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Comment by tszyn | Hacker News Reader

I just put my phone in the refrigerator and called it -- works perfectly. Then I tried the same thing with a microwave oven (following pilsetnieks's suggestions) -- didn't even lose one bar on the signal indicator. It did kill the Wi-Fi reception, so clearly the oven is blocking the 2.4 GHz frequency as it should.

Edit: Also tried a cookie can. (And anti-static bags -- see below.) Faraday cage my ass.

Clarification: I use a Nokia. Maybe the technique works only on iPhones? I've heard they have some reception issues.

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Get a friend to call you on your mobile. Answer it. Put the phone in the fridge. Close door. Walk to middle of room. Speak. Can your friend hear you?

Probably not, but they also won't be able to hear me if I put the phone in another room. Or if I leave it in a car.

Yeah but "can't hear you" is not the same as "sophisticated signal processing techniques will be thwarted;" putting the phone in another room might not be good enough. Leaving the phones in the car might work, though I suspected that the risk of the phones being removed from the car while everyone is inside would be pretty high in Hong Kong.

All in all, I think the fridge's insulation probably acts as a good sound barrier, and the hum from the compressor probably adds quite a bit of noise. Probably a smart thing for someone in Snowden's position to demand.

Or being removed/replaced and outfitted with an independent bug complete with radio and power supply. Perhaps it exists as part of a covert, replacement spy battery that powers the phone!

The "refrigerator" had a false back. When the phones were placed inside and attention directed elsewhere, NSA agents opened the back of the refrigerator and installed bugging devices (complete with reserve power cells) in the phones. The goal has always been to identify and entrap counter-intelligence targets.

Snowden is a false flag exercise, which would be obvious to anyone who read Catch-22.

You can never be paranoid enough, because they have a million people with TS clearance thinking of ways to get into your heads. That's just in the US.

> though I suspected that the risk of the phones being removed from the car while everyone is inside would be pretty high in Hong Kong.

The crime rate in Hong Kong is relatively low, especially compared to Snowden's country of origin.

And a freezer acts as a surefire way to not catch some conversation, if you end up walking around the apartment or into the room where the cell phone was tucked away.

Had visitors not brought cell phones in the first place, it wouldn't be an issue.

What is safer in this scenario? Sealed fridge or coffee table in another room?

Car wise, well, that could be impracticable: 1) does one have a car in HK? Or did one fly in and hire a cab? 2) Has one just walked up a tower block to get to Snowden? All go down, and come up again? That would be annoying. 3) If I take all the phones and put them in a box, I know I have the phones. I have control.

In a normal apartment, I reckon a fridge is possibly the best place to put turned off phones to isolate them as best you can at short notice with out super spy resources. Not prefect, but looking round my place, I don't see something better than a fridge for the purpose.

I found this pretty awesome explanation for why a microwave oven doesn't block cell phone reception: http://skeptics.stackexchange.com/a/5410

Clearly, this whole "cell phone in a fridge" thing is a clever attempt to make Snowden lose his nerd cred.

A solid box of metal is going to be a pretty effective Faraday cage regardless of what (sane^) frequency you're using. Microwaves on the other hand are specifically designed to be Faraday cages, so they're tailored specifically to the frequency they're blocking.

(How exactly they do this I'm not quite sure, given that the frequency of a phone and the frequency of phones is quite similar.)

^: Where "sane" means "not gamma waves"

Microwaves aren't Faraday cages as it's too difficult to manufacture a perfect seal, instead the size of the holes in the door act as a tuned RF choke.

Why on earth would you need a "perfect" seal?

People sometimes like to watch the food being cooked, for one. At KW levels of power a few dB is still enough to do harm. Lots of nasty reports in the medical literature of this type of exposure, typically unlucky repairmen.

The eye has very poor heat dissipation mechanisms, and they showed in the 70's the RF exposure could cause cataracts in rabbits. (I think dogs were exposed too -- it was a new field, so they didn't know what would happen)

https://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=45025...

Not an RF engineer here, though I know a few- is a nearly-but-not-quite-perfect Faraday cage really only going to attenuate a few dB?

It all depends^TM, mostly on the wavelength and the size and shape of the gap and the phase of the moon.

dB is a logarithmic scale, so this kind of back-of-the-envelope reasoning is sometimes made easier and sometimes counterintuitive.

> the frequency of a phone and the frequency of phones is quite similar

(I'm assuming you meant frequency of a phone and microwave oven.) That's not quite accurate. Cell phones use many frequency bands [1]. I only see one frequency on the list (2500 MHz) that is close to the frequency used by consumer microwave ovens (2.45 GHz) [2]. I don't think 2100 MHz is close enough to be attenuated by a microwave oven, but of course I'm just speculating because it depends on the design of the oven's shielding.

Don't forget that phones switch to a frequency on which they have signal. If you don't have 4G coverage, your phone will use 3G, etc. These operate at different frequencies. So attenuating one or two of the frequencies on the list is probably not good enough.

[1] https://en.wikipedia.org/wiki/Cellular_frequencies

[2] https://en.wikipedia.org/wiki/Microwave_oven#Principles

> I don't think 2100 MHz is close enough to be attenuated by a microwave oven, but of course I'm just speculating because it depends on the design of the oven's shielding.

I assumed that the design was that of a Faraday cage, which blocks the frequency it's designed for and all lower frequencies. anotherhue noted however that it's a tuned RF choke, which will only block a certain frequency.

For reference, I'm in Australia with a 3G phone on Optus, so the frequencies here are a little less scattered [1].

[1]: http://whirlpool.net.au/wiki/mobile_phone_frequencies

One of my old professors developed radiation-hardened orbital sensors for NASA. From what I remember of his lectures, I think a refrigerator is probably a terrible Faraday cage.

Here are some excerpts from a nifty Analog Devices tutorial about RF shielding[1]. This is probably the most relevant part:

    The longest dimension (not the total area) of an opening is used
    to evaluate the ability of external fields to enter the enclosure,
    because the openings behave as slot antennas.

Remember those big rubber seals on the doors of most refrigerators?

It then goes on to describe an equation for approximating the shielding effectiveness at a given frequency:

    Let λ = wavelength of the interference
    Let L = maximum dimension of the opening

    Shielding Effectiveness (dB) = 20 * log10(λ/2L)

And here's a nice rule of thumb:

    A rule-of-thumb is to keep the longest dimension less than 1/20
    wavelength of the interference signal, as this provides 20 dB
    shielding effectiveness.

There's other stuff in there, such as how to properly shield cables entering or exiting the box. (Unshielded wires penetrating the box can also act as antennas.)

Worth a read, if you're curious.

[1] "EMI, RFI, and Shielding Concepts", Analog Devices, http://www.analog.com/static/imported-files/tutorials/MT-095..., Page 10

Quick chart showing wavelength for typical bands: http://www.hottconsultants.com/techtips/freq-wavelength.html

I just repeated this, while streaming (with no buffer) over wifi. The wifi connection dropped out almost instantly, and the phone signal dropped out after about a minute. My guess on the phone signal is that it assumed it was still connected momentarily while trying to reconnect. I know from experience this happens when between towers, and appears to be a sort of UI smoothing to avoid showing "no signal" frequently.

Also, from experiments I've conducted, a cookie can will work for certain frequencies; for example, the RF transmitters in anti-theft devices. I suspect the material is too thin to stop phone transmission though.

The poster didn't just look at the UI signal indicators, the poster also called the phone and it rang. I'm pretty sure a signal is getting through! =)

Indeed. Might be something to do with the differing frequencies in different countries; I'm in Australia, so my 3G would either have been on the 900MHz or 2.1GHz bands (with GSM on 900MHz/1.8GHz).

Will a GSM phone ring if only the paging frequency is received? It's possible it couldn't bring up the CS session but still rang.

Maybe better to set the "fridge" phone to call the "outside" phone, then answer it after a few rings from the icebox?

Got an anti-static bag to try, as others suggested? Would be interested in your result.

Tried two different types of anti-static bags -- even if I use two layers, the phone doesn't lose a single bar of signal strength.

I've recently tried the static bag using a samsung tablet - wifi and bluetooth both showed no attenuation with two different bags. A "farraday cloth" that was suggested to me is priced at about $24 a linear foot (8'x1') so I wouldn't expect these cheaper solutions to be very effective. I would expect a microwave oven cavity to effectively reduce signals in 2.4ghz, though not signals in the lower gsm and related bands.

An anti-static bag (even the metallized type) probably won't be the best idea. Faraday cages depend on the conductivity of the outer shield to exclude electric fields from the inside, but anti-static bags aren't very conductive (100k-10G ohm square resistivity[1]).

You might even be better off wrapping the phone in aluminium foil, but the problem is then going to be ensuring that all seams in the "cage" are actually conductive.

[1]http://www.esdjournal.com/techpapr/ryne/esdbags.htm

I've seen police investigators using some sort of Faraday bags to preserve evidence while working with cell phones.

A couple of years back a friend and I tried the same thing with my stainless steel colored LG fridge. It totally blocked the signals on our phones. He had a blackberry and I had an el cheapo LG smartphone.

I actually tried the same thing. The fridge/freeze failed miserably; the microwave worked like a charm.

"Although all fridges don’t function this way, those constructed with more metal have the potential to serve this purpose."

Maybe your refrigerator and the other objects you're using to test this theory don't have a thick enough layer of metal?

you forgot the most classical one: tinfoil.

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