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Microwave oven shielding

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ccurtis

Well-Known Member
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?
 
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Sceadwian

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

New Member
Forum Supporter
No, ccurtis has it right. Maybe he should check the RF seal on the microwave!
 
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Sceadwian

Banned
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

New Member
Forum Supporter
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. :rolleyes:
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.
 
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Sceadwian

Banned
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

New Member
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.
 
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Sceadwian

Banned
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

New Member
Forum Supporter
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

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

ccurtis

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

That is what I am thinking. :eek:

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

Banned
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

Banned
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

Super Moderator
Most Helpful Member
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

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

Sceadwian

Banned
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.
 
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