You guys are smart, and some quick googling got me nowhere. Why baby monitors, WiFi and Bluetooth … All use same 2.4GHz frequency? I understand this is quite good frequency for home, indoor use and wall penetration etc…
But why is it exactly 2.4GHz? Why does WiFi not use 2.2GHz, bluetooth 3GHz and baby monitors 2.6GHz or 1.7GHz?
I have two hipothesis but I cannot validate them:
- For some reason this specific frequency is the only allowed for commercial use and other are used for military or other purposes?
- Interference would still happen if they would use mentioned frequencies since they are “close” together. If they use the same frequency, they can communicate and segment space more efficiently. But I do not know if this is actually happening.
Since radio spectrum is shared globally because radio waves don’t follow political boundaries, it’s coordinated by a UN body called the ITU, the International Telecommunications Union.
There are a few frequency ranges set aside for so-called ISM (Industrial Science and Medical) use.
The 2.4 GHz band was set aside for such use and together with Wifi, Bluetooth and baby monitors, it’s used for all manner of other things.
Since the range at low power is quite short, many devices can share the same frequency and not interfere with each other.
Here’s some background.
It’s because of microwave ovens. These were designed to work at 2.45ghz, because water absorbs energy efficiently at that frequency. Thus when the laws were written about what parts of the spectrum could be used for what and what sort of licenses were required a band around 2.45ghz was cleared for all use. The idea was anything in that band would be microwave ovens and having to license your oven would be a bit annoying. But then when consumer electronics wanted to do extremely short range radio, they all flocked to that band because they wouldn’t need to license it. So it saved on costs and regulations they would need to follow. So wifi became 2.4ghz and many other things as well.
A similar thing happend to the 433mhz band, except that one was specifically deregulated to encourage innovation and usage. So that band is also used a lot.
You’re pretty close. Yes water absorption and therefore microwave ovens, but it also means that 2.4GHz is a terrible frequency to use for commercial radio. Leaves are full of water, as are clouds, and rain, which means long range propagation generally sucks. So it was reserved legally for Industrial, Scientific, and Medical uses, and it’s mostly a free for all with regards to transmission and interference.
You’re also correct that the band being unlicensed led to wide adoption in short range consumer radios due to lower costs. There is still compliance testing, but it’s done for things like the right to use the official WiFi logo, not to appease the FCC.
Spectrum management. Your option 1.
It’s also a pretty good middle ground for signal propagation and energy use and speed and that sort of thing as far as I know.
Because of law. The law is that you can’t just transmit radio. The governments of the world have enclosed the electromagnetic commons. Transmitting radio is illegal except as the law allows it. 2.4Ghz is a carve out in the law for unlicensed use. In the USA we call that portion of the law “Part 15”
https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A/part-15
Bluetooth because it’s named after Harald Blåtand and it was two point from end to end so the first and last letters B and D are 2nd and 4th in the alphabet so 2.4 and WiFi because it is Bi Directional so that is 2 and D=4 as above, also 2.4. Funny coincidence, really.
Also the width of the pyramids and the distance to the far side of the moon.
