Are ammo cans sufficient for EMP protection?

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With the lid overlapping the way it does I can't see much getting in. This is primarily for storing electronics and digital media.
 
Not worth a crap as a faraday cage. The metal enclosure must be continuous. The gasket is too big of a hole. Also, anything not insulated from the can (touching the inside wall) would have no protection.
 
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Can you do a basic test at home by taking your cell phone and your homemade faraday cage to a location that normally has good service, putting your phone in the faraday cage, and then calling to see it if rings? You'd have to ground the faraday cage.

Not sure that this would tell you how much protection from an EMP it would provide.
 
EMF is line of sight. It doesn't go around corners, but it does reflect. To enter an ammo can it seems to me it would have to reflect back and forth in the tiny space between the can and the overlapping lip until it reached the gasket it could penetrate.

It like to see one put to a test, but have no idea how to simulate anywhere near the amplitude or frequency range, etc.
 
The Boeing CHAMP non nuclear EMP Weapon
 
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Can you do a basic test at home by taking your cell phone and your homemade faraday cage to a location that normally has good service, putting your phone in the faraday cage, and then calling to see it if rings? You'd have to ground the faraday cage.

Not sure that this would tell you how much protection from an EMP it would provide.

That's a pretty good test for RF shielding.

Tin foil actually does great. Those metallized bags don't.

They don't need to be connected to ground for this to work in my experiments....
 
EMF is line of sight. It doesn't go around corners, but it does reflect. To enter an ammo can it seems to me it would have to reflect back and forth in the tiny space between the can and the overlapping lip until it reached the gasket it could penetrate.

It like to see one put to a test, but have no idea how to simulate anywhere near the amplitude or frequency range, etc.

There's more to it than that. Frequency has a lot to do with what can make it through certain gaps.
 
EMF is line of sight. It doesn't go around corners, but it does reflect. To enter an ammo can it seems to me it would have to reflect back and forth in the tiny space between the can and the overlapping lip until it reached the gasket it could penetrate.

It like to see one put to a test, but have no idea how to simulate anywhere near the amplitude or frequency range, etc.


Gaps need to be < 0.1 wavelength to be good barriers to RF. An ammo can is what, about a foot long maybe? That means it's relatively leaky above 100 Mhz or so?
 
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How about a small obsolete microwave oven with a good earth ground and power cord disconnected?
 
That makes a lot of sense. I'd chop the cord to prevent it from being an antenna, but since shielding is reciprocal it should work!

(I'm making evil plans for my next (( if ever )) slow day at work)
 
On phone, but will type more tomorrow about slot antennas. Short answer has been given already: the gasket lets RF in. The cell phone test could give a false sense of security. More tomorrow.
 
OK.
A "real" Faraday cage is solid metal - no holes - in a good conductor. Most metals we use are thick enough we don't have to worry about the skin depth.

A solid metal box is not very useful in the real world except for storage; even then, it needs a removable lid (unless we are talking about a spam can or similar). This brings us to the ammo can. To understand this, we need a bit about antennas.

A basic antenna type is a half-wave dipole, which is a piece of wire one-half wavelength long at its resonant frequency. Ignore the feed in the middle for now (that's where it is broken and fed with a transmission line). Picture it - it is a long, thin conducting rod, surrounded by air. We commonly refer to it as an "electric" antenna because it responds quite well to the electric field portion of an electromagnetic wave.

Now there is a powerful concept called "duality." Using this concept, one can take one equation and switch variables around, and still have the math apply. In antennas, there is a type of antenna called the slot antenna. Imagine a huge sheet of metal. Cut a long narrow slot in it. This is a slot antenna. The dipole and slot are duals: replace metal with air, and air with metal. They have the same field configuration, if you exchange electric E and magnetic H fields. This means that a dipole has E parallel to the length of the wire. The slot has H parallel tot he length of the slot. A lot antenna is referred to as a "magnetic" antenna.

If you are old enough (ahem) to remember some of the old IBM PC/XT cases, where the lid lifted up to reveal the motherboard and guts inside? That case had a big EMI problem. Where the edges of the metal case top met the bottom left a narrow slot. It was a WONDERFUL broadband antenna. Stuff could get out (and did), and stuff could get in (and did). The solution to something like that, where you have to have a seam, is to have an EMI gasket. Common forms include a conductive elastomer (a type of O-ring with metal embedded, usually) or finger stock, which is a springy metal with some cuts in it that resemble lots of fingers.

A word about polarization. Remember duality? A wire antenna prefers electric field parallel to the wire. It has a preferred polarization (orientation of the field). A slot antenna prefers magnetic field parallel to the long dimension of the slot. So, if an EM wave comes in at the preferred orientation, it will GO RIGHT THROUGH the slot. It will even have gain, at the right frequencies (same gain as the dipole - duality again). If you rotate the wave or antenna ninety degrees, it will reduce the penetration - by a lot, say 20 dB or so - but not eliminate it.

So...we could replace the O-ring in ammo cans with a conductive elastomer, and have a much better Faraday shield.

If you want to, we can talk about hole sizes, how to protect wires that go inside metal boxes, etc.
 
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@RFMan I don't think I'm as skilled as you in this field, but I am an EE that has done electronic (embedded systems) design including correcting bad designs that suffered from EMI issues and in the process studied a lot of Howard Johnson and Henry Ott (a couple of the EMI greats). This leads me to question some of the hoopla and paranoia about an EMP and its ability to wipe out everything electrical and electronic.

I think much of the fear comes from earlier days, but much of the equipment back then was significantly more primitive in its design in that it worked at much lower frequencies in terms of rise/fall time bandwidth. Modern electronics in order to even work have a lot of mitigating factors that older ones would not; bypass capacitors, ground planes, surface mount packaging, smaller foot prints, etc. Coupke this with the idea that a field passing through a device would have a gradient over the size of the device and I have to question whether or not you would have enough volts/meter or amperes/meter respectively to induce a killing voltage/current in modern devices? This is similar to how it's easy to zap a part by ESD but much harder to once it's on a circuit board.

Granted much consumer stuff is junk in terms of RF and as you pointed out, there is the duality that also says a good emitter (cheap noisy electronics) are also going to be susceptible to interference. Still it doesn't take a whole lot to reasonably harden a device, but it needs to be designed that way. There is still the question of how consumer junk would be useful following a catastrophic event, in other words, what would you save that isn't already resilient by design?
 
@RFMan I don't think I'm as skilled as you in this field, but I am an EE that has done electronic (embedded systems) design including correcting bad designs that suffered from EMI issues and in the process studied a lot of Howard Johnson and Henry Ott (a couple of the EMI greats). This leads me to question some of the hoopla and paranoia about an EMP and its ability to wipe out everything electrical and electronic.

I think much of the fear comes from earlier days, but much of the equipment back then was significantly more primitive in its design in that it worked at much lower frequencies in terms of rise/fall time bandwidth. Modern electronics in order to even work have a lot of mitigating factors that older ones would not; bypass capacitors, ground planes, surface mount packaging, smaller foot prints, etc. Coupke this with the idea that a field passing through a device would have a gradient over the size of the device and I have to question whether or not you would have enough volts/meter or amperes/meter respectively to induce a killing voltage/current in modern devices? This is similar to how it's easy to zap a part by ESD but much harder to once it's on a circuit board.

Granted much consumer stuff is junk in terms of RF and as you pointed out, there is the duality that also says a good emitter (cheap noisy electronics) are also going to be susceptible to interference. Still it doesn't take a whole lot to reasonably harden a device, but it needs to be designed that way. There is still the question of how consumer junk would be useful following a catastrophic event, in other words, what would you save that isn't already resilient by design?
Let's see...first, yep, Johnson and Ott are great references.

The older TUBE stuff was reasonably immune. Soviets had tube radios in their military well into the later part of the cold war. Some folks said it was for EMP resistance. I think there was an economic factor too ("Radio still work? No replacement with latest and greatest!")

Solid state is interesting. Early solid state was pretty susceptible (low breakdown; heck, it was mostly GERMANIUM based, with REALLY low breakdowns). As solid-state has matured, inherent immunity has increased (better materials, processing, etc), and designed immunity has increased (to reduce ESD issues, and to not tick off customers with early mortality), which you mentioned. Designs ARE better. Our RF environment has more junk in the ether, and more devices in closer proximity, so engineers have HAD to take some steps. The fight often comes over budget, because that stuff is not free...but it's less expensive than leaving it out. Standards are a little stricter too, and those are a minimum bar, or you don't get to go to market (for consumer, anyway).

I think another aspect of this is the changing face of nuclear warfare. Early days, we tended to make larger and larger weapons, with higher yields. And there is a "sweet spot" of where you detonate a given size weapon to get maximum EMP at a target. Later strategy scaled down the individual warhead yield and upped the number of warheads, to counter dispersal of targets...and those would tend to be used at lower altitudes, therefore lesser EMP... The history is interesting, but you can reason your way to almost any answer, by tweaking the assumptions :D

For now, I will say that one might start with a reasonably hardened BOX, and then hook it to a system, and completely destroy the hardness :oops: Ground loops, improper connection of cable shields, non-single-point ground... I've fixed a number of problems by changing how AC cords are plugged in, or how shield grounds are connected (or NOT connected, which can be a mind-blower, until you draw the ACTUAL schematic...). I will also say, that EMP hardness is one thing. RF espionage is something else (I'm thinking about a little thing called "TEMPEST"... :D ).
 
There are no emp weapons. If you are experiencing an EMP issue, you are experiencing a nuclear one as well.
There ARE HPM weapons...

But I agree about the nuclear issue. If nuclear-based EMP takes down a bunch of electronics, it's taking down LOTS of electronics. I'm not going to miss most of those, and I'll probably have other, more immediate things on my mind...
 
To the original question:

Are ammo cans sufficient for EMP protection?

Yes, your ammo is safe. :D

I don't fear a direct EMP/CME impact as much as the indirect impact of taking the grid down with a long recovery cycle. A national grid down scenario has been estimated to have a 2+yr. recovery time. Estimated human attrition in the first year, 98%.

Of course the high density areas will suffer the bulk, due to competition for limited resources and overall collapse of infrastructure.

:(
 
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For cheap and easy as stated before just grab a microwave oven.
 
For cheap and easy as stated before just grab a microwave oven.
A microwave has a grid on the front that is opaque at microwave frequencies, but passes visible light. It's all about wavelength versus the barrier. A microwave may not work as a faraday cage, but it certainly won't hurt.
 
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A microwave has a grid on the front that is opaque at microwave frequencies, but passes visible light. It's all about wavelength versus the barrier. A microwave may not work as a faraday cage, but it certainly won't hurt.

Not the end all be all by any means. Obviously a microwave was only really designed to keep certain frequencies in. A little work on an older oven and I'd call it good. If it were gonna take more than that, then we got much bigger issues to worry about and electronic crap isn't gonna be really high on priority list at that time. But... that's just me
 
Why is everyone so concerned about protecting their phone during an EMP event? Hell I doubt the towers will be much good, so what good will the silly-assed phone be?

There are much more probable scenarios to be concerned about and certainly more important items to be concerned about, other than phones and digital media storage.
Do you know what isn't affected by and EMP?

God old fashioned books.
 
Can you do a basic test at home by taking your cell phone and your homemade faraday cage to a location that normally has good service, putting your phone in the faraday cage, and then calling to see it if rings? You'd have to ground the faraday cage.

Not sure that this would tell you how much protection from an EMP it would provide.
You would only be testing the frequency of your phone but not all frequencies...
 
Why is everyone so concerned about protecting their phone during an EMP event? Hell I doubt the towers will be much good, so what good will the silly-assed phone be?

There are much more probable scenarios to be concerned about and certainly more important items to be concerned about, other than phones and digital media storage.
Do you know what isn't affected by and EMP?

God old fashioned books.

Even if the towers are down you would still have local lan connection, ad-hoc and gps.. these have nothing to do with your cellular service.
 
Even if the towers are down you would still have local lan connection, ad-hoc and gps.. these have nothing to do with your cellular service.
So, you have backup Wi-Fi routers and servers? GPS is an interesting question. Are the satellites hardened?
 
So, you have backup Wi-Fi routers and servers? GPS is an interesting question. Are the satellites hardened?
Mil things tend to be, because they are mil. But keep in mind that GPS sats are in MEO, with both Van Allen (trapped particle) and cosmic ray exposure. They are hardened for the environment; not for prompt stuff, but for SEU (single event upset), total dose, and similar effects.
 
So, you have backup Wi-Fi routers and servers? GPS is an interesting question. Are the satellites hardened?
Ad-hoc is a 1to1 connection no router required. Yes I can enable a wireless hotspot from my phone so I have a built in router.
 
So, you have backup Wi-Fi routers and servers? GPS is an interesting question. Are the satellites hardened?
Even if the Sats weren't hardened and within range... it's not like a single or even multiple emps would knock them all out. You need to think about how many sats are up.. This ranges from google earth, tv and independently owned.
 
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