Posted on 05/15/2012 2:04:08 PM PDT by Kartographer
This may well be the best Faraday cage you can make on the cheap and the best Faraday cage at any price. I'm not saying that to brag but I really believe in this design. If you are at all concerned about protecting your electronic equipment from either a man made or solar EMP please consider making a Faraday storage cabinet like this. The cabinet cost me $100, the tape $32 and the aluminum insulation was free. You might be able to make this Faraday cage for less if you stop by construction sites and get left over aluminum insulation and ask for thirteen feet of aluminum duct tape.
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“I don’t think a Faraday Cage is sufficient to stop an EMP.I believe you need a mu-metal cage, to block the magnetic part of the pulse.”
Steel will do the trick, as it has sufficient permeability.
You say even mu-metal wouldn't work, but RFEngineer says even steel is good enough to work. One of you is clearly wrong (or both). However, I think that you are probably correct. A high-current pulse with a very short pulse-length is bound to produce a very high magnetic field.
Wouldn’t you say the cage fails if it doesn’t pass the ‘radio’ test? Could you please prove an alternative test.
The unclassified EMP states up to 50,000 V/m from about 5kHz to 1GHz
The pulse rise time is about a nanosecond (that’s where the top end 1 GHz comes from)
“The officer wasn’t sure ANYTHING would be sufficient to shield against such a pulse.”
Steel will do it - low frequency permeability ducts magnetic flux around the shield. At higher frequencies it’s conductive enough to suffice.
“The brother set up an experiment where he created a Mu-metal cage around a detector on the theory that the cage would route the field around the detector.”
Not at the very low frequencies, low magnitudes involved in your described test scenario - the low-level magnetic pulse will blow through the shielding because there isn’t enough magnetization energy. This is a big problem for sensitive instruments such as newer electron microscopes.
At even slightly higher frequencies, passive eddy current shielding works very well for magnetic fields (such as 60Hz)
“Would being underground, say in a cave, bunker, or fallout shelter protect one’s electronic gear or electrical conductors?”
No. in “nominal” soil conditions you’d need to be between 100 and 200 ft in depth to be sure you met the MIL SPEC.
However, there are exceptions. For instance, if you are under 200ft of granite, you will not get significant shielding effect, because granite (or dry sand) is very low conductivity and hence, does not significantly dissipate an EMP.
“Wouldn’t you say the cage fails if it doesn’t pass the ‘radio’ test? Could you please prove an alternative test.”
That would be true.
To be sure, you’d need multiple tests and many frequencies that could measure across an up to 80dB dynamic range. Not an easy thing to accomplish. Google “MIL-STD-188-125-1” for a lot of detailed information on a test apparatus.
The dynamic range is a key factor.
“One of you is clearly wrong (or both). “
The military usually uses steel for fixed facilities.
“A high-current pulse with a very short pulse-length is bound to produce a very high magnetic field.”
This would create a broad-spectrum pulse with frequency content up to the inverse of the pulse-length.
At low frequencies magnetic fields dominate, at higher frequencies electric fields dominate.
Electric fields at all frequencies can be shielded with foil-thickness shielding. magnetic fields are different. The lower the frequency the more it will penetrate. Think about it...the earths magnetic field goes through the entire earth. you never have to worry about the earths electric field- it never makes it anywhere close to the surface.
The most energetic part of an EMP does not have low frequency components - though the later stage EMP DOES have a geomagnetic component - but it is not highly energetic - so you need a very large collector for it to impact (e.g. power transmission lines, or pipelines).
Then I would use a radio test multiple frequencies on the FM, AM and SW band and then use a couple of cellphones to the GSM and UMTS bands. You could even use a small TV and test UHF and VHF bands. I would say that after such a test you could be fairly sure you were covered.
How about just using a multifrequency scanner?
The problem is both frequency, and dynamic range.
You may think you have a good Faraday cage using receivers but you could fall well short of performance because the signals weren’t strong enough to show your true performance under EMP conditions.
It’s better than nothing - but it’s also a false confidence builder if you get a radio to block a station.
It’s a matter of how sure you want to be. The best inexpensive ad-hoc shield I can think of is the trash can - but use steel wool fragments around the full circumference of the lip and hammer it down solid with a rubber mallet.
No guarantees, but it’s much simpler in its uncertainties than this cabinet thing.
Please add me to your ping list!
Thanks!
You could also place a transmitter inside and see if you get a signal.
“You could also place a transmitter inside and see if you get a signal.”
That is how it’s usually done - though to achieve the dynamic range you need fairly high radiated power, and you need to sweep through a decent sample of frequencies.
There are companies that make a decent living doing only that.
ok, thanx
That pic looks like a pic from one of those facebook “Find the objects” games like Garden of Time.
Cheapest faraday cage you can find: Old, dead microwave ovens. I use several right now!
That’s cool, now if I could just find one big enough to put my solar panel in I’d be set.
The conductivity of the standard cage should take care of the magnetic field component, at least to the extent it handles the electric component.
Besides, the mu-metal would saturate anyway in response to an EMP, blocking only a fraction of it.
So really - what we each said doesn’t contradict each other ;-) You have a higher magnitude than my memory has stored in it - but we’re in the same ball park.
My statements are from memories at a ham club meeting I attended in 1982 - so please forgive the slight variance ;-) The officer had come down from Vandenburg AFB to Santa Barbara.
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