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I want to produce a Fourier series of microwave harmonics in a cavity. My first thought was to order a bunch of magnetrons tuned to each harmonic, up to some limit and get our lab tech to modify a cavity. The problems with this are:

  1. there isn't enough space to attach more than 5 or 6 magnetrons, so I will have an extremely bandwidth limit Fourier series;

  2. If I could get up to the 10, 15 or maybe more frequencies I would like, then it starts to become expensive. Probably not prohibitive, but if a cheaper way existed, I would go for that instead;

  3. I imagine there are constraints on the positioning of the magnetrons to get waves that will have the right phase to usefully interfere to produce the standing wave-shapes I want... I guess that further reduces the number of magnetrons I could realistically incorporate into a given size of cavity.

So, do you know of a better way to form Fourier series of standing wave harmonics in a microwave cavity? Is there a device that can produce the fundamental microwave frequency as well as several harmonics with a way to change the amplitude of the harmonics?

toolic
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holyjoly
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  • You haven't mentioned power. – Andy aka Aug 12 '23 at 12:42
  • magnetrons tend to pull (frequency variation) due to changes of load impedance and reflected power - so coupling a large number (ie > 1) into a cavity seems to be unwise. – D Duck Aug 12 '23 at 12:42
  • Thanks. Total power <10kW, probably ~2 to 3 kW. – holyjoly Aug 12 '23 at 13:10
  • So it's a bad idea to use magnetrons. Thanks. Is there something else I could use to do what I want, or should I just forget about it? – holyjoly Aug 12 '23 at 13:14
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    Varactors and step recovery diodes are used to generate harmonics in frequency multipliers, you might look at that. – GodJihyo Aug 12 '23 at 13:18
  • I would expect magnetrons of different frequency bands would not interact, but I'm far from a specialist in them. It also seems an expensive solution (so many unique parts). But I have no idea what you're doing, or how much it's worth doing it. BTW, note that the waves need not be harmonic, because a cavity may be dispersive. And needless to say, the entire waveform (at a point, and spacially i.e. amplitude vs. position) depends critically on the materials and geometry placed within the cavity. – Tim Williams Aug 12 '23 at 14:32
  • What frequency are you working at? And how big is your cavity - is it the size of a room, with many modes, or just enough for 5 modes. CW or pulsed? And where are you going to get these magnetrons at any old frequency? Typically they come at just a few discrete frequencies (like 896, 915, 922, 2450 MHz) – D Duck Aug 12 '23 at 16:43
  • If you're not too particular about signal purity, a spark-gap in the cavity will create harmonics... and possibly other frequencies, too. BTW, rusty connections on radar antennas can act as rectifiers, creating harmonics. https://en.wikipedia.org/wiki/Nonlinear_junction_detector – DrMoishe Pippik Aug 13 '23 at 03:22

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