Semi-Scientific Microwave experiment

[kb0nes]

So the other day I was perusing the Internet and I came across a posting that stated if you placed a cellular telephone in a microwave oven it would still ring when called even with the door shut. This was used as proof that a microwave oven has inadequate shielding. Of course I had to try it...

Yes indeed it did ring. But two thoughts occurred to me. First, my work is only a couple hundred yards from a T-Mobile cell site. Secondly, the design frequency for the door trap in a microwave oven ~550Mhz above that of my cell phone. Nothing has 100% shielding, but how much isolation does a microwave oven actually have?

I needed a better test but unfortunately any RF test gear that I have signs off above 1Ghz so that was out. Then it hit me that 802.11B WiFi frequencies are in the range of the oven! The usual microwave design frequency falls into WiFi Channel 9. So I decided to test with my existing wireless network.

 

[kb0nes]

First test; Door Open

So to run this test I used the WiFi Analyzer App on my Kindle Fire tablet. I realize that this isn't a lab quality instrument but for a relative measurement it should give some idea of levels we are seeing.

I did the test with my existing network which is on WiFi channel 6 which would be just a bit below the design frequency of the microwave oven (but far closer then PCS cell frequencies). The access point was in the next room about 15 feet from the microwave.

As indicated by the display I got a signal level of -45db with the door open. The Kindle was in the middle of the cavity, standing waves would alter these numbers.

 

[kb0nes]

Second test; Door closed

OK now with the door closed.

We see -87db

So effectively we are measuring an isolation of 42db with the oven closed. This number could vary a bit with placement of the device inside the oven.

If we go on to do the math assuming some numbers for sake of discussion:

900 watts RF power energizing the cavity less the 42db isolation means that ~.057 watts (57mw) of RF energy would reach the outside world. Pretty minimal all in all.

For comparison sake, the cellular device that many of us hold against our heads emits ~200mw at its highest output. Also keep in mind that RF field density falls off rapidly with the square of the distance as we move away from the source. From this we can see that the risk from RF exposure from the cell phone is likely many time any possible risk from a microwave oven. Interesting paradox that the original post I read was using a cellular telephone to "prove" a microwave oven is an exposure risk!

73, Phil

 

[philr]

That's an interesting test!

I tried the cell phone in a microwave some time ago and my old cell wouldn't ring inside one but the new cell phone does ring... I tried with old and new microwave ovens and got the same results (new phone rings in every of microwave oven I tried and the old one didn't!).

 

[kb0nes]

Another microwave tested

I repeated the same test at home in a more modern Kitchenaid rangehood convection microwave.

This time I only got 29db of isolation! If I assume the same 900 watt level inside the cavity there is the potential to have 1.13 watts of RF leakage. That almost seems a bit excessive. I'll have to borrow a friends spectrum analyzer and see if I can see what the true levels of leakage are. In any event it would be interesting to validate my numbers!

I tried the cell phone trick at home also and it rang when I called it. I'm about 1/4 mile from the T-Mobile gap filler site here at home. The signal display on the phone dropped from full scale to one bar when the door was closed.

 

[philr]

I have to say that I re-tested a few microwave ovens since my last post with another older cell phone and it also rang on all the microwave ovens I have...

 

[combo52]

Cell Phones In MW Ovens

I tried my new I-Phone in all three MW ovens in my kitchen and it would not ring in any of them, where I live I do not get really strong reception from my provider, Sprint. Maybe I should retry the test with the MW oven running, LOL.