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Comments on Microwave oven interfering with WiFi on the 2.4GHz band

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Microwave oven interfering with WiFi on the 2.4GHz band

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As I understand it, microwave oven magnetrons operate at 2.45GHz, which is an unlicensed band in most of the world. When 2.4GHz technologies such as WiFi (802.11), Bluetooth and Zigbee were launched, there were concerns that these would collide with microwave oven frequences. I remember a very early (year 2000-ish) Bluetooth evaluation project where I amused myself by placing two Bluetooth modules on each side of a running microwave oven and couldn't get any form of Bluetooth traffic working.

But are microwave ovens really allowed to cause interference at longer distances, such as jamming out WiFi located in the same room or building? My take is that they should count as non-specific SRD (short range device). I'm located in Europe, so in that case the 2400-2450MHz unlicenced, harmonised band should apply - this is standardized by the European Radiocommunications Committee within the whole of EU. If so the SRD standard EN/ETSI 300 440 for non-specific SRD (sorting under the new RED directive) should apply. Is this correct so far? Or do microwave ovens have their own specific technical standard?

ETSI standards are conveniently available for free, so I checked v.2.1.1 of EN/ETSI 300 440. Chapter 4.2.4.4 regarding limits of unwanted emissions in the spurious domain are limited to 1uW for frequencies > 1 GHz. I'm assuming this is 1uW E.R.P, so roughly -30dBm E.R.P. The spurious domain is defined in complex ways by this standard depending on bandwidth from carrier. I don't think a device that disturbs with -30dBm E.R.P. would affect WiFi or any other radio technology much at all. Or would it? Would the "listen before talk" RSSI limit of WiFi cause any problems here?

Could any radiated emission from a microwave be regarded as an intentional radiator? Does it have a designated carrier and occupied band, where it may act just as any other SRD? I'm assuming that the microwave oven occupied band is very wide, several 100MHz.

If so, then what happens when the microwave occupied band (+/- x MHz from carrier) collides with the WiFi occupied band? There are a couple of (European) WiFi channels that overlap with 2.45GHz specifically. Since WiFi (and Bluetooth) use spread spectrum technologies, would that mean that WiFi skips certain channels, resulting in reduced bandwidth?

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General comments (1 comment)
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Microwave ovens work on the frequency they do because that's one of the main resonant frequencies of water molecules. The radiation excites water molecules, which then transfer some of their extra energy to other surrounding molecules.

No, microwave ovens are not intentional radiators, at least not in the legal sense here in the US. Yes, they use radiation internally, but there is not purpose or intent of that radiation getting outside the unit. So unless there is a specific exemption for microwave ovens in the law (haven't checked), they have to comply with the same maximum radiation limits any other device does. I don't have time right now, but maybe later I'll look the applicable FCC part 15 rules and see what that radiation limit is and exactly how it is defined.

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From a very cursory look, it seems like a home microwave oven in the United States is not allowed to produce external radiation exceeding 500 µV/m at a distance of 3 m at the microwave frequency.

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General comments (7 comments)
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Lundin‭ wrote about 4 years ago · edited about 4 years ago

I read some loose rumour on the net about FCC having a special rule for microwave ovens, something about allowing 5mW in the near field, no idea if that's true (and I can't find anything about a similar rule in Europe either). However FCC part 15 generally allows a carrier of ~0.75mW ERP for short range devices on most licence free bands. I don't remember the exact number, it's likely expressed in dBuV/m or some tricky unit like that.

Lundin‭ wrote about 4 years ago

Bit of research about the equivalent rules in the US. https://www.fcc.gov/oet/ea/rfdevice claims that "these devices fall under the FCC rules 47 CFR Part 18". I'm not quite able to weed out which parts of Part 18 that are applicable though, §18.305 speaks of field strength of emissions for "Induction cooking ranges" and says 300uV/m (30m). If that's the applicable rule, then they are allowed to leak a very low field strength. Which seems unlikely to affect WiFi.

Lundin‭ wrote about 4 years ago

That is, assuming that the oven and WiFi router both have FCC approval. Which I wouldn't automatically assume to be the case, some may just have "Ali Baba approval".

Bruce Hansen‭ wrote about 4 years ago

Microwave ovens operate by dielectric heating of water and fats. The 2.4GHz operating frequency has nothing to do with water molecule resonance - in fact "industrial" microwave ovens often operate at around 900MHz.

TonyStewart‭ wrote about 4 years ago

false assumption on resonant frequency of water. -1

TonyStewart‭ wrote about 4 years ago

@Olin's assertion to resonant frequency is totally bogus. Water has absorptive properties over many decades of f without this resonance. http://www1.lsbu.ac.uk/water/microwave_water.html#loss This is a good reference but fails to discuss the ionic behaviors of contaminants.