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More frequent microwave ovens

Higher microwave frequencies. Shorter wavelengths.
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A common misconception is that household microwave ovens operate at 2.45 GHz because water resonates at that frequency, leading to the water getting heated. Actually, water isn't especially sensitive to that frequency, and can be heated just as well by other frequencies. I don't know why that frequency was used initially, but the main reasons, AFAIK, that that frequency is still used are that the magnetrons and waveguides for it are already developed and available and that it's already an ISM band and therefore legal to use for this purpose. Actually, the 2.45 GHz ISM band was created because ovens were already producing a lot of interference there, rendering it less useful for communications.

But there are higher ISM bands, such as 5.8 GHz, 10.525 GHz, 24.125 GHz, 61.25 GHz, and so on. (Actually, I haven't been able to find anything saying there's a 10.5 GHz ISM band, but the availability of unlicensed radio products (such as radar modules) using that band seems to suggest it's one. Can anyone confirm/deny?) So, let's use those for food heating as well. The shorter wavelengths could have several advantages. One would be smaller magnetrons. Another would be smaller waveguides, which could be routed to more outlets around the cooking chamber for more even heating. Another might be that the door could be shielded using a (semi)conductive coating or a fine mesh like a window screen rather than a coarse mesh, improving visibility.

A microwave oven could even be made with two magnetrons operating at different frequencies, which would enable some degree of control over the distribution of heat between the surface and the deeper regions of the food.

N/A [2019-10-07]

notexactly, Oct 10 2019

Scroll down to the “Penetration depth” heading https://www.ncbi.nl...rticles/PMC4657497/
[hippo, Oct 10 2019]

More on microwave penetration depths https://www.pueschn.../penetration-depths
[hippo, Oct 10 2019]

[link]






       //two magnetrons operating at different frequencies//
That bit I like.
Something I always thought was weird about microwaves: when you have it on less than 100%, the magnetron simply switches on-and-off to regulate the power. But (AFAIK) they do it on the order of (quite a few) seconds; where-as I would have thought switching at 10th's (or faster...) would be better. Or can magnetrons not switch that fast (ie. takes time to start up)?
neutrinos_shadow, Oct 10 2019
  

       Shorter wavelengths wouldn’t cook food all the way through. I think you need the distance you want heat to penetrate into the food to be about half a wavelength - the wavelength chosen for microwave ovens is about the distance from the outside of a chicken to the centre of a chicken. See links.
hippo, Oct 10 2019
  

       5G has already started and the FR2 millimetre wave bands have frequencies from 24GHz to 52GHz. There are also lower frequency bands below 6GHz, likely to be used by handsets.   

       The higher frequencies are already being used for Fixed Wireless Access. An example is Verizon in the US which uses 28GHz. The Fixed Wireless Access idea is to have something on the outside of your house which is your connection to the Internet. This re-broadcasts to the inside of your house. Cunning beam forming technology allows each outside receiver to be pinpointed with a beam. This means that rather than wastefully radiating in all directions into space an in annoying Physics inverse square law like way, the beam concentrates its energy, a bit like a virtual wire, to the intended recipient. The end result is more bandwidth. People in rural communities in the US can, at last, have decent Internet connectivity.   

       Why, we ask does it have to be on the outside of the house? It's because the frequency band does not do so well through solid objects. It turns out higher frequencies in general also don't do so well through leaves (clutter in the jargon of radio planning) or in fact the air in general. There's something in the air, a pesky greenhouse gas, water vapour. It limits the range of the signal and forces the carriers to build more micro cells.   

       All we have to do is to get a load of Fixed Wireless Access external antennae, power them up, and place them in a Faraday cage. They'll try to phone home and, with a bit of luck, there's a chance they will act like a microwave oven. Does it match the resonant frequency of water vapour? Who cares, crank up the power until it does!   

       Handsets, like the new Samsung S10, are 5G capable but these currently use the lower frequency bands.   

       The future may see higher frequencies in the hand and therefore nearer the head. Luckily, my head is full of air so there's no danger of blowing my mind.
DenholmRicshaw, Oct 10 2019
  

       // Or can magnetrons not switch that fast (ie. takes time to start up)? //   

       Magnetrons can be gated very quickly ; if they couldn't, pulsed radar wouldn't be possible. And they can operate in the megawatt range.   

       No, the reason for the long pulse durations in domestic ovens is simply that the power is switched by a simple relay, because it's a cheap and well-proven technology, and relay contact life is directly calculable from voltage, current, and switching cycles. Since it makes no difference to the performance of the unit, high-PRF magnetron switching doesn't happen in home ovens.   

       It's entirely and simply possible to modify such an oven, using a triac, to zero-crossing switch on half-cycles, so you can pulse the energy at 25 Hz or derivatives thereof with a little microcontroller. It doesn't make the thing cook any better, just makes it more complex and less reliable, but it does score a whole load of Geek Points.   

       With an Arduino or a Raspberry Pi running it, you can control the energy over a 2% - 100% range, pretty smoothly, by selective cycke dropping.
8th of 7, Oct 10 2019
  

       // But (AFAIK) they do it on the order of (quite a few) seconds; where-as I would have thought switching at 10th's (or faster...) would be better. Or can magnetrons not switch that fast (ie. takes time to start up)? //   

       Panasonic Inverter microwave ovens, with their modern high-frequency magnetron drivers instead of MOTs, were (maybe still are) advertised as having smooth power adjustment rather than on/off over several seconds, which I took to mean they used a somewhat more reasonably high (i.e. not absurdly low like everyone else's) PWM frequency, but could also mean that they're doing analog regulation on the magnetron power. I think they said it results in more even heating, or something.   

       // Shorter wavelengths wouldn’t cook food all the way through. //   

       Yes—that was the point.   

       // I think you need the distance you want heat to penetrate into the food to be about half a wavelength - the wavelength chosen for microwave ovens is about the distance from the outside of a chicken to the centre of a chicken. //   

       Interesting. So the higher-frequency magnetron, if it is to cook only the outside of the food, might need to use a much higher frequency. Fortunately, there are 122.5 and 245 GHz ISM bands, though we might need an array of microscopic magnetrons.
notexactly, Oct 10 2019
  

       [8th of 7], [notexactly]; thanks for the info.
neutrinos_shadow, Oct 10 2019
  
      
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