[scottdupoy]

> While the cookie tin performed much better than the mylar bags, and could provide a modicum of useful attenuation under some circumstances, it was not sufficient to provide meaningful assurance of signal isolation at any frequency. However, it was unique among the containers tested in providing tasty snacks during measurements

Back in the day working for Symbian I used to regularly have to run a load of automated Bluetooth tests. Unfortunately there were 200+ engineers in the close vicinity of my test setup and they all had smartphones and all had Bluetooth turned on, causing loads of the test to timeout. We were very pleased with ourselves when we thought of using a biscuit tin as a Faraday cage to try and improve things. Didn't work as well as we hoped, but got the tests running within their timeouts. I always thought it'd be interesting to try using an old microwave.

Best thing was that we got to eat all the biscuits first :)

[aurizon]

Biscuit tins and aluminium foil have layers of insulation on them, the varnish of the tin and the Al2O3 oxide layer for the foil. It is true that the gap in each case is small, but when you look for 10 orders of magnitude in attenuation - a little leakage goes a long way. Silver has the unusual property in that both the metal and it's oxide are both conductors = true silver foil made of thin metallic silver will make a good wrap around Faraday bag. Even better would be true gold metal foil - note they often use gold plated shields in space uses, even though gold foil is like fly paper in space = contact welds readily, much more so than most other metals (many of which have also contact weld problems that have to be mitigated). Those Ramsey boxes are excellent, far better than many costlier, but crappier boxes, obviously made by someone who knows an ohm from a volt...

[formerly_proven]

I think the biggest issue with cookie tin cans is that they usually don't close all the way around, because the lid or the box is always slightly oval and not round (they always "stick" just in two or three spots when opening), so it looks like there's a small gap around half the circumference long - far bigger than the wavelengths in question.

Square tin can works better because those seem to usually scratch on the corners. The smallest square tin can you can find should work best. A tin can sealed with adhesive cooper tape should work pretty well even if the copper tape is isolated by the paint on the can.

[mcshicks]

I had to do some testing with ble temp sensors once we're I needed to isolate them and then put them in a freezer. They were in advertising mode so I could tell at least if I could see them with a phone. A round tin that was used for green tea from Japan was the most reliable method I was able to improvise. It fit very snug and the lid was about an inch deep.

[aurizon]

Yes, a one inch compressed gap like that is a tortuous attenuative path in it's own right...

[redleader55]

The simplest test you can make to verify that the microwave oven & biscuit tin can are good Faraday cages is to put your phone in one and try to call it - every time I did that in the past they failed to function as a Faraday cage.

Edit: clarification.

[jmnicolas]

Did you wrap your phone with something non conducting so that it doesn't touch the tin or microwave?

It works for me with a cookie tin, the phone is wrapped in a towel.

[OJFord]

That's an easy way to verify failure (phone rings) but not success - unless the only thing you care about blocking is phone calls I suppose.

[dzdt]

The Faraday cage failed (the phone rang?) or the phone failed to ring (Faraday cage succeeded)?

[redleader55]

Clarified the text of my original comment.

[kingcharles]

When I was in jail I had a cellie who imported drugs from China. They were always shipped inside defunct microwave ovens. I wondered if it was just because it looked like a harmless kitchen appliance or because the microwave provided shielding from an x-ray scan?

[habi]

This really depends on the X-ray machine, a powerful x-ray source might easily penetrate a small appliance.

A faraday cage/bag does not protect x-ray since it ‘only’ blocks electromagnetic fields.

So I guess in the case you mention it’s more hiding in the insulation/empty space of an appliance.

[BenjiWiebe]

X-rays _are_ electromagnetic radiation.

[duskwuff]

Technically speaking, yes. But they're way off on the other side of the EM spectrum, giving them dramatically different properties. You can shield against radio with a shell of any conductive material -- even a wire mesh for longer wavelengths -- but X-rays are only absorbed by high-atomic-weight atoms, like lead.

[habi]

Of course the are, but - if I'm correct - they don't produce an electromagnetic field. And are not shielded by a bit of mesh wire (I work with desktop microCT machines...).

[refurb]

X-rays are the exact same thing as any other electromagnetic radiation, just at a different frequency.

Generally a mesh can block an EMR signal if the holes are < 1/2 wavelength in size. X-rays have nanometer wavelengths so the mesh wouldn’t even be visible to the naked eye.

[Terry_Roll]

How many different biscuit tins did you try? :-)