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Author Topic: Why does my phone work in the microwave?  (Read 18607 times)

KyunaKyuna

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Why does my phone work in the microwave?
« on: 27/10/2011 09:39:35 »
I was listening to Dr. Karl and Dr. Rhod on a recent podcast.  Dr. Karl stated that a microwave oven was a type of Faraday cage and if you put your cell phone in the oven, it would not receive calls.

Foolishly, I believe everything Karl says, so over lunch at work, I tried to impress a couple of my team mates.  I put the phone in the microwave oven, closed the door and then called the phone with a different mobile.

The phone in the oven rang.

So, what is going on?  Is my oven not really a Faraday cage?  It is pretty new, so I don't think it is broken.  Is the cage designed only to stop microwaves (optimization for the relevant wave lenghts does make sense)?  Or, perish the thought, has Dr. Karl missed something?

Thanks,

Dave

imatfaal

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Why does my phone work in the microwave?
« Reply #1 on: 27/10/2011 09:52:34 »
Kyuna

Welcome to the forum.  Cool experiment - I am gonna try it pretty soon.  I think you might be onto the correct thing about the wavelengths - I presume your microwave has a honeycomb see-through door; I will have to think about this, and hopefully one of the actual electrical engineers on the boards will have a look as well

I might move this question to general science or physics where more people read the messages - we will see if it gets any responses

Cheers

imatfaal

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Why does my phone work in the microwave?
« Reply #2 on: 27/10/2011 09:57:43 »
OK results of the test are in...

my office microwave totally blocks any signal to my phone.  Within seconds of shutting the door the signal goes from 5 bars to "searching", no calls will connect, and as a clincher I connected a call outside the microwave and as soon as the phone was shut in the box it was disconnected.

mmm - perhaps time to check your microwave

KyunaKyuna

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Why does my phone work in the microwave?
« Reply #3 on: 27/10/2011 13:44:32 »
Hmm, more data.  I went and tested again.

First five calls, zero.  Zero bars, zero light, zero ring.

The honeycomb you describe is what we have.  I would also add that there is a cell phone tower in direct line of sight of the microwave.  We are on the third floor and the tower is about a block away.  But which network is the tower?  I realized that my two phones are on different networks.  I had put a different phone in the microwave on the second test.

I put the original phone in the microwave and it rang.  It showed two bars in the microwave and five while out.

I dragged another person in and we put his phone in.  It was on the same network of the phone that would ring.  We called it and it rang.  It also showed two bars instead of five.

So, data now shows that the bars decline for both networks when in the microwave, but that for one network, enough signal reaches the phone to ring.

Additional data that may have some bearing.  We are in Kenya, which I think is the same frequency as Europe and the UK.  The US uses a different frequency.

I am leaning towards my original idea, that the microwave oven sheilding is optimized for microwaves.  However, I am out of my depth.


KyunaKyuna

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Why does my phone work in the microwave?
« Reply #4 on: 27/10/2011 14:51:18 »
Ok, home from work, so new microwave.  Also, this microwave came from the shop only two weeks ago.  However, same results.  One network (the same as in the office) rings and the other does not.  No cell tower in line of sight this time either.
« Last Edit: 27/10/2011 16:23:51 by KyunaKyuna »

KyunaKyuna

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Why does my phone work in the microwave?
« Reply #5 on: 27/10/2011 16:35:20 »
I did a bit of a search and it seems that the frequency of microwaves and of cell phones is quite different. 800 or 1900 Megahertz for the phones (depending on location) vs 2450 megahertz for microwaves.  The standard method of blocking microwaves in is subject to a patent, and is very specific to that frequency.  Have a look at http://message.snopes.com/showthread.php?t=67936  The poster Alchemy has some good detail.

So, it seems that most microwave ovens will reduce the transmission of the energy waves cell phones use, but it is better at stopping microwaves.

Back the the podcast, Dr. Karl was talking about a Carrington Event, where the sun shoots out a unusually high level of radiation, potentially cooking almost all electronics.  He was discussing ways to protect your equipment, including setting up a Faraday Cage made of mosquito netting.  I now think that we need to know more about what frequency of radiation before we can determine how to best protect equipment.

Geezer

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Why does my phone work in the microwave?
« Reply #6 on: 27/10/2011 18:57:29 »
OK results of the test are in...

my office microwave totally blocks any signal to my phone.  Within seconds of shutting the door the signal goes from 5 bars to "searching", no calls will connect, and as a clincher I connected a call outside the microwave and as soon as the phone was shut in the box it was disconnected.

mmm - perhaps time to check your microwave

You forgot to turn the microwave on, Silly!

Geezer

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Why does my phone work in the microwave?
« Reply #7 on: 27/10/2011 19:01:20 »
I tried it too. No bars on the phone when it's in the microwave. (Mind you, the signal where I live is so crappy that there are no bars when it's not in the microwave. I was sort of hoping the microwave might fix that.)

RD

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Why does my phone work in the microwave?
« Reply #8 on: 28/10/2011 01:47:01 »
The type of microwave oven may be relevant: there may be a difference between the basic bog-standard no-frills microwave, and the deluxe versions with an electric grill, which has dozens of inch-wide* ventilation slots in the metal top.

* or bigger ...

http://www.antenna-theory.com/antennas/aperture/slot2.php
« Last Edit: 28/10/2011 02:24:48 by RD »

techmind

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Why does my phone work in the microwave?
« Reply #9 on: 12/11/2011 22:43:34 »
I'm sure we've visited this subject before. I too discovered soon after I had my first mobile phone (in about 2002) that it still received a signal inside a (not switched on) microwave oven.

A true Faraday cage should screen out all frequencies, although if it's made of a mesh then the mesh needs to be perhaps one tenth the size of the shortest wavelength you wish to block out.

However, a true Faraday cage needs to be properly conducting the whole way around (this is why when you see a real radio test-chamber it has little metal gold fingers and contacts all around the doors). Your microwave over does NOT have gold contacts all around the door, so it isn't a proper Faraday cage. What it does normally have is a special 'slot' around the door-seal (often hidden behind a piece of plastic trim) which works to block the radiowaves... but only within a narrow frequency band - 2.45GHz (I believe it's known as a "quarter wave choke joint"). As others have said, GSM mobiles normally work at ~900MHz (O2 and Vodafone) and 1800MHz (Orange and T-mobile, and O2/Voda fill-in). 3G works at 2.1GHz ... which is getting closer to microwave-oven frequencies.

So... if your phone signal is weak, then it may stop working, but if you've got plenty of signal from a nearby mast (base station) then enough signal gets in and out to make a call.
« Last Edit: 12/11/2011 22:57:28 by techmind »

techmind

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Why does my phone work in the microwave?
« Reply #10 on: 12/11/2011 22:47:29 »
You also test putting a phone inside an all-metal biscuit-tin with a metal lid. Usually you'll find the phone still works - again because the seal around the lid is not a good quality continuous electrical contact so RF potential differences can still arise and the signals escape...
If you were to scrape off all the paint, and solder the lid to the tin all round (and solder across any original seams that weren't electrically sound) then it should fully block the signals.

imatfaal

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Why does my phone work in the microwave?
« Reply #11 on: 13/11/2011 09:17:16 »
Thanks for that Techmind - it didn't seem to make an sense but I think that clarifies it

Geezer

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Why does my phone work in the microwave?
« Reply #12 on: 13/11/2011 20:27:48 »
You also test putting a phone inside an all-metal biscuit-tin with a metal lid. Usually you'll find the phone still works - again because the seal around the lid is not a good quality continuous electrical contact so RF potential differences can still arise and the signals escape...
If you were to scrape off all the paint, and solder the lid to the tin all round (and solder across any original seams that weren't electrically sound) then it should fully block the signals.

Depends  ;D.

That should block the electric component, but it might not block the magnetic component. It shouldn't really be necessary to solder the thing shut either. There can be plenty of gaps just as long as they are not too large in relation to the wavelength.

If my $8 a month T-mobile phone had any coverage where I live, I would try the experiment.

RD

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Why does my phone work in the microwave?
« Reply #13 on: 20/11/2011 03:47:07 »
Looks like the metal foil bags used for a five finger discount are folded over , i.e. a complete seal ... https://en.wikipedia.org/wiki/Booster_bag
« Last Edit: 20/11/2011 03:57:06 by RD »

Stephany

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Why does my phone work in the microwave?
« Reply #14 on: 26/11/2011 19:02:28 »
May be change of a type of cell from GSM to CDMA will change the result.

techmind

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Re: Why does my phone work in the microwave?
« Reply #15 on: 30/01/2013 23:00:41 »
You also test putting a phone inside an all-metal biscuit-tin with a metal lid. Usually you'll find the phone still works - again because the seal around the lid is not a good quality continuous electrical contact so RF potential differences can still arise and the signals escape...
If you were to scrape off all the paint, and solder the lid to the tin all round (and solder across any original seams that weren't electrically sound) then it should fully block the signals.

Depends  ;D.

That should block the electric component, but it might not block the magnetic component. It shouldn't really be necessary to solder the thing shut either. There can be plenty of gaps just as long as they are not too large in relation to the wavelength.

If my $8 a month T-mobile phone had any coverage where I live, I would try the experiment.

Even leaving aside that a perfect conductor would also block an AC magnetic field (wouldn't it?) - an EM wave *requires* both an electric and magnetic component to propagate. If you stopped the electric part and kept the magnetic, you'd still only have a near-field RF magnetic field - not a radio wave which travels for any distance.

 

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