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Bluetooth and WiFi both borrow their spectrum from microwave ovens. It's typical and expected that microwave ovens will cause some interference with other users

Bluetooth and WiFi both borrow their spectrum from microwave ovens. It's typical and expected that microwave ovens will cause some interference with other users of the 2.4GHz ISM band that are very nearby. Microwaves operate at very high power levels and are required to be shielded for human safety, but the permissible leakage power is relatively high compared to typical WiFi and Bluetooth devices---there's a simple reason why. From a legal perspective, Bluetooth is essentially pretending to be a microwave oven and making use of the permitted leakage power. This is the cost of the historical regulatory situation that most of these unlicensed radio services use the ISM bands originally allocated for microwave heating. One of the advantages of newer WiFI standards, particularly WiFi 6E, is that they finally change this situation by using the U-NII bands allocated specifically for unlicensed short-range digital communications, rather than for microwave heating. Mind that this is all in the context of US spectrum regulations, although other countries have largely harmonized their approach. I have a lengthier treatment of the topic here: https://computer.rip/2022-04-14-unlicensed-radio.html https://computer.rip/2022-04-14-unlicensed-radio.html

p_l 3y ago
The international allocation of 2.4GHz spectrum for microwaves happened at lobbying of USA, but is global - and exists in order for airplane galleys to be equipped with microwave heaters and be allowed to travel internationally.
- ThePowerOfFuet 3y ago
  Are they actually microwave instead of convection?
  - p_l 3y ago
    Depends on equipment of the plane - the original reasoning for making 2.4GHz the international dumpster spectrum was to install microwaves
    - jbverschoor 3y ago
      But we need to switch off WiFi and Bluetooth
      - p_l 3y ago
        That's because the equipment installed in the galley is going to be tested for operation with the specific aircraft. Meanwhile phone radios (not WiFi, not Bluetooth) used to have interference issues with various aviation systems (with some 5G bands still causing issues). Additionally, outside of interference issues from phone systems (and various other non-ISM band radios), the order helps preventing people from doing stupid things with electronics during takeoff and landing, where sudden deceleration can cause things to go awry.
        nucleardog 3y ago
        The restriction on cellular phone use on a plane comes from the FCC not FAA. As far as the FAA is concerned as long as you can demonstrate (through standard tests) that a class of aircraft is tolerant of electronic devices (which the manufacturers can do), it’s up to the airline if they want to allow it. They’ve even greatly simplified the process due to the explosion of different electronic devices over the last couple decades. This covers your cell phone in the same way it does an e reader or anything else. [0] There are basically no substantiated instances of portable electronics interfering with aircraft. Certainly not the number you’d expect given the massive number of devices that transit on a massive number of planes every day at this point. The restriction on usage comes from the FCC, applies specifically to using land mobile frequencies from aircraft, and is not out of concern for safety of the aircraft. Things getting up into VHF, UHF and above are “line of sight” propagation. If you can draw a straight line between two points without hitting anything that will attenuate the signal… it’ll pretty much go anywhere. Voyager 1 is transmitting 15 billion miles to us on 8GHz with a 23 watt radio. Your cell phone can put out around 3 watts. People called in to ham radio nets in Vancouver, BC from Seattle, WA on a 5W handheld radio just by going to the top of the Space Needle. People regularly make contact with the ISS or satellites on similar power. A large limiting factor to the distance is the height of the transmitter. Thanks to the curvature of the Earth, over enough distance, that straight line between the two points will begin to intersect the Earth which works great to attenuate the signals to nothing. Getting higher up lets you "shout" further out. Your cell phone transmitting at 3 watts from that kind of altitude is potentially hitting dozens or hundreds of cell towers. Frequencies which are intended to be shared among inhabitatants in a maybe dozen mile radius now need to be shared across hundreds of miles, reducing the bandwidth available for everyone. And the weak signal will cause interference substantially further. The FCC limited transmissions while on an aircraft to prevent interference with ground communications, and continues to because there isn’t enough evidence that they won’t. [0] https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_91.21-1D.pdf https://www.faa.gov/documentLibrary/media/Advisory_Circular/...
- jjtheblunt 3y ago
  it's kind of funny because water resonates in that band, international consensus or not?
  - Hello71 3y ago
    https://en.wikipedia.org/wiki/List_of_common_misconceptions https://en.wikipedia.org/wiki/List_of_common_misconceptions: > Microwave ovens are not tuned to any specific resonance frequency for water molecules in the food, but rather produce a broad spectrum of frequencies, cooking food via dielectric heating of polar molecules, including water. Several absorption peaks for water lie within the microwave range, and while it is true that these peaks are caused by quantization of molecular energy levels corresponding to a single frequency, water absorbs radiation across the entire microwave spectrum.
    - jcrawfordor 3y ago
      I would contest Wikipedia's description here a bit because I think it confuses the issue of microwave emissions. A number of ISM bands were allocated for RF heating applications originally, 6.78MHz, 13.560MHz, 27.12MHz, 40.68MHz, 915MHz (commonly referred to as 900MHz), 2.45GHz (commonly referred to as 2.4GHz), 61.25GHz, 122.5GHz. These are just examples taken from my NTIA chart which doesn't call all of them out. The lower ones are largely historic, the very first RF heating experiments used HF and VHF low band which were easier to produce with the radio transmitter technology of the time, but not very efficient at all. The invention of the magnetron changed that, suddenly it was much easier to produce microwave radiation at high power levels, and so the inefficient HF/VHF RF heating devices have all but faded away (there are some specific technical applications that remain exceptions, as usual). The low frequencies are inefficient and difficult to produce with compact electronics, the high frequencies aren't very attractive for food heating applications because of the limited skin depth. So every microwave oven you're likely to run into operates at 2.4GHz, which is pretty much the sweet spot for food heating among the allocated ISM bands. That band is defined as 2.4GHz through 2.5GHz. So I quibble with describing microwaves as "broad spectrum." Magnetrons do not produce very narrow output, one of the reasons they aren't often used for radio transmitters today, but microwave ovens are required to constrain their meaningful output to within 50MHz of 2.45GHz. 100MHz is a lot of bandwidth from a modern radio communications perspective, but isn't really that wide from a perspective of physical effects. 2.45GHz was chosen as an ISM band in part because it had good properties for heating, but it wasn't put exactly on a resonance frequency for water or anything like that. The exact details of the selection process are obscure but 2.45GHz was already being used for experimental microwave heating before the ISM band was allocated, and I would imagine came out of some combination of ease of magnetron construction and reasonably good heating properties. It is documented, for example, that the EHF bands were never popular for consumer microwave heating because of poor efficacy with food, although they do have industrial applications (especially in welding). Given the history of the topic there's a decent chance that 2.45GHz came about because it was being used by experimental radar at the time, the main thing that magnetrons were being built for. Microwave heating was basically a byproduct of radar development during its early days of development.
ethbr1 3y ago
And on the follow-up, "Why use 2.4(5) GHz for microwaves?" Because it happens to be a convenient frequency that water absorbs readily, which is the easiest way of heating up what we'd want to heat with a microwave (read: food). https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water#Microwaves_and_radio_waves https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_... And once we'd polluted the frequency for any stable commercial use, why not use this weird carve-out for a little thing called WiFi? Rabbit hole: Apparently the precise mechanics of EM heating of molecules are surprisingly complex, and offer a number of frequency options. But this would have been the 1950s(?), so I'd assume they empirically determined a balance of functionality + technical feasibility for water, called it a lunch, and went to have five martinis in a practical fashion.
- LarsAlereon 3y ago
  To build off your rabbit hole, I've read about using the way microwaves selectively heat molecules in cooking. In particular, they can heat oil molecules in a way that increases flavor extraction without causing thermal breakdown of the flavors like uniform heating would.
- sleepybrett 3y ago
  To further build off your rabbit hole, this is also why 2.4ghz long range wifi connections (like via a yagi antenna) is also problematic. Foliage fucks with your range. Also why central air conditioners and wet walls create wifi shadows.
  - 2rsf 3y ago
    But the rabbit hole goes further. Higher frequencies are more susceptible to wall in the middle problems, you can see that easily with 5GHz WiFi, lower frequencies might be better but require larger antennas and might suffer from other limitations. As everything else in engineering this is a compromise.
- jodrellblank 3y ago
  > "But this would have been the 1950s(?), so I'd assume they empirically determined a balance of functionality + technical feasibility for water, called it a lunch, and went to have five martinis in a practical fashion." Or maybe they just wanted to revive frozen hamsters more humanely: "I promise this story about microwaves is interesting." by Tom Scott, about scientist James Lovelock - https://www.youtube.com/watch?v=2tdiKTSdE9Y https://www.youtube.com/watch?v=2tdiKTSdE9Y
pcthrowaway 3y ago
I've read that the way to determine if your microwave is properly sealed is to put a phone inside of it, close the door, and then try calling it. I've been unable to get the phone to ring last time I did this ~12 years ago, though if there's some expected leakage, I suppose proximity to the cell tower is a factor
marcus_holmes 3y ago
Could I make a legal bluetooth jammer that is actually a small microwave oven then?
- quickthrower2 3y ago
  Probably not legally.
discordance 3y ago
If anyone’s interested in seeing the leakage on a spectrum analyzer: https://youtu.be/tToMYkO7Gkw https://youtu.be/tToMYkO7Gkw
jchw 3y ago
Also, to the best of my understanding, the only known risk to human safety of a microwave without proper shielding is injury due to the heating of human tissue. Not like that's a good reason to ignore any potential health concerns, but I used to have the misconception that it was much more dangerous, e.g. could cause cancer, but, of course, ionizing radiation is much, much higher frequency.
- alexdbird 3y ago
  The heating part can still be very bad news for your eyes, being the most delicate exposed tissue.
kalleboo 3y ago
And now my new microwave actually has Wi-Fi built-in, we've come full circle.
- deleted 3y ago
  [deleted]
- stephen_g 3y ago
  That's probably a good thing, because it means they have to take extra care to make sure the shielding is good enough that microwave energy used for cooking won't interfere!
  - kalleboo 3y ago
    Except it disconnects from the Wi-Fi when running, and only re-connects afterwards to send the notification about completion! A total cop-out...

p_l 3y ago

The international allocation of 2.4GHz spectrum for microwaves happened at lobbying of USA, but is global - and exists in order for airplane galleys to be equipped with microwave heaters and be allowed to travel internationally.

ThePowerOfFuet 3y ago
Are they actually microwave instead of convection?
- p_l 3y ago
  Depends on equipment of the plane - the original reasoning for making 2.4GHz the international dumpster spectrum was to install microwaves
  - jbverschoor 3y ago
    But we need to switch off WiFi and Bluetooth
    - p_l 3y ago
      That's because the equipment installed in the galley is going to be tested for operation with the specific aircraft. Meanwhile phone radios (not WiFi, not Bluetooth) used to have interference issues with various aviation systems (with some 5G bands still causing issues). Additionally, outside of interference issues from phone systems (and various other non-ISM band radios), the order helps preventing people from doing stupid things with electronics during takeoff and landing, where sudden deceleration can cause things to go awry.
      - nucleardog 3y ago
        The restriction on cellular phone use on a plane comes from the FCC not FAA. As far as the FAA is concerned as long as you can demonstrate (through standard tests) that a class of aircraft is tolerant of electronic devices (which the manufacturers can do), it’s up to the airline if they want to allow it. They’ve even greatly simplified the process due to the explosion of different electronic devices over the last couple decades. This covers your cell phone in the same way it does an e reader or anything else. [0] There are basically no substantiated instances of portable electronics interfering with aircraft. Certainly not the number you’d expect given the massive number of devices that transit on a massive number of planes every day at this point. The restriction on usage comes from the FCC, applies specifically to using land mobile frequencies from aircraft, and is not out of concern for safety of the aircraft. Things getting up into VHF, UHF and above are “line of sight” propagation. If you can draw a straight line between two points without hitting anything that will attenuate the signal… it’ll pretty much go anywhere. Voyager 1 is transmitting 15 billion miles to us on 8GHz with a 23 watt radio. Your cell phone can put out around 3 watts. People called in to ham radio nets in Vancouver, BC from Seattle, WA on a 5W handheld radio just by going to the top of the Space Needle. People regularly make contact with the ISS or satellites on similar power. A large limiting factor to the distance is the height of the transmitter. Thanks to the curvature of the Earth, over enough distance, that straight line between the two points will begin to intersect the Earth which works great to attenuate the signals to nothing. Getting higher up lets you "shout" further out. Your cell phone transmitting at 3 watts from that kind of altitude is potentially hitting dozens or hundreds of cell towers. Frequencies which are intended to be shared among inhabitatants in a maybe dozen mile radius now need to be shared across hundreds of miles, reducing the bandwidth available for everyone. And the weak signal will cause interference substantially further. The FCC limited transmissions while on an aircraft to prevent interference with ground communications, and continues to because there isn’t enough evidence that they won’t. [0] https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_91.21-1D.pdf https://www.faa.gov/documentLibrary/media/Advisory_Circular/...
jjtheblunt 3y ago
it's kind of funny because water resonates in that band, international consensus or not?
- Hello71 3y ago
  https://en.wikipedia.org/wiki/List_of_common_misconceptions https://en.wikipedia.org/wiki/List_of_common_misconceptions: > Microwave ovens are not tuned to any specific resonance frequency for water molecules in the food, but rather produce a broad spectrum of frequencies, cooking food via dielectric heating of polar molecules, including water. Several absorption peaks for water lie within the microwave range, and while it is true that these peaks are caused by quantization of molecular energy levels corresponding to a single frequency, water absorbs radiation across the entire microwave spectrum.
  - jcrawfordor 3y ago
    I would contest Wikipedia's description here a bit because I think it confuses the issue of microwave emissions. A number of ISM bands were allocated for RF heating applications originally, 6.78MHz, 13.560MHz, 27.12MHz, 40.68MHz, 915MHz (commonly referred to as 900MHz), 2.45GHz (commonly referred to as 2.4GHz), 61.25GHz, 122.5GHz. These are just examples taken from my NTIA chart which doesn't call all of them out. The lower ones are largely historic, the very first RF heating experiments used HF and VHF low band which were easier to produce with the radio transmitter technology of the time, but not very efficient at all. The invention of the magnetron changed that, suddenly it was much easier to produce microwave radiation at high power levels, and so the inefficient HF/VHF RF heating devices have all but faded away (there are some specific technical applications that remain exceptions, as usual). The low frequencies are inefficient and difficult to produce with compact electronics, the high frequencies aren't very attractive for food heating applications because of the limited skin depth. So every microwave oven you're likely to run into operates at 2.4GHz, which is pretty much the sweet spot for food heating among the allocated ISM bands. That band is defined as 2.4GHz through 2.5GHz. So I quibble with describing microwaves as "broad spectrum." Magnetrons do not produce very narrow output, one of the reasons they aren't often used for radio transmitters today, but microwave ovens are required to constrain their meaningful output to within 50MHz of 2.45GHz. 100MHz is a lot of bandwidth from a modern radio communications perspective, but isn't really that wide from a perspective of physical effects. 2.45GHz was chosen as an ISM band in part because it had good properties for heating, but it wasn't put exactly on a resonance frequency for water or anything like that. The exact details of the selection process are obscure but 2.45GHz was already being used for experimental microwave heating before the ISM band was allocated, and I would imagine came out of some combination of ease of magnetron construction and reasonably good heating properties. It is documented, for example, that the EHF bands were never popular for consumer microwave heating because of poor efficacy with food, although they do have industrial applications (especially in welding). Given the history of the topic there's a decent chance that 2.45GHz came about because it was being used by experimental radar at the time, the main thing that magnetrons were being built for. Microwave heating was basically a byproduct of radar development during its early days of development.

ethbr1 3y ago

And on the follow-up, "Why use 2.4(5) GHz for microwaves?" Because it happens to be a convenient frequency that water absorbs readily, which is the easiest way of heating up what we'd want to heat with a microwave (read: food). https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water#Microwaves_and_radio_waves https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_... And once we'd polluted the frequency for any stable commercial use, why not use this weird carve-out for a little thing called WiFi? Rabbit hole: Apparently the precise mechanics of EM heating of molecules are surprisingly complex, and offer a number of frequency options. But this would have been the 1950s(?), so I'd assume they empirically determined a balance of functionality + technical feasibility for water, called it a lunch, and went to have five martinis in a practical fashion.

LarsAlereon 3y ago
To build off your rabbit hole, I've read about using the way microwaves selectively heat molecules in cooking. In particular, they can heat oil molecules in a way that increases flavor extraction without causing thermal breakdown of the flavors like uniform heating would.
sleepybrett 3y ago
To further build off your rabbit hole, this is also why 2.4ghz long range wifi connections (like via a yagi antenna) is also problematic. Foliage fucks with your range. Also why central air conditioners and wet walls create wifi shadows.
- 2rsf 3y ago
  But the rabbit hole goes further. Higher frequencies are more susceptible to wall in the middle problems, you can see that easily with 5GHz WiFi, lower frequencies might be better but require larger antennas and might suffer from other limitations. As everything else in engineering this is a compromise.
jodrellblank 3y ago
> "But this would have been the 1950s(?), so I'd assume they empirically determined a balance of functionality + technical feasibility for water, called it a lunch, and went to have five martinis in a practical fashion." Or maybe they just wanted to revive frozen hamsters more humanely: "I promise this story about microwaves is interesting." by Tom Scott, about scientist James Lovelock - https://www.youtube.com/watch?v=2tdiKTSdE9Y https://www.youtube.com/watch?v=2tdiKTSdE9Y

pcthrowaway 3y ago

I've read that the way to determine if your microwave is properly sealed is to put a phone inside of it, close the door, and then try calling it. I've been unable to get the phone to ring last time I did this ~12 years ago, though if there's some expected leakage, I suppose proximity to the cell tower is a factor

marcus_holmes 3y ago

Could I make a legal bluetooth jammer that is actually a small microwave oven then?

quickthrower2 3y ago
Probably not legally.

discordance 3y ago

If anyone’s interested in seeing the leakage on a spectrum analyzer: https://youtu.be/tToMYkO7Gkw https://youtu.be/tToMYkO7Gkw

jchw 3y ago

Also, to the best of my understanding, the only known risk to human safety of a microwave without proper shielding is injury due to the heating of human tissue. Not like that's a good reason to ignore any potential health concerns, but I used to have the misconception that it was much more dangerous, e.g. could cause cancer, but, of course, ionizing radiation is much, much higher frequency.

alexdbird 3y ago
The heating part can still be very bad news for your eyes, being the most delicate exposed tissue.

kalleboo 3y ago

And now my new microwave actually has Wi-Fi built-in, we've come full circle.

deleted 3y ago
[deleted]
stephen_g 3y ago
That's probably a good thing, because it means they have to take extra care to make sure the shielding is good enough that microwave energy used for cooking won't interfere!
- kalleboo 3y ago
  Except it disconnects from the Wi-Fi when running, and only re-connects afterwards to send the notification about completion! A total cop-out...