[u-u] microwave ovens vs. WiFi

[Greg A. Woods]

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.

-- 
					Greg A. Woods <gwoods at acm.org>

+1 250 762-7675                           RoboHack <woods at robohack.ca>
Planix, Inc. <woods at planix.com>     Avoncote Farms <woods at avoncote.ca>