Maybe EMP weapons?
I suppose I might think about something like this if I lived in proximity to China or Russia, but not so much if I lived in Kansas.
“Current loops” in the shield are not the problem; the point of a Faraday cage is that the charge is essentially universally distributed and there is no net charge within the cage due to external fields. The reason that having a uniform connection around all joints is because even small gaps allow high frequency electromagnetic waves to ‘slip’ through the shield. A galvanized or anodized contact surface will not provide very reliable conductivity so you really want to have a metallic mesh or foil bridging any joint between sections of the cage. There are textiles that are ‘metallized’ or conductive; that is, the fibers are either intertwined with very thin wires or are made of a conductive substrate such as polyacrylonitrile (PAN) carbon fiber and interfacial enhancers such as graphene microparticles or pyrolized carbon black. However, anything that is “squishy” is by definition going to have variable and likely poor conductivity just because the compliant texture indicates a lot of porous volume.
Agreed with the ‘fingers’; shielded braid cables often have connectors with little interlacing fingers to assure that there is a reliable shield around the wires inside. Even with this, care has to be taken to assure a complete connection and there are very rigorous conductivity tests with extremely tight tolerances to assure that there is a secure shield in the connection. The “halo ground” loops are actually to protect against the E2 component of high altitude electromagnetic pulse (HEMP), the effects of which are more akin to lightning and is what will cause large sparks or other visible phenomena, versus the E1 (very high frequency pulse) component that damages delicate electronics and which a good Faraday cage will protect against. If you don’t have anything attached to an antenna or the electrical grid, the E2 pulse will probably not damage it unless you actually get an electrical arc from some nearby ‘antenna’.
There is a common misunderstanding that all nuclear explosions result in large EMP effects. Low altitude detonations don’t really produce much in the way of electromagnetic pulse just because the high energy X-rays don’t have a very long mean free path before they are absorbed and converted into thermal and blast effects; thus, their ionization potential is limited and mostly absorbed by the atmosphere. If you are close enough to actually experience EMP from a low altitude nuclear detonation you have the bigger problems of being burned to a crisp and crushed by powerful pressure waves. HEMP results from detonations at high altitudes that excite free electrons in the thermosphere. These electrons absorb the X-rays and oscillate in the Earth’s magnetosphere, emitting a semi-coherent high frequency pulse that is then propagated through the atmosphere up to the 50 kV/m dielectric breakdown limit. Because this occurs over a wide area it can affect large regions on the ground. This is not an incidental effect; it is assumed to be a deliberately targeted attack; space warfare or the use of nuclear-tipped interceptors for mid-course destruction of ICBM-launched weapons could also incidentally cause HEMP effects but they are more likely to occur over broad ocean area or over the poles where their field of effect would be away from inhabited areas and major military installations.)
Realistically, if we were in a shooting match with an opponent capable of launching an EMP attack you’d probably have bigger concerns than whether you could use your smartphone, and while HEMP can damage automotive and appliance electronics, your bigger problem is going to be that the power grid has utterly collapsed and you are now having to scour the wasteland roadways for precious ounces of gasoline in leather fetish gear with your hogleg shotgun and faithful Australian Cattle Dog in a gas-guzzling muscle car with…doesn’t really make much sense but still more rational than delivering precious water to your faithful followers by deluging them with it in a muddy pit.
I had a point when I started this but it has since escaped me like a wander roaming the radioactive wastes in search of redemption and a water purifier chip but just having to take on an interminable number of side quests. “War…war never changes.”
Stranger
Wikipedia defines microwave as 1mm to 1m wavelength, (less than that as infrared) so presumably this implies the gaps need to be ¼mm or less. Does infrared induce currents?
Ha ha - perhaps the best defense is to invert the garbage can and fill the rim of the lid with salt water. Mercury would be better I assume but brings up other problems.
Also the quality of the box needed I assume is inverse to the distance from the source EMP…
And at what distance/strength need one worry about the device itself (as opposed to say, much longer network and power cables) being capable of producing a destructive current? Would th new Chinese EMP missiles be as bad as a nuclear blast?
Well, again, the E1 component of HEMP is actually a result of interactions between the X-ray yield and free electrons in the thermosphere; the E2 component is mostly driven by neutron emission that produces secondary gamma scattering through ionized gases. In the atmosphere X-rays are mostly absorbed within a few tens of meters (especially once the atmosphere starts to compress with thermal heating) and even fast neutrons will be absorbed pretty quickly, well within the subsequent blast effects that will physically destroy any delicate electronics and the people using them before they have time to notice that their Gameboy is bugging out. The range of effects depend upon the X-ray and neutron yield of the weapon. HEMP effects are more about the position of the device relative to the ground range and interactions with the Earth’s magnetic field so there is no simple function to calculate this, and there are people who make entire careers researching the physics of atmospheric electrodynamics and nuclear effects trying to make better predictive models but it isn’t the sort of thing you can assess purely from first principles.
I don’t know anything about “Chinese EMP missiles” and a cursory look at online sources makes be believe that nobody writing histrionic articles about them do either. The US has been working on both EMP effect weapons and systems to do practical EMP testing without having to explode a “flux compression generator” (basically a big solenoid or hollow tube that is compressed to generate a powerful magnetic transient) but so far such devices have not been fielded at the scale to actually knock out weapons or vehicles on the battlefield at useful ranges, so I’m a little doubtful that these missiles are as much the vaunted threat they are imagined to be.
Stranger
How about a biscuit tin? Something like this:
I think it’s painted or coated where the lid meets the rim, but I would think this could be overcome by sanding and applying some conductive tape. Do you need to open and close it frequently, or is it for long term storage?
Thanks for all the useful well thought out replies! It sounds like we can’t make a perfect box. But if the EMP is far enough away I’ll be better off than most.
As I posted - microwaves are in the range of 1mm+ so a quarter wave gap is ¼mm or better. My experience with assorted tins is you cannot rely on them to be that tight all along the seam, so foil tape sounds like the thing to use. Or… cover to top with aluminum foil before pressing on the lid?
From what I’ve read over the years, the biggest danger is from currents induced on network phone or power cables travelling into the device (or an unplugged USB cable still attached to a peripheral.) Some items suggest the pulse if strong enough will generate enough charge on circuit board traces to cause damage. Nobody can say what is what because there’s only one sure way to find out.
As I understand, the trick with EMP weapons is to have the nuclear device go off high enough that when the interaction with the upper atmosphere happens to generate the pulse, it is spread over a fairly wide range for maximum damage.
Of course, it doesn’t have to be 100%. Frying a few input gates on a chip is the same as frying the whole chip for most electronics. Damaging one chip may cause the whole device to fail. The issue is that modern chips running on 5V or less and very low currents are most susceptible to damage from a high voltage spike.