Microwave oven shielding

[ccurtis]

As I understand it, the perforated metal screen on the door of a microwave oven serves to attenuate the great microwave power (hundreds of watts) present inside the oven down to trivial levels outside the oven. The screen is a grid of circular waveguides beyond cutoff. The microwave oven operates at about 2.5 GHz, or so.

However, when I place my wireless phone inside the microwave and close the door, I can easily see that the phone rings when I call it from another phone. Since the phone operates at even lower frequencies than the microwave, I would expect the attenuation of those lower frequencies to be even greater than that for the magnetron in the microwave.

What's up with that?

 

[Externet]

Very good one !

 

[Birdman Adam]

When I first read your original post, I thought you said you put your phone in the micro and turned it on and it started to ring when you called it!!! I would expect to see a lot of sparks...

"Is it a good idea to microwave this?"

I think I saw them do a phone once...

 

[Sceadwian]

You have it backwards ccurtis, low frequencies penetrate better than high frequencies do.

 

[kchriste]

No, ccurtis has it right. Maybe he should check the RF seal on the microwave!

 

[Sceadwian]

Given equivalent shielding, a lower frequency will penetrate better than a higher one. A cell phone receiver is designed to pick up very weak signals. Microwaves ovens will effect Wifi and 2.4ghz cordless phone reception pretty heavily even shielded.
The attenuation is probably higher, the receiver is just sensitive enough to still receive.

 

[kchriste]

Given equivalent shielding, a lower frequency will penetrate better than a higher one

You can say it as many times as you like. It doesn't make it right.
Given your reasoning, you shouldn't be able to SEE your food inside the microwave then since the frequency of light is higher than microwaves. Or have you ever noticed that, as you drive over a metal suspension bridge, FM radio reception is OK, but AM radio reception can be messed up due to it's longer wavelength.

 

[Sceadwian]

With really large metal grids yeah sure kchriste, microwave ovens aren't exactly made of the structural ibeams of 1/4 inch thick steel. They block enough radiation to prevent microwaving of people on the outside, but most definitely not enough to prevent the RF that gets out from upseting my cordless phone or local wifi signals, and if you have decent signal strength on your cell phone even the high degree of attenuation the shielding of the microwave provides isn't enough to kill communication completely.

 

[sfink06]

okay so i just tried it an my phone does ring inside my friends microwave. The material MUST me set up so that low frequency signals like cell phone signals pass through, and high frequency signals like microwaves don't.

 

[Sceadwian]

sfink06 I think you're confused. Microwaves are 3 or 4 orders of magnitude LOWER in frequency that visible light, not higher.

Visible light can pass through the shielding because it has holes in it, the holes are small enough to not let too much of the microwave energy pass through, some does. The high sensitivity of cell phone receivers are what let it receive signals. Also the shielding on a microwave oven isn't foolproof because of gaps larger than a wavelength mainly the door edges. Try wrapping your cell phone in a sheet of tin foil and folding the edges, you won't get anything.

 

[kchriste]

With really large metal grids yeah sure kchriste, microwave ovens aren't exactly made of the structural ibeams of 1/4 inch thick steel.

It is not the thickness of the ibeams, it is the spacing of them that matters. AM radio is in the 0.5-1.6Mhz range which gives it a wavelength of apx 1968ft to 615ft which is too big to "fit" between the girders. Now the FM band is from apx 88 to 108Mhz which has a wavelength of 11ft to 9ft which "fits" between the girders.
Your microwave puts out a RF field with a wavelength of apx 4.5" which is too big to "fit" through the tiny screen door holes.

 

[Sceadwian]

It does however fit between the air gap in the door.

 

[ccurtis]

No, ccurtis has it right. Maybe he should check the RF seal on the microwave!

That is what I am thinking.

I assume that the field strength at the outside of the microwave is supposed to be at about the same order of magnitude of a wireless phone, by regulation for safety reasons. Given that the field strength inside the microwave is HUGE the metal screen should provide several tens of db of attenuation. I find it hard to accept that a wireless phone can receive signals several tens of db below that which it receives under ordinary conditions, though I haven't researched that.

Maybe the screen effectiveness is a near-field, and not a far field issue?

I did put the wireless phone in a metal box, just as a sanity check, and it does not ring inside the box. The same applies if a wrap the phone in aluminum foil.

 

[Sceadwian]

Well if you look at it that way, a microwave oven is say 1000 watts, to make it safe outside the microwave you need to drop that to 1watt, that's 1/1000th the power. Only 10db attenuation required. 10db is not bad from a communication standpoint as long as the available signal strength is modest, cell phone towers are everywhere and are capable of putting out decent amounts of power, and again modern receivers have come a long way.

Also think about the edges of the microwave door, they're basically long narrow slits, more than long enough to let relatively decent RF power through.

 

[Sceadwian]

Played around with my cell phone a bit more, got it into a debug mode that showed signal strength, it was within a few decibels of what it was outside, why that is I don't know, but it is. I know the frequencies that my cell phone provider uses is 1900mhz

 

[Nigel Goodwin]

You have it backwards ccurtis, low frequencies penetrate better than high frequencies do.

Perfectly true - the higher the frequency the far easier it is to block - as any radio amateur will know.

But as the door mesh of a microwave is designed to block a single specific frequency, and nothing else, then it doesn't really matter.

 

[Sceadwian]

I'm curious as to how the hole spacing was determined and what the bandwidth is.

 

[Nigel Goodwin]

I'm curious as to how the hole spacing was determined and what the bandwidth is.

No idea, I've never been that interested

But you can tear a one inch hole in it, and it still bearly leaks at that - oven leakage is a VERY overe exaggerated 'problem'.

 

[Sceadwian]

Nigel I just wanna know why it can get 20db attenuation at 2.4ghz and only a few at 1.9.... The cell phone reception where I live is pretty poor, the phone was registering only around 92db and only went up to 94-96 when I placed it in the microwave. The network here is a hybrid 1.9ghz/800mhz system.

 

[Nigel Goodwin]

Nigel I just wanna know why it can get 20db attenuation at 2.4ghz and only a few at 1.9....

I presume the mesh is tuned to 2.4GHz?