Comment by ahartmetz
3 days ago
I wonder why microwaves can't work like modern radio transmitters. Magnetrons generate ~2.4 GHz radio waves using resonance and a strong magnetic field acting on free electron orbits. That was necessary in the 1940s for radar transmitters. But today, solid state electronics generate 2.4 GHz (and higher) waves without any trouble - cf. WiFi and Bluetooth hardware. I'm not the first to have this question, and it looks like there is some ongoing work. https://www.digikey.com/en/blog/will-the-microwave-ovens-mag...
Because transistors for generating even very low microwaves like 2.4GHz are extremely expensive comparatively speaking, and don't produce much power. They're good, though, because you can produce very precisely tuned and modulated signals and very precisely controlled output powers - as long as they're less than a couple of watts.
A cavity magnetron is a block of metal with some holes drilled in, two bits of glass glued on, and all the air sucked out. They're hard to tune to exact frequencies and hard to regulate to exact powers, and modulation is as you've already discovered kind of limited to just turning them on and off - but they're extremely cheap to make, last a very long time, and require minimal support circuitry to generate double-digit kilowatts of RF.
You don't need to be cock on frequency to heat up a pie.
Now there are gallium nitride microwave transistors that can produce very high microwave power at very high efficiency. So that is no longer a limitation.
Microwave ovens with such transistors have been demonstrated, which have the advantage of modulating the microwaves in such a way as to achieve a more uniform heating throughout the oven, than can be achieved with the fixed-frequency magnetrons.
At least for now, such microwave ovens with transistors might be encountered only in some professional applications, because these transistors together with the associated control circuits remain much more expensive than magnetrons.
They would be ridiculously expensive compared to a cavity magnetron and have no real advantage though.
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Ah, thanks, good to know. I thought solid state power electronics had come down in price more than they apparently did. I guess it's high frequency plus high power that's still expensive. For not so high frequencies (< 1 MHz), mass production for ubiquitous switched-mode power supplies and electric cars has surely brought down the price.
(Side note, modern very high power radio transmitters might also still use some vacuum tube technology - my latest information is that there's a slow transition going on)
Magnetrons are cheap, reliable, and work well enough. Their one and only job is to produce energy at about 2.4GHz, and to make a lot of it. You merely put in electricity and through the magic of geometry you get RF out.
A transistorized solution is, simply, inappropriate. The RF section alone will cost more than an entire microwave off the shelf, and plus that you need all the control circuitry and high speed crap to generate a 2.4GHz tone. Not to mention that putting kilowatts through silicon creates a much higher risk of catastrophic failure: transistors blowing out into a plume of plasma and such.
You're asking why we haven't reinvented the wheel. There's simply no improvement to be had.
You want high power RF in a given frequency band, you want a magnetron. It's just a lump of iron with some wires. You don't need any electronics. At all.