a 
country like Iran to try it.  But why would we encourage it - maybe just so 
we knew who had tried it and could be reasonably sure of our retaliatory 
target.

Gary J
N5BAA

Gary J
N5BAA

----- Original Message ----- 
From: "Kerry Sandstrom" <kerryk5ks at hughes.net>
To: "Gary and Arlene Johnson" <qltfnish at omniglobal.net>
Cc: "HCARC Reflector" <HCARC at mailman.qth.net>
Sent: Monday, October 01, 2012 7:51 PM
Subject: Re: [HCARC] Faraday Cage

> Gary,
>
> I've spent many years working in and on shielded enclosures.  There are 
> several reasons why it wouldn't work.  First of all, EMP and geomagnetic 
> storms are low frequency events.  The geomagnetic storms have large 
> frequency components of perhaps a few Hertz.  EMP from a high altitude 
> nuclear detonation has frequency components almost entirely below 1 MHz 
> and largely below 100 kHz.  The only way to shield from these low 
> frequencies is with a large amount of ferrous metal, steel!  Copper won't 
> do nor will aluminum.  Commercial (and expensive) shielded enclosures are 
> only rated down to 100 kHz, and even at 100 kHz the attenuation is several 
> 10's of dB below what it is at 1 MHz.  Modern shielded enclosures are 
> often constructed of particle board sandwiched between sheets of copper 
> plated steel.  Even with the steel, they provide almost no attenuation of 
> magnetic fields. Shielded enclosures also have power line filters, 
> waveguide below cutoff filters for air vents, and very strict methods for 
> bringing coaxial and fiber optic cables in.  Telephone lines are seldom 
> brought in but if they are they also require filters.  The filters are 
> typically rated for 100 dB attenuation above 14 kHz.  Traditional copper 
> wire screen rooms are even less effective at low frequencies.  I suspect 
> aluminum screen would be less effective than copper screen.  The screens 
> I've seen were about 1/8" or less.
>
> I am quite familiar with pulse weapons.  Trust me, they are not a problem 
> for us.  Their range is extremely limited unlike a high altitude EMP which 
> covers a radius of 1000's of miles.  Unless you are an important target or 
> the weapon is inaccurate by a 100 miles I don't think we have anything to 
> worry about.  Incidently, the field strength level normally used for EMP 
> testing is 50 kV/m.  For comparison, the electric field suceptibility 
> level (MIL-STD-461) is about 200 V/m, some 48 dB less.
>
> The main threat from both EMP and magnetic storms is conducted energy on 
> transmission lines, power lines and telephone lines that are perhaps 100's 
> of feet to miles long.  Unless you have excellent filters on all those 
> lines and the filters are properly installed on a shielded enclosure, they 
> will be ineffective.  The most effective way to protect your ham station 
> from EMP is to diconnect everything: power cords, microphones, keys, 
> antennas, etc. Several years ago FEMA or CD or somebody like that did 
> testing on ham gear and basically concluded that it is survivable if all 
> wires are disconnected.
>
> By the way, a magnetic compass works just fine in a sheilded enclosure. 
> Yes, a General did ask!
>
> This information is all available in the technical literature on EMI/EMC 
> (electromagnetic interference/elctromagnetic compatibility).  QST has had 
> a few articles on EMP effects and protection during the cold war years. 
> The last real US test of EMP was a device called "Starfish" about 61-62 
> time frame.  It was launched from Johnston Atoll and was above the horizon 
> in Hawaii.  A 1976 Los Alamos Scientific Laboratory publication lists 
> Starfish as 1.4 Megatons at an altitude of 400 km.   This was the most 
> powerful and except for a series of very small experiments called Argus, 
> highest EMP test.  The various test ban treaties have prevented any more 
> test of high altitude EMP by all parties.
>
> That should keep you busy for a while!
>
> Kerry
>
>
>
> -----
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