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Comment by flatulent1 | Hacker News Reader

I'm surprised the article didn't mention that a ham band partly overlaps the 2.4 GHz WiFi frequencies. In theory at least, anyone operating WiFi gear isn't allowed to interfere with licensed services, including hams. Also, our unlicensed WiFi gear must accept any interference including that which "may cause undesired operation"... from anything, including the neighbors WiFi. (see FCC notice in U.S. WiFi product manuals)

Another related tidbit: Microwave ovens getting water molecules so excited at 2.4 GHz ties in with WiFi performance. It's a tidbit the head tech at a local wireless ISP didn't even know... 2.4 GHz WiFi is particularly prone to fading in fog/rain because water absorbs energy so efficiently at that frequency.

If they really want to get into strange frequency trivia, they should explain why Channel 37 has never been used by any over-the-air television station in Canada or the United States. Hint: little green men

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NASA publications indicate a microwave absorption peak due to water vapor occurs at 22.234 GHz and peaks due to oxygen occur lear 60 GHz and 118 GHz (..). Below 10 GHz absorption caused by atmospheric gases is small.

The associated absorption graphs do indicate there is a small peak of 2 dB per kilometer around the 2.4 GHz band due to water. (I'd expect to have a larger drop through a wall.)

From: NASA Reference Publication 1108(02) 1987; "Propagation Effects on Satellite Systems at Frequencies Below 10 GHz; A Handbook for Satellite Systems Design; Second Edition" by Warren L Flock.

I wonder what level of humidity that 2 dB per kilometer figure is for? Is that typical or worst case?

At 20 kilometers (about 12.4 miles), a distance this ISP is trying to serve some customers at, that 2 db becomes 40db. Overcoming an additional 40 db loss would require increasing the power by a factor of 10,000 times. They're using amplifiers and directional antennas but the fade margin is nowhere near 40 db. And they do have fading problems. Of course refraction/reflection from temperature/humidity inversion gradients and other propagation effects come into play too. It isn't just simple absorption.

Are we still talking about WiFi? The one with coverage limited in fractions of a kilometer? Achieving 20 km coverage radius (or even targeted coverage) sounds like a sisyphean task to me.

I dont know how they do it but the longest range wifi link is 382km[1]. And 10 km with good directional antennas is not that special, really.

[1] http://www.slideshare.net/1ereposition/long-distance-wifi-tr...

Wow, that's actually kind of exciting (and disturbing!), even though the link is unstable. Thanks for the info.

Hoff, which graph are you looking at? I'm looking at page 20 of chapter 3 (http://descanso.jpl.nasa.gov/Propagation/1108/1108Chapter3.p...) - it does show a small peak but it looks to me as if it's at ~22 GHz, not 2.4. Note that it lies between "10" and "10²" on the X-axis. Also the magnitude of even that peak is only 2 x 10 to the minus 1 - i.e. 0.2 dB per km, and the nearby freqs are really not much different. The graph on page 21 agrees.

This article: http://www.martin.chaplin.btinternet.co.uk/microwave.html suggests that 2.4 GHz was chosen for microwave ovens as a NON-optimal absorption frequency, as this allows some of the energy to penetrate past the outermost layers of food!

>Another related tidbit: Microwave ovens getting water molecules so excited at 2.4 GHz ties in with WiFi performance.

Does this have safety implications? I mean, if I stay too close to my wifi router can I be damaged?

No. Because it doesn't work that way. See comments above.

And that would be your Part 97 divide: amateur radio.

Your consumer gear runs under Part 15. You cant generate interference, and you accept all interference. It's cut and dry, as in you have no recourse.

Amateur radio, on the other hand, have legal powers to stay on air, and even kick you off. Legally, a ham can command under Part 97 and your Part 15 to turn off your receivers (a good receiver is also a weak transmitter). Now the common courtesy in the ham community is to troubleshoot their problems so everybody benefits. But prior is the law.

As per H2O attenuation, that's way up there in Spark Gap frequencies... around 100GHz. What you're facing is perhaps at most +12dBi from your antenna and -40dB loss due to the good ol inverse square law.

The Water heating in the microwave and associated to 2.4GHz is false. When you have a polar molecule (liquid water) and expose it to alternating EM fields, they gain kinetic energy and warm up. Any polar molecule will exhibit this behavior when exposed to an alternating EM field.

Microwave ovens getting water molecules so excited at 2.4 GHz ties in with WiFi performance. It's a tidbit the head tech at a local wireless ISP didn't even know... 2.4 GHz WiFi is particularly prone to fading in fog/rain because water absorbs energy so efficiently at that frequency.

No, no, no, no, no.

http://earth.esa.int/applications/data_util/SARDOCS/_icons/c...

Well, that's not very helpful, because he's not talking about the standard atmosphere, but about a particularly humid atmosphere.

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