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Yupper… :+1: :smiling_imp:

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Last I looked at this, which was a while ago… a gap up to 1/10 of the wavelength will still offer some shielding. Overlapping edges (such as an ammo can) will also offer some shielding.

One thing that frustrates me about faraday cage discussions is lack of “engineering/science” based solutions. There’s a lot of arm waving and forum discussions about microwave ovens and aluminium foil without much discussion about likely wavelengths and energy which needs sheilding.

I had to do some more googling as it’s a long time since I read up on this, following is from Google AI summary:

An electromagnetic pulse (EMP) contains frequencies across the entire electromagnetic spectrum, from direct current (DC) to gamma rays, though its frequency composition depends heavily on the source.

For example, high-altitude nuclear detonations produce a “fast-pulse” (E1) with frequencies from DC to 1 GHz and a “slow-pulse” (E3) with very low frequencies (MHD-EMP).

Conventional EMPs using High Power Microwave (HPMW) technology often have frequencies in the range of 100 MHz to 100 GHz.

EMP Frequency Components

E1 EMP (Fast Pulse):
Created by the interaction of gamma rays with stratospheric air, this component has a broadband power spectrum from direct current (DC) to approximately 1 GHz.

E2 EMP (Intermediate Pulse):
Caused by scattered gamma rays and neutrons, this pulse lasts from about one microsecond to one second. It has characteristics similar to lightning strikes and is easier to protect against, though the preceding E1 pulse may have already damaged protective devices.

E3 EMP (Slow Pulse):
Amagnetohydrodynamic (MHD) phenomenon caused by the distortion of Earth’s magnetic field from a nuclear explosion, this component induces low-frequency currents in long conductors.

So, 1Ghz iz wavelength of c.0.3m, 1/10th is 2.9cm.

Working the other way, assuming you had electrical connection between lid and body but also had a 5mm gap this should be okay up to 6Ghz.

FWIW, microwave ovens run at 2.45Ghz.

My take was a faraday cages are perhaps a good idea for things with longer wires which will act as antenne (radios, chargers, etc) but I’m not convinced flashlights will be particularly susceptible. It looks like cells ( not batteries) will be immune also.

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I’ve used it on spark plugs and headlight bulbs without a problem when I’ve changed them which hasn’t been very often. The auto parts stores have been selling it in small pouches up by the counter around here as “bulb grease” that they recommend when people buy bulbs.

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Set of 4: U.S. Military Surplus .50 cal Ammo Cans - Grade A $59
:wink:

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I got an easy way to do it…Assuming all continuity is on the rim and the lid and the hinge is a moot point, pick a stationary point on the lid, quick solder a contact wire from the lid to the rim. Then with a dmm, pick a second stationary contact point on the lid, apply one probe. With the second probe, trace the grinded area of the lid. Reverse and repeat for the rim. If at any point continuity is broken, you’ll have to grind off a little bit more.

I should probably have recommended how you can make sure the lid and rim contact surfaces are mated…I used a chalk to generously line the rim, then closed the can so the chalk left residue on the lid. Then I just sanded off wherever the chalk residue made a line, then sanded a bit off the rim.

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I keep forgetting that we are talking AC at RF. Not just DC. So the size of the gap becomes real important. I guess we should talk about AC Impedance at specific frequencies. The problem still is measuring it. But a perfect seal would still be preferable.
Which you won’t get with crumpled foil or even the stainless steel wool that is often recommended.
Either of which make using the chalk method for checking the seal impossible.

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This would tell you if you have all of the pint off… But with the chalk you couldn’t tell if you have a true continuous contact … SIGH

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SIGH. The assumption is that you’re proficient at what you’re doing, so you’re checking with a square that the contacts are straight, and that they are indeed making contact.

the chalk is for the initial marking of where to grind, it does nothing for the continuity test.

I saw you typing and deleting answers repeatedly, and was ready for whatever you were gonna toss my way, LOL. I’m only responding now cuz it took you SO DAMNED LONG! :laughing:

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The mated edges should still clamp down tightly if since you’re basically removing only a few microns of paint…I did mine by hand, much easier to control the surface. Even, single stroke passes. A P.I.T.A., but well worth it for a flush, squared surface.

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“All my goodies can withstand a 1.2 megaton aerial burst, as little as 5km overhead!”

“Ummm, yeah, but can you??”


Point being, what would generate such a strong EMP that would otherwise fry all your goodies, where a hyu-mon would also be expected to survive?

Wouldn’t a bone-stock ammo can reduce an EMP’s effects by, say, 90%-95%, and still be good enough, vs trying for nearly 100% elimination?

RF-shielding baggies might take up more of the slop. And stick the ammo-cans in a metal filing cabinet.

“Dress in layers.”

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It’s scary but I was wondering about layering after my last post. Perhaps my tin foil helmet needs more layers to keep you from stealing my thoughts.

Depending on how big or what type the ka-boom is I may not worry about anything but running around outside to absorb as much radiation as I can.

I also was thinking about how rooms for taking X-rays for humans are required to be shielded to protect people outside the room yet the electronics in there continue to function. Ours used a lightbulb used in movie projectors for adjusting the beam before taking X-rays and it was on for 10 seconds or so at a time. By the time it burned out it was harder to find a replacement as the market for them was a lot smaller after movie projectors were replaced by whatever they use now and the original was probably 30 years old then. The overhead lights continued to work as did the small refrigerator we had in there.

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I’m in the process of tracing the linkage between a good AA light and ammunition boxes :⁠-⁠)

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I ordered a Convoy T3 for the box. Yes, that’s the second flashlight I don’t need I’ve ordered since coming back here.

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X-rays are directional. I worked with those tubes (still got one somewhere), the beam coming out is like a laser pointer. You have to diffuse it to get coverage for, eg, a chest x-ray.

When the beam hits something, it can scatter and even backscatter, which is why everyone in the room hides behind those Pb aprons and huge shields, because you don’t want a cumulative dose from being around it all day long.

Think of it as Ceiling Bounce From Hell™.

Anyhoo, point being that the electronics would get almost no “dosing” from it, and would be unlikely to get damaged by it.

At worst, SEUs (single-event upsets) would flip a 0/1 to 1/0 in RAM or something, by just depositing or removing electrons from a memory cell, but that’s it. You’d need gamma or cosmic rays to do actual hardware damage.

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Or neutrons…the highest energy frag humans have produced, and the frag can slip between atomic structures and fracture DNA…scary stuff.

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The light was the pretense, but the box is actually the premise.

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The cumulative effect is what I was referring to. The dose from most clinical X-rays is less than a flight to the coast from the Midwest. The bulb I mentioned wore out from use over many years and the rest of the electronics in the columnator were ok, same for the bucky that’s as close to the film cassette as you can get. And the cassettes (the film holders) wear out gradually with use as the radiation slowly reduces the effectiveness of the crystals that intensify the image.

Having written all this I’ve forgotten what the subject was. Maybe the radiation has reduced my memory.

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I write thoughtful replies, and my typing sucks. More time in corrections than in the writing. But my thought was, if I am going to use metal foils or steel wool, does flatness and perfect contact make any difference with RF. With those there will be lots of areas without perfect contact. I guess the thought is that the gaps would be small enough so that the frequencies of concern would be blocked. I think the internal isolation from the metal and adding layers of foil around each item will be as good as I am willing to do. All that will just make getting in and checking batteries and function testing a PIA.

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Security and “defensive” measures, by definition, are supposed to be inconvenient…just a little less inconvenient for you than whatever or whomever you’re protecting from.

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I don’t think this is true. But in any case the easier and less time consuming a task is, the more likely it will be accomplished regularly. Especially if the one accomplishing the task is me… :japanese_ogre:

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