Hacker News Reader

Comment by lucb1e | Hacker News Reader

Sameish here. I get the significance, like, it's cool in general to be able to receive other things when the rain somehow scatters it, but I understand very little about why rain makes it possible. Isn't rain supposed to block 2.4GHz and nearby frequencies? Is 10GHz also affected? Why doesn't it block but reflect? Does the moving of the rain have anything to do with it, is that why they're seeing a 20m/s doppler effect? Speed of light in water is maybe different, does it refract and then come out on the other side after having moved down? If someone could give a "Too Dumb; Didn't Understand" version of the article, I'd be interested!

View on HN

https://en.m.wikipedia.org/wiki/Radio_propagation covers it a little bit. The short answer is that wavelength(aka frequency) vs raindrop size causes signals to scatter instead of attenuate and you can pick up a fraction of the original signal.

To add to what others have said, it's a common misunderstanding that water somehow has a peak absorbtion of radio waves at 2.4GHz because microwave ovens use that frequency. There is, however, nothing special about 2.4 GHz and in general attenuation of radio waves in water increases with frequency.

Since nobody else answered you, yes, the falling rain creates the observed Doppler frequency shift. This guy must have some pretty good instrumentation to be able to accurately detect a 700Hz shift in an X-band carrier.

Rain itself doesn't "block" 2.4GHz signals per se, though any obstruction will cause some degree of radio wave attenuation. Attenuation is higher at 10GHz and above, since the wavelength is shorter: higher frequency = shorter wavelength, and shorter wavelengths are obstructed by smaller objects.

That being said, wet trees are more effective at attenuating non line of sight radio signals than dry ones, even at 2.4Ghz and below. So while the raindrops themselves have minimal effect for sub-10GHz signals, you can see signal fade in wet conditions.

Reply on news.ycombinator.com