6 ms·

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 avai

by pdkl95 5y ago

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/Wine-Cocktail-Shaker-Set-Mixing-Making-Drinking-Container-Barware-Bartender-Drink-Party-Bar-Tools-Stainless-Steel.jpg_Q90.jpg https://ae01.alicdn.com/kf/HLB1yRyhXZTxK1Rjy0Fgq6yovpXaZ/Win... [2] Holes of sufficient size. How big are the waves you are trying to block?

kurthr 5y ago
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 5y ago
  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 5y ago
    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.
sumtechguy 5y ago
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.
causi 5y ago
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?
- 2rsf 5y ago
  Radio waves (well, light and radio are the same thing just with different wavelength) are bounced from conductive materials, light is absorbed
  - relaxing 5y ago
    Light bounces too. As you said, it’s the same thing.
    - alecdibble 5y ago
      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 5y ago
  Does the right-angle corner trick still work if the container surfaces are made of mirrors?
- rocqua 5y ago
  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.
adolph 5y ago
> 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 5y ago
  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...
  - speedgoose 5y ago
    Hilton is a tiny bit more fancy than the average.
  - adolph 5y ago
    She's not an average ship, Jim!
MR4D 5y ago
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?
14 5y ago
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 5y ago
  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 5y ago
    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).
ncmncm 5y ago
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 5y ago
  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_depth_by_Zureks.png https://upload.wikimedia.org/wikipedia/commons/f/f3/Skin_dep...
  - amluto 5y ago
    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 5y ago
    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!
- ghaff 5y ago
  I imagine I've seen them now and then but they're hardly common. I'm grateful when I get real glassware.
- geewee 5y ago
  I'm from Europe and I've never seen one either.
GordonS 5y ago
> 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 5y ago
  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 5y ago
    The Fairmont Royal York in Toronto has shaker cups. It is 5-star though.

by pdkl95 5y ago

kurthr 5y ago

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 5y ago
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 5y ago
  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.

sumtechguy 5y ago

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.

causi 5y ago

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?

2rsf 5y ago
Radio waves (well, light and radio are the same thing just with different wavelength) are bounced from conductive materials, light is absorbed
- relaxing 5y ago
  Light bounces too. As you said, it’s the same thing.
  - alecdibble 5y ago
    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 5y ago
Does the right-angle corner trick still work if the container surfaces are made of mirrors?
rocqua 5y ago
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.

adolph 5y ago

> 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 5y ago
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...
- speedgoose 5y ago
  Hilton is a tiny bit more fancy than the average.
- adolph 5y ago
  She's not an average ship, Jim!

MR4D 5y ago

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?

14 5y ago

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 5y ago
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 5y ago
  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).

ncmncm 5y ago

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 5y ago
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_depth_by_Zureks.png https://upload.wikimedia.org/wikipedia/commons/f/f3/Skin_dep...
- amluto 5y ago
  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 5y ago
  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!
ghaff 5y ago
I imagine I've seen them now and then but they're hardly common. I'm grateful when I get real glassware.
geewee 5y ago
I'm from Europe and I've never seen one either.

GordonS 5y ago

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