Microwave ovens and cell phones

[claudiocamera]

A concerned guy told me that the sinal strenght of a cellular radio station in his neighborhood is so intense that when he puts his cell phone into the microwave ovens, close the door and call it, it rings even inside the microwave oven.

I laught at him and told him that a microwave oven has a frequency higher than cell phones either 800, 900 or 1800 MHz, and it is, shilded to electromagnetic fiels with frequency smaller than its, so it was impossible to a signal with smaller frequency penetrate into the oven cause it is like a Faraday cage.

He challenged me to doing the experiment. I did and for my surprise the cell phone rang, not because the strenght of the base station as he tought, it rang in every oven I tested. I did the test for several ovens manufactures and in different neighborhoods, and the cell phone rang in all of them.

So, isn't the microwave ovens shielded ? If its not shielded, does its frequency scapes outiside it ? Why cell phones ring inside a microwave oven ?

 

[PRodrigues]

A concerned guy told me that the sinal strenght of a cellular radio station in his neighborhood is so intense that when he puts his cell phone into the microwave ovens, close the door and call it, it rings even inside the microwave oven.

I laught at him and told him that a microwave oven has a frequency higher than cell phones either 800, 900 or 1800 MHz, and it is, shilded to electromagnetic fiels with frequency smaller than its, so it was impossible to a signal with smaller frequency penetrate into the oven cause it is like a Faraday cage.

He challenged me to doing the experiment. I did and for my surprise the cell phone rang, not because the strenght of the base station as he tought, it rang in every oven I tested. I did the test for several ovens manufactures and in different neighborhoods, and the cell phone rang in all of them.

So, isn't the microwave ovens shielded ? If its not shielded, does its frequency scapes outiside it ? Why cell phones ring inside a microwave oven ?

The reply of the question you wrote , is that a cellphone has a very low lever sensitivity (103 dBm).
A microwave oven operates in the 1.8 - 2 GHz band like in celular bands (Dual and Tri band GSM 1.8 and 1.9Ghz).Though a microwave oven is shielded there's some
wave propagation that can´t be shielded (higher signal harmonics), thats why it's not advisable to pregnant women to infront of a microwave oven when it's cooking.
I've made the test that you described with a iron box (maximum shielded) and without any breach, the mobile phone rang.
Though if you pick up an old mobile phone of first genereations GSM for example with a higher level sensitivity the mobile phone will not ring or there's a higher probability that the mobile phone won't ring.

 

[claudiocamera]

I put the cell phone into a pan and closed it, the cell phone didnt ring. In the elevators with iron ceiling mobiles phone loose signal, the Faraday cage doesnt give any level inside. Are you sure that the microwave ovens is shielded, or does it rely on propagation modes?

 

[mr_ghz]

I did an additional test:
I put a 2.4GHz transmitter with an output power of 0dBm inside the microwave oven. With a selective receiver I measured outside the oven the recevied energy. With an opened door I received -42dBm. The I closed the door and the received power has gone down to about -85dBm. So the shielding of my microwave oven (a cheap one!) is about 40dB.

OK, lets think about the reception power of an GSM phone: In my opinion an additional loss of 40dB (increasing the distance to the base station by a factor of 6!) can't be handled of such a phone.... But it's ringing!

Why, why, why......

 

[claudiocamera]

Dear mr_ghz.

Your experiment reveals that the microwave ovens is not a faraday cage as it was suppose to be in theory . So, definitely eletromagnetic waves scape from the compartment, as it is bidirectional eletromagnetic waves outside penetrates in the compartment. According to the attenuation presented it was not possible to the phone ring, unless this attenuation is smaller in one particular direction. In order to the experiment be complete it would be a good idea to perform the measure of received power isotropically around the oven. Maybe there is a particular direction on which te atenuation is smaller ... Are you gonna try ?

 

[mr_ghz]

Hi claudiocamera

OK, I will try it, but give me some days. I also have another theory: The microwave oven is not a 'proper' faraday cage, it is a cavity exactly for 2.44GHz (the working frequency of the oven). On other frequencies the damping is much worse.

 

[dz99]

OK, lets think about the reception power of an GSM phone: In my opinion an additional loss of 40dB (increasing the distance to the base station by a factor of 6!)

Hello!

I didn't really understand your idea about distance factor! I've done a small calculation. 40 dB difference by the transmit power (ok, or recive power, let's consider reciprocal system) caused 100 times distance factor.
http://dz99.fw.hu/propag.pdf

Any idea?
Bye!
Z.

 

[mr_ghz]

Hi dz99

Yes, you are right: Doubling the distance decreases the reception power by 6dB. I assumed (for an easier calculation without a calculator) 6 times 6db = 36dB which gives 2^6 times the distance to the base station (not 6 times).

 

[throwaway18]

OK, lets think about the reception power of an GSM phone: In my opinion an additional loss of 40dB (increasing the distance to the base station by a factor of 6!) can't be handled of such a phone.... But it's ringing!

Wrong. Lets say the telephone needs at least -110dBm of signal. You just need at least -70dBm outside the oven. I used to use a phone with a test mode that give the signal strength of cells it could hear. It often picked up signals stronger than -70dBm. I think the strongest signal it showed was -40dBm which I sometimes saw when standing outside near a cell.

 

[mr_ghz]

Hi throwaway18

Hmm, I did the test with the 'conventional' RSSI bargraph of a nokia phone. Outside the microwave oven I get 4 from 5 bars, inside the oven 3 from 5 bars. I can't believe that this would be 40dB ....

 

[jallem]

That graph bars does not mean any linearity...!!!!
it is useless to compare power level.....

 

[throwaway18]

The main use of the signal strength indicator on a phone is to move somewhere else when you don' thave much signal. It seems plausable to me that the lower bars are much closer together in terms of received signal.

 

[claudiocamera]

How about the Microwaves ? The waves are suppose to reflects inside the oven, as it is somekind of hazard, it shouldn't get out of it, so, why there is a level inside it that is able to do the cell phone receive a call ? What happens at all ? Isn't the oven a Faraday's cage ? Is it a ressonator cavity ? Is the attenuation really 40 db ? What goes in on?