[u-u] microwave ovens vs. WiFi

[Adam Holland]

Well we are not going to use this microwave anymore.  (That's why I said 
you could have it to block all use of wifi)  Our microwave is nearly a 
decade old and it did not use to cause this interference.  So while all 
microwaves will leak a tiny bit, we feel this is now a health concern.

-Adam


On 2018-09-17 12:08 AM, Greg A. Woods wrote:
> At Sun, 16 Sep 2018 22:23:29 -0400, Adam Holland <uu at vl-entropy.trade> wrote:
> Subject: Re: [u-u] Suggestions for stopping occasional spurious use of commercial wi-fi
>> This is a complete coincidence, but yesterday my housemate and his
>> girlfriend realized that our microwave in the kitchen has become very
>> leaky!  When I was heating up some hot dogs, both of their cell phones
>> streaming video dropped to completely zero, and came back as soon as
>> the microwave turned off 1 minute later.  We repeated this twice with
>> shorter time spans, and the correlation is direct.
> I'm not at all surprised.
>
> In the USA the federal limit for microwave oven emissions is 5
> milliwatts (mW) of radio waves per square centimetre at approximately 2
> inches from the oven surface; and the equivalent European (CENELEC),
> limit is 50 watts per square metre (W/m2) at any point 5 cm away from
> the external surfaces of the oven (these are basically the same value,
> at lease if you equate ~2 inches with 5cm).
>
> This limit is far below the level known to harm any living thing (at
> least in terms of the effects of any short term exposure as one might
> get from staring through the oven window watching as your soup warms or
> your old CD-ROMs making pretty lightning patterns).
>
> In practice most manufactures apparently manage to keep their ovens far
> below these limits (though it has been decades since I've seen any
> actual real-world measurements).
>
> Thanks also to the inverse square law, radio wave energy decreases
> dramatically as you move away from the source of emission.  A
> measurement made 20 inches from an oven would be approximately 1/100th
> of the value measured at 2 inches from the oven.
>
> Of course that's all talking about the raw energy in the emitted
> microwave radio emissions.
>
> However if you think about it in terms of signals and interference of
> signals using the same radio frequency band, a radically different
> picture soon emerges.
>
> Regulators watching over microwave ovens are primarily concerned about
> the heating energy emitted from the device, not necessarily the radio
> noise that also ensues from these emissions.
>
> Microwave ovens are basically unmodulated carrier wave generators (and
> I'm not so sure about the "unmodulated" part -- that's more a side
> effect of their design than a goal).  I suspect some microwave designs
> are a bit more broad-band than others too, thus their wider affect on
> what should normally be multi-channel spread-spectrum WiFi radios.
>
> I think one of the reasons WiFi was initially allocated to the 2.4GHz
> ISM (industrial, scientific, and medical) radio band was because of the
> fact that the 2.4GHz band is basically a wild wild west full of noise in
> the first place (but limited by the nature of radio at those frequencies
> being unable to travel too far), so adding some computer nerds (and
> cordless phones, and baby monitors, and so on and so on and so on) to
> the mix wouldn't cause anyone else any issues, especially at the
> extremely low emission levels allowed for WiFi.
>
> If you can, you should use the 5.8GHz WiFi band to avoid the noise
> pollution almost universal in the 2.4GHz band; and make sure you place
> your access points in your house to give good coverage of the area you
> want your WiFi users to be able to range over since 5.8GHz is much more
> line of sight and is unable to penetrate the materials that 2.4GHz can
> easily get through.  There are still lots of non-WiFi ISM devices in the
> 5.8GHz band, but typically a lot of the noisier signal-interfering
> abusers like microwave ovens are not as common (mostly because it
> doesn't have such great propagation characteristics, nor does it have
> uses such as heating food).
>
>> P.S. I was going to look to borrow a microwave testing unit, but
>> actually I think I want to have one to keep, since you can't really
>> tell if a microwave is leaking until it gets that bad.  I mean, we
>> have smoke detectors and carbon monoxide detectors, so why not test
>> our microwave once per month or something?
> A microwave oven test meter will measure energy, and you can only infer
> the radio noise pollution as a side effect.
>
> Normally a microwave oven won't change its emission characteristics
> after manufacture unless its frame is bent, its door or the door hinge
> is damaged or bent, its cover removed or partly opened, or some similar
> physical structural alteration is made to it.  You can see through the
> window of the oven because the screen in the window is designed as a
> Faraday cage for the specific frequency band the oven generates, but of
> course not for visible light.
>
> I suppose you could go to an appliance store with a pair of WiFi
> tethered devices and ask to test various models by heating a large cup
> of water in them to see which ones might have the least affect on the
> WiFi connection between the devices at some reasonable operating
> distance from the oven.
>