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"For the EMP "experts" and nuclear preppers" update
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12:44 AM
You are subscribed to the thread "For the EMP "experts" and nuclear preppers" by paramilusmc, there have been 1 post(s) to this thread, the last poster was Dave_C.
http://www.survivalistboards.com/showthread.php?t=387592These following posts were made to the thread:
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http://www.survivalistboards.com/showthread.php?p=18025714#post18025714Posted by: Dave_C
On: 03-18-2018 09:00 PM
I tried to read all of the more technical posts in this thread. Although I might have missed some details, it seems to me that most of them discuss electromagnetic shielding in terms of electric and/or magnetic field shielding--charge cancellation, eddy currents, magnetic saturation, etc. And that's how I originally understood EM shielding--that a good shield must handle both the electric field (E field) and the magnetic field (H field). But contrary to this view, I learned that the E and H components of *far-field* electromagnetic radiation don't act independently--in fact, they don't exist apart from one another. Although dipole and loop antennas interact with an EM wave differently (the E field induces current in a dipole, and the H field induces current in a loop), shielding acts by a different set of principles.
Shielding of far-field electromagnetic radiation works through reflection and absorption, not through field cancellation and/or redirection of magnetic lines of flux. When far-field EM radiation traveling in air, like an EMP, hits a conductive barrier, part of the radiation is reflected, part propagates through the shielding material, and part is re-transmitted into air at the inner surface of the shield. Reflection occurs when there's an impedance mismatch between two materials--the bigger the difference, the bigger the reflection. Metals provide good reflection because their wave impedance is very low relative to air. Reflection is not affected by field strength, so even a very thin, metal shield will provide good E1 EMP protection. Radiation that propagates through the shield material is attenuated according to its thickness. If the material provides good reflection, thickness need not be considered.
EMP is far-field radiation. In the far field (i.e., at a distance from the source that's greater than a particular minimum distance), the E field and H field are inseparable--they exist in a constant ratio (which is 377 ohms, the wave impedance of free space), and wave attenuation can be accomplished as I described above.
HOWEVER, not all of the radiation that hits an object in a HEMP event comes directly from the source--from the area directly below the blast. Some of it comes in the form of near-field radiation. This occurs because when the EMP wave hits an unshielded conductor, like a power cord or extension cord, the conductor becomes an antenna that both receives and re-radiates the incoming wave. Depending on the length of the conductor and the distance between it and a shielded enclosure (your "Faraday" bag or box) the enclosure might lie within the near field of the radiating conductor. The near field is an area near an antenna where the E and H field can exist independently and where either the E or H field can dominate.
So, if your shielding bag, box or can is located some relatively close distance from unshielded conductors, it's possible that significant near-field energy could make it to the enclosure. In that case, your enclosure's ability to cancel an AC electric field or redirect the lines of flux in an AC magnetic field would come into play as some of you have discussed. And, as you've discussed, this can be difficult, particularly in the case of an AC magnetic field at 1 MHz with a field strength in the neighborhood of 133 A/m. Off the top of my head, a centimeter-or-more-thick shell of copper or aluminum (or a thinner piece of steel) would be required to protect your items.
I haven't done this yet, but I plan to look at the wavelengths of radiation in the frequency range of EMP in order to figure out the lengths of conductors that could cause near-field problems as described above, and the minimum distance that a shielding enclosure must be stored away from them. I think this is vital information that some of the online EMP experts don't discuss (although I think Jerry Emanuelson discusses how damaging a tangle of wires can be in an EMP at his website, futurescience.com--or, maybe he mentioned that in an email).
I'm not an engineer, but I've made digging into the physics of electromagnetic shielding a nearly full-time effort over the past couple of months. A Google of "electromagnetic shielding theory" provides some good resources. The website, electronics.stackexchange.com, has been a good learning tool--it and a lot of online research helped me go from newbie questions to a useful understanding of basic theory that comes from and is supported by authoritative sources. The accepted answer to this question (
https://electronics.stackexchange.com/questions/361022/electromagnetic-shielding-and-emp/361442#361442) at stackexchange summarizes what I've learned about electromagnetic shielding.
Of course, I'm open to correction from some of you physics guys on all of this.
All the best,
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