Mesh Size and Thickness for Faraday Cage?

I've been considering a double faraday cage of 1/4in 23-gauge wire and 1/8in 27-gauge wire. With this setup would the 1/4in on the outer shell and 1/8in on the inner or the 1/8in on the outer shell and the 1/4in on the inner yield better shielding from an external EMP source? Is there any difference or is this based on other factors entirely?

To a large extent, it will not matter. If you consider two infinite sheets of the two sizes, the transmission of a plane wave through the pair of sheets is identical in the two directions, due to reciprocity. Any difference will be due to the finite size of the cage.

For a pair of spherical Faraday cages, the two spheres will have slightly different radii, and transmission will again be very similar.

If you have a rectangular box, any difference would be due to energy getting through at the corners and edges. It's still probably still similar transmission either way, though it's hard to say for certain.

One thing that may be different is that there will be evanescent fields, which will fall off exponentially away from the mesh surfaces. The evanescent decay will be faster on the side with the smaller mesh spacing, so if one side will have other things closer to it, that should have the tighter mesh. For your mesh spacings, once you're more than 1/2 inch to an inch away, that won't really matter much either.

I guess one other thing, since you mention EMP. If it will have enough energy that it might melt the wire mesh, you should also compare the power handling ability of two of the 27-gauge wires in parallel versus one of the 23-gauge. Hopefully that isn't a concern...

• So there's no secondary effects wherein the thicker mesh should go on the side closer or furthest from the source of EMP if you're trying to insulate from an EMP? I'm thinking of bulletproof glass with this question, wherein the hardness vs flexibility of the materials makes it one-way from plexiglass-to-glass without letting someone shoot from glass-to-plexiglass due to the brittle+flexible natures absorbing shock. Is there no similar effect with EMF which would make it more beneficial for shielding to have the larger or smaller mesh facing outward? Commented Apr 26 at 5:54
• For electricity, there isn't really an analog to brittle or flexible. The only difference will be if the wires closest to the EMP melt. 23 gauge wire has about 2.5 times the maximum current capacity of 27 gauge wire. There will be two strands of the 27 gauge wires for every strand of 23 gauge wire, so having the 23 gauge wire on the EMP side will let your two-layer system handle about 50% more power before melting. But that would require really high power, like from a nuclear bomb, not just someone with a radio transmitter. Commented Apr 28 at 13:06
• Thanks, that was kind of the analog (there's also evanescent waves when you get into inductance but I was mostly referring to what you mentioned: the physical breakdown of the material in the event of a nuke.) Commented Apr 29 at 20:10