[pdkl95]

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 together; this should cause the inner cup to slightly cut into and/or deform the outer cup along the circle where they join together. The seal between the cups should work sort of like the knife-edge seals used in vacuum chambers.

A metal box or conductive bag is only a Faraday cage if it is fully closed/sealed. Any imperfection in the seal or hole[2] might allow the radio signal to leak out. Most improvised items (freezer, random metal box, etc) have poor seals. Making a high quality cage that actually block a modern phone can be done without much trouble, but the drink shaker method is the only method I know of that will do the job using widely available (free) or very cheap ($10-ish?) parts.

[1] https://ae01.alicdn.com/kf/HLB1yRyhXZTxK1Rjy0Fgq6yovpXaZ/Win...

[2] Holes of sufficient size. How big are the waves you are trying to block?

[adolph]

> The average hotel room in any medium/large hotel chain probably includes a shaker as a standard item as part of the minibar.

Maybe the hotels you stay in, Bond, James Bond.

[pdkl95]

I admit the vast majority of my experience with hotels consists of a single repeated data point: the staff[1] suite(s) we shared in the Hilton attached to the SJ convention center. They always had glasses/shaker/etc. You could probably ask for a shaker from roomservice or the hotel bar, if necessary.

[1] If you registered for FanimeCon 2000-2011, the weird software you had to use was my fault...

[adolph]

She's not an average ship, Jim!

[speedgoose]

Hilton is a tiny bit more fancy than the average.

[GordonS]

> The average hotel room in any medium/large hotel chain probably includes a shaker as a standard item as part of the minibar

I've stayed in dozens of hotels all over the world, and never once have I seen a cocktail shaker in the minibar?!

[ghshephard]

I've stayed in over a hundred, frequently in suites (upgrades), often at 4* hotels, in 22 countries (Full set of brands - Marriot, Hyatt, JW, etc...) Ice Buckets are pretty common - but I have never once seen a drink shaker in a room mini-bar.

[srcreigh]

The Fairmont Royal York in Toronto has shaker cups. It is 5-star though.

[ncmncm]

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.

[ejolto]

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...

[amluto]

The tin is painted, and paint is nonconductive. A very thin long slit in an otherwise conductive surface or mesh can be extremely leaky.

[yosito]

Reminds me of a joke my dad used to tell:

Dad: What's five plus five?

yosito: Ten!

D: How many fingers do you have?

y: Ten!

D: What's an aluminum can made out of?

y: Tin!

D: Wrong! Aluminum!

[geewee]

I'm from Europe and I've never seen one either.

[ghaff]

I imagine I've seen them now and then but they're hardly common. I'm grateful when I get real glassware.

[MR4D]

Now would be a great time to sell a ton of mints that just happens to fit an iPhone max with a case inside.

“Security Mints - sweet & secure”.

Maybe a kickstarter?

[kurthr]

If you're interested in simple cages similar in effectiveness to the EDEC Window Pouch (they drop off above 3GHz)... try a Microwave Oven. They are highly available, reasonably convenient and well designed, with pretty good quality control. You can even still see your phone inside has zero bars. I do, however, recommend unplugging the microwave (or even cutting the power cord) as a fire would likely result from cooking your hardware.

To answer [2] How big are the waves you're trying to block? - it's actually crack length not width that usually determines the effective re-radiating antenna length. They can be very narrow, but a long (a few cm) crack will allow one polarization through almost completely.

I've run this test with a GSM base station and confirmed the effectiveness. If you want to test communication with a device to a base station, but have many other local antennas interfering and want to fit many devices inside... it's a decent choice. The alternatives are less convenient and usually thousands of dollars to construct.

[Johnythree]

The door seals in most microwave ovens consist of tuned slots (eg half-wave stubs) to prevent the RF escaping. These are ineffective at frequencies other than 2.4Ghz.

The reason for the tuned slots is that it's basically impossible to make a conventional metal joint which will seal sufficiently well.

[kurthr]

I encourage you to test a few. I found that the cheap ones blocked 2.45GHz ~60dB, but that they were still down 40dB at 600MHz... the real problems were at higher frequencies and 5GHz was only down ~20-30dB at the worst case polarization. I tried using RF absorbing tape, but while it's possible to improve another 10-20dB it's painful and not repeatable without testing, if you need to remove it.

I've also worked in RF Antenna test rooms with seals that are expensive, finicky, and easy to damage... they're also at least $10k and usually much much more. In a corporate environment that's not a problem, but for a home lab it's unrealistic.

If these bags were big enough to put a few phones and a small base antenna in they'd have other uses.

[causi]

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?

[newaccount74]

There are two important differences between light and radio waves:

Difference 1 is the wavelength. Light has a wavelengths of 400-700nm, whereas radio waves used by phones have wavelengths in the mm and cm range.

If structures are of similar size to the wavelength, then waves can "bend around" obstacles. This effect is called diffraction and can be observed with sound waves, electromagnetic waves, water waves, etc.

Difference 2 is the photon energy / frequency. How well radiation is absorbed depends on the photon energy. There are lots of molecules that absorb visible light very well, so it's easy to make surfaces that absorb light. But there are few things that absorb radio waves as well, so it's hard to make things that neither reflect nor transmit radio waves.

[2rsf]

Radio waves (well, light and radio are the same thing just with different wavelength) are bounced from conductive materials, light is absorbed

[relaxing]

Light bounces too. As you said, it’s the same thing.

[alecdibble]

How much EM radiation bounces and how much it gets absorbed is highly variable depending on the material and, most importantly, the frequency of the EM radiation. A great example in this article is the faraday bag that was visibly transparent but blocked the EM radiation in the 1-6gHz range. The metal tin was basically the opposite, it blocked visible radiation very effectively but not radiation in the radio frequency range.

[foobarian]

Does the right-angle corner trick still work if the container surfaces are made of mirrors?

[hammock]

A couple of right angles not really make it light tight either.

I remember going caving and we would have to go 100m or more through many different turns before we all turned our lamps off and truly experienced perfect dark.

[devb]

Are you familiar with daylight tanks for film processing? They work as advertised using right angles.

[pdkl95]

The last time I used a daylight tank was in the mid '90s so maybe newer models are different, but I believe they are made out of black/dark plastic? Several right angle turns of black plastic provides multiple opportunities for a visible-light photon to be absorbed into a black body where it should be re-radiated as infrared (heat).

A hole/leak in the metal wall of a Faraday cage is going to be a lot more reflective and can easily act like a crude waveguide[1].

[1] https://en.wikipedia.org/wiki/Waveguide_%28radio_frequency%2...

[newaccount74]

There's an old man made cave near me that consists of two small connected chambers, and once you are in the second chamber it is pitch black.

[rocqua]

Also, 20-30 Db of attenuation for light is already quite a lot. Whilst for a radio signal it is still very conceivable that 30Db of attenuation still allows for a signal to be received.

Our eyes simply aren't very sensitive instruments. And the visible part of the spectrum is uncharacteristically full of 'noise', so it makes some sense that our eyes don't need to detect any signals that are too far below the noise-floor.

That makes me wonder. How much 'darker' is any given bit of radio spectrum as compared to the visual spectrum earth at night.

[elbasti]

> our eyes aren't very sensitive instruments

I take umbrage with that statement! Our eyes are exquisitely sensitive, and most importantly, have staggering dynamic range.

Our eyes are capable of perceiving a single photon [1], albeit noisily (I've been lucky enough to have performed this experiment myself!).

But the greatest thing about our eyes is the dynamic range: the difference in brightness between a moonless, starry night (which we are perfectly capable of navigating by eyesight) and a bright sunny day is nine orders of magnitude. A bright day is a billion times brighter!

Show me an RF receiver or light camera with that dynamic range!

The one place our eyes are limited is in frequency range.

[1] https://www.nature.com/articles/ncomms12172

[rocqua]

I knew the dynamic range was large. I did not know about the sensitivity! That is quite impressive.

The magnitude of the dynamic range is even more impressive if converted to 'stops' from photography, yielding about 30 stops (1 stop halves the light). Whereas a really good camera will do about 15 stops. Though I suppose that the camera gets 15 stops in a single 'scene'. Whilst the 30 stop figure for the human eye does not hold up if half your vision is taken up by daylight and the other half by a night sky. For a single 'scene' I think it becomes hard to define the dynamic range of a human eye though.

[joshspankit]

As well: because of reed solomon a modern digital signal can be reconstructed from flickers and fragments of the radio waves

[effie]

What does this mean? A fragment of a radio wave is still a radio wave and can carry information irrespective of whether reed solomon or other encoding is used, no?

[joshspankit]

Think of it like trying to receive morse code:

If the signal gets choppy and you miss some of it, how do you know what you missed?

`..--- -.... ----- -----` (2600) could come through as `..- -. —` (Uno). Uno is a valid word, so passes validation, but it’s the wrong message.

What Reed-Solomon allows us to do is pad the message with n% of error correcting ‘bits’. That way, if >100-n% of the message gets through, the whole message can be reconstructed from whatever bits that made it. And if not enough of the message made it, it immediately fails the validation check and so you know you must resend the message.

It’s so handy and so solid that it’s used almost everywhere. From optical discs to ECC ram to radio communications and a bunch more.

[nabla9]

Microwave wavelength lower than visible light. It interacts with atoms differently by interacting with magnetic fields in atoms more.

There is:

- lines-of-sight propagation in free space,

- reflect from the surfaces like light (spectacular reflection)

- microwaves can be channeled trough tubes (think sound waves). If you fold a conductive material like tin foil multiple times, it can still work as a wave guide and escape.

[14]

I think the issue is then you have to wait until you are already inside the hotel to use those shakers, which as others have said don’t actually exist in most hotels, and at that point you have been tracked to your exact location. Then it would be trivial to surveil you with other methods without needing your phone.

[JacobThreeThree]

Pretty sure Google uses dead reckoning geolocation tracking when no signal is available. So as long as your phone is outside of the bag at some point, you're being tracked even when the phone is in the bag, and Google can correct any errors in their dead reckoning calculations once a signal is found.

[BenjiWiebe]

It's possible, but I'd be surprised. Dead reckoning is terribly inaccurate. To get briefly acceptable accuracy, you need extremely precise sensors that you are reading very very quickly (which would make battery life plummet).

[sumtechguy]

At one company I worked at we had an 'out of coverage bucket'. It was a zinc plated steel bucket. Worked almost every time. Some dude had found at an arts and craft store a very small version of the same thing. It fit over phones perfectly. Basically put device on ground. Put bucket over it. If you had any gaps or any cables hanging out it was sort of iffy and you would get coverage sometimes.