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Testing Phone-Sized Faraday Bags

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Re: Testing Phone-Sized Faraday Bags

#21

Post by kortilla »

op00to wrote: post #12
bborud wrote: post #7

At my former job we had a rather large faraday cage for testing telco equipment. Two things surprised me: 1) It is more fiddly to ensure a room-sized faraday cage doesn't leak RF than I thought. It is really easy to make mistakes that compromises experiments. 2) There was a bath-towel sized piece of cloth that came with one of the expensive Rohde & Schwarz instruments that was astonishingly effective at blocking sign…

I see one of the founders (Ulrich Rhode) of Rhode and Schwarz on amateur radio some times, and I always ask for test equipment and he always says no.

Maybe stop asking then?

Re: Testing Phone-Sized Faraday Bags

#22

Post by formerly_proven »

aurizon wrote: post #16
scottdupoy wrote: post #8

> 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 B…

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…

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.

Re: Testing Phone-Sized Faraday Bags

#23

Post by redleader55 »

scottdupoy wrote: post #8

> 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 B…

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.

Re: Testing Phone-Sized Faraday Bags

#24

Post by ncmncm »

pdkl95 wrote: post #15

I really like Snowden's recommended Faraday cage for phones or other devices of similar size: two drink-shaker cups[1]. Drink shakers are cheap and widely available. The average hotel room in any medium/large hotel chain probably includes a shaker as a standard item as part of the minibar. The simple two-cup style (as seen in [1]) is preferred over the fancier "strainer top" style because you can press the two cups t…

It is hard to believe this would perform better than a Lazzaroni tin.

And I don't recall ever finding one of these things in a hotel room, even in Europe.

Re: Testing Phone-Sized Faraday Bags

#25

Post by whiw »

> Faraday cages are non-directional.

Per-se, yes, agreed, but I was wondering whether the transmitter and receiver antenna couple into the surrounding screen differently. The degree of coupling may de-tune them differently. If so then the setup which most accurately represents the intended use would be with the receiver inside the screen.

Re: Testing Phone-Sized Faraday Bags

#26

Post by causi »

pdkl95 wrote: post #15

I really like Snowden's recommended Faraday cage for phones or other devices of similar size: two drink-shaker cups[1]. Drink shakers are cheap and widely available. The average hotel room in any medium/large hotel chain probably includes a shaker as a standard item as part of the minibar. The simple two-cup style (as seen in [1]) is preferred over the fancier "strainer top" style because you can press the two cups t…

What's the physics behind being able to make a container light-tight with a couple of right angle corners yet radio waves seem to leak out like water?

Re: Testing Phone-Sized Faraday Bags

#27

Post by ncmncm »

I wish it covered (so to speak) aluminum foil more thoroughly.

In particular, foil is cheap, so rolling up in several layers (3? 5? 10?) and twisting the ends cracker-style ought to be more reliable and practical than trying to do it with one layer and fiddly seams. A suggested minimum number of layers and number of twists would be useful.

Re: Testing Phone-Sized Faraday Bags

#28

Post by ejolto »

ncmncm wrote: post #24
pdkl95 wrote: post #15

I really like Snowden's recommended Faraday cage for phones or other devices of similar size: two drink-shaker cups[1]. Drink shakers are cheap and widely available. The average hotel room in any medium/large hotel chain probably includes a shaker as a standard item as part of the minibar. The simple two-cup style (as seen in [1]) is preferred over the fancier "strainer top" style because you can press the two cups t…

It is hard to believe this would perform better than a Lazzaroni tin. And I don't recall ever finding one of these things in a hotel room, even in Europe.

Depends what the tin and shaker are made of. The tin is probably made of aluminum which has a higher skin depth than stainless steel, which I assume the shaker cups are made of. A Faraday cage of continuous surface needs walls that are thicker than the skin depth of the material the cage is made of. Since the skin depth of stainless steel is lower, the cage wall thickness can be lower while still creating an effective Faraday cage compared to an aluminum cage.

Ref skin depth of different materials at different EM frequencies:https://upload.wikimedia.org/wikipedia/commons/f/f3/Skin_dep...

Re: Testing Phone-Sized Faraday Bags

#29

Post by Kaibeezy »

taneq wrote: post #6
0xdeadb00f wrote: post #5

Wrapping your phone in aluminum foil also works. I've made a few DIY Faraday bags that way. They're not perfect of course, but less expensive than these bags most of the time. Edit: Ah, I see this was covered in the article. I didn't read it before posting this comment.

It also keeps them from drying out when you microwave them while still maintaining a crispy skin.

Also, caution: If your phone is made from a tomato or other acidic substance, the foil may corrode, further reducing RF containment.

Re: Testing Phone-Sized Faraday Bags

#30

Post by catlikesshrimp »

djmips wrote: post #19

If you are sufficiently concerned, why not mod the phone to add a physical battery disconnect? Would this not provide 100% protection?

If you are concerned enough, but a phone with physical switches

https://www.androidpolice.com/2020/08/22/this-smartphone-has...

Or at least phone with a removable battery

I know I will never buy a phone without a 3.5 audio jack

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