this post was submitted on 29 Nov 2024
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The real reason is that that range is reserved for consumer devices so that it doesn't interfere with actual ISM sanctioned communications as enforced by the FCC. We just also decided to put wifi in the same range cause they're stingy releasing frequencies for public use.
https://en.wikipedia.org/wiki/ISM_radio_band#Frequency_allocations
But research was done on it cause of course it has been.
https://iopscience.iop.org/article/10.1088/0031-9120/39/1/006
So you're likely right that it's not water resonance, but chassis cavity resonance. I can't say that I've read deeply into it. And thinking about it I remember hearing something about some of the high level stuff that I just read in relation to this article. I probably ran into it in passing and just failed to recall it. But to be frank, I'm okay just calling it voodoo wizardry in of itself. But I have to understand wireless communications stuff for my profession, and it's well known that it's basically the same range as wifi 2.4ghz/bluetooth/other consumer standards that sit in the same crowded space.
There's several ISM bands, though, pretty evenly spaced. The 13.5MHz one is used for passive RF chips like on credit cards, for example. They're skinny, but for purposes where bandwidth doesn't matter they can be. For other purposes bandwidth is scarce enough there has to be tight regulation.
Actually high water absorption happens in mm wave bands up in the hundreds of GHz (and THz too, if we could make a decent transmitter). Those fucked up riot control devices that make your skin feel on fire work based on that principle, because the heat will only go deep enough to hit pain receptors. Presumably, they stop working if you get a water mixture of any kind on the window, too.