# What size of hole do microwaves eventually pass through?

I've been searching high and low across the internet and haven't found an answer for this.

I have a microwave I got from a friend who wants me to repair it (he REALLY does not want a new one). It had a bad magnetron, which was a simple fix. I closed it all up and tested out the radiation being emitted from it using a simple little device that measures microwave radiation (I got it here). It appeared to be emitting a very unhealthy amount of radiation.

Upon closer inspection, the top of the microwave has several tiny holes on it for heat dissipation?, I think? These holes were put in by the manufacturer.

I was wondering, should I patch these holes? And using what? Is screwing on a metal plate over those safe enough?

EDIT: The microwave is 900 watts. And here is a photo of the holes in question: https://s2.postimg.org/7ft2p174p/IMG_0176.jpg

• Are you sure you replaced the bad magnetron with the right one? It's possible that you have a more powerful magnetron in the microwave that it's not rated for. Also, a picture of the holes would help. – Envidia Mar 16 '17 at 19:01
• I feel this is not clear enough from my answer: THIS IS ACUTELY DANGEROUS. Your questions indicate you have very little idea of what you're doing, but are still handling very poisonous materials (magnetrons when broken), high voltages, devices that can cook your brain, eyes and reproductive organs, and fire hazards. LEAVE THE MICROWAVE OVEN ALONE. – Marcus Müller Mar 16 '17 at 19:13
• they usually pass thru any tiny gap in long edges. how well does it pickup your mobile? – Tony Stewart Sunnyskyguy EE75 Mar 16 '17 at 19:46
• It is unlikely (with the caveat that I am not a physician) that you will get berylliosis, but do take note of the symptoms if you touched anything vaguely pink in the magnetron which was physically broken or abraded. Try to avoid beryllium compounds from now on. On the bright side, it might have been a cheapo magnetron with no Be in it. – Andrew Morton Mar 16 '17 at 19:56
• I suspect your "leak" is related to your work. Do you know what a waveguide coupler is? If not, then I wonder that you installed the replacement magetron correctly and/or re-tuned it properly with the waveguide. – jonk Mar 16 '17 at 20:00

Wikipedia "cutoff frequency" provides formula[along with derivation of formula] $$omega(cutoff) = C * 2.4088/diameter$$

Dividing speed-of-light by a 1cm hole (0.01 meter), we have 300Million meter/second divided by 0.01 = 30Billion radians/second.

Then restore the 2.4, and you have 72Million radians/second.

Divide by 6.28, and find 12GigaHertz is the lowest that comes thru 1cm hole.

Hence those holes in the front view port are << 1cm.

• would you have the actual link to said wikipedia page? Notice that with cutoff frequency, we do usually mean the minimum frequency that can losslessy propagate through a wave guide, not what fits through a hole and then spreads into a room. totally different things, so I'd say, without knowing better, that your answer does give the right order of magnitude of wave frequencies that can successfully pass through such holes, but neither for the right reason, nor actually the right value (since I'd assume it's the wrong formula, but can't tell without your source). – Marcus Müller Mar 17 '17 at 10:18
• go to Wikipedia "cutoff frequency"; the article is for a waveguide; is a hole in a sheet of metal a waveguide? Must the waveguide be 5 wavelengths long, or 50 wavelengths long, for the formula to be useful? – analogsystemsrf Mar 17 '17 at 14:21
• Sometimes one must spend 2 days in a lab, with quiet night time nightmares, before the random neural firings create new perceptions of what is possible, what is unknown, with some vague comments by one's professer who mentioned some new phenomena you chose to ignore, not doing the homework problems, and the night time neural firings -----led to a daytime literature search. Again, the lab time is needed, as is quiet time, and discussion time, and time to read, and time to scribble on chalkboard. – analogsystemsrf Mar 17 '17 at 14:43
• By the way, I got to "cutoff frequency" by first reading "evanescent waves", which are how totally-reflected waves behave: the waves penetrate and create a complex-number-propagation wave that rapidly fades out in the new media. – analogsystemsrf Mar 17 '17 at 15:27

Wah! WAH!

First of all, your 29\$ device is not a substitute for proper RF qualification. It simply isn't. You might be able to find a concentrated leak in the expected frequency range with that, but all you said is that you tweaked around with the magnetron, probably replaced it, and might have to deal with different frequencies now. All in all, you're also putting your health in the hands of a device without any specs on sensitivity whatsoever. I hope you don't do that with medicine, cars, building material… or else, people will die. This device is snake oil, nothing more. So, whilst I strongly recommend you do not take it displaying "everything's OK" as an OK for things being actually OK, it displaying "danger" might be a good indication to quickly back up, turn off the microwave (eg by opening the circuit breaker), unplugging it, and putting it on the trash, where it belongs as a device that can, literally, fry your brains or cook your gonads.

Then, magnetrons are dangerous, as they contain very poisonous elements that they expose when broken. Please dispose of the broken Magnetron carefully.

Also note that microwave ovens have high-voltage generators inside. It's not very likely, but your modification might lead to those generating discharge that leads X-Ray irradiation. Please be careful.

You're clearly not qualified for this kind of work, so please avoid harming yourself and others. I know this sounds harsh, but right now I'm mostly worried about your and your friend's health.

These holes were put in by the manufacturer. I was wondering, should I patch these holes?

How on earth do you think that's a good idea. If your microwave is leaking radiation, then surely not through the holes the manufacturer made, but through some defect (corrosion? your unqualified labor?) or other effect. Plugging holes that you think are for thermal regulation will, at the very best, lead to a fire hazard.

Seriously, leave the microwave alone.

• Although it's the right thing to do, your warning is also stealing a very competent contender from the claws of this year's Darwin Awards. – Enric Blanco Mar 16 '17 at 19:28
• What are the very poisonous elements that magnetrons contain? – Peter Mortensen Mar 16 '17 at 20:09
• @PeterMortensen At a minimum, beryllium oxide (c.f. aluminium oxide). – Andrew Morton Mar 16 '17 at 21:17
• Ok, wow, a lot has happened here since I last checked. When I took it apart, I did discharge the capacitor and my replacing the magnetron was done using gloves, eye protection and a respirator mask. I made 100% certain that my replacement did not alter the case at all. The magnetron was properly installed, and my replacement was done correctly. I will admit, I am not an expert at this stuff, but I've repaired microwaves before without this happening. And when I said "patch the holes" I meant screw a piece of metal over them. As I said, I think they are for heat dissipation, I am not positive. – ditheredtransparency Mar 17 '17 at 2:56
• The microwaves I've worked on in the past never had this. Lastly, I talked to my friend about this. He said, "I don't f****** care, do whatever you can so I can use this thing safely." I hate asking this a second time, but how bad would it be if I just screwed that plate on? That seems to me to be the most effective solution, as that's where the most radiation appeared to be leaking from. Also, what about my question I asked? What size holes do microwaves eventually pass through? – ditheredtransparency Mar 17 '17 at 3:10