How small can a magnetron be made?

  • Thread starter Thread starter Guineafowl
  • Start date Start date
Click For Summary

Discussion Overview

The discussion revolves around the feasibility of miniaturizing magnetrons, particularly for applications requiring lower power outputs (50-100W) compared to standard models (1000W). Participants explore the implications of size reduction on performance, design, and alternative technologies.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that the physical size of magnetrons is largely determined by heat sinking, bulky magnets, and resonant cavities, which are constrained by the wavelength of microwaves.
  • One participant references research indicating that the key to miniaturization lies in the wavelength, mentioning terahertz magnetrons as a potential area of exploration.
  • Another viewpoint emphasizes that common cavity magnetrons are limited by the dimensions of their resonator cavities, which could be mitigated by alternative designs with smaller or folded cavities.
  • Some participants propose that for smaller applications, alternatives like gunn diodes or gyrotrons might be more suitable than miniaturized magnetrons.
  • Concerns are raised about the scaling of internal electromagnetic fields and the potential for arcing at higher frequencies, which could limit power output in smaller devices.
  • It is noted that smaller magnetrons would have lower breakdown voltages for the same current, complicating their stability and control due to negative resistance characteristics.
  • One participant humorously describes the challenges of tuning magnetron cavities and the effects of thermal expansion on frequency stability.

Areas of Agreement / Disagreement

Participants express a range of views on the feasibility of miniaturizing magnetrons, with no clear consensus on whether it is a viable option. Some argue against the practicality of miniaturization, while others explore alternative designs and technologies.

Contextual Notes

The discussion highlights limitations related to the definitions of magnetrons, the dependence on specific design parameters, and unresolved questions regarding the performance of smaller devices.

Guineafowl
Messages
910
Reaction score
418
I was asked the other day about a miniature magnetron for a ‘thought-invention’ by a friend. Putting aside (for now) safety and power supply considerations, can they even be miniaturised? Say you wanted a 50- 100W one instead of the usual 1000W.

My intial thoughts, unqualified, are that much of the physical size is related to heat sinking, bulky magents and the resonant cavities, whose size are dictated by the wavelength of microwaves. So no, in other words.

A search online revealed many normal-size magnetrons and an awful lot of unrelated devices called ‘magnetron’ because it sounds cool. Exercise bikes, for example.
 
Last edited:
Engineering news on Phys.org
  • Like
Likes   Reactions: Guineafowl
The answer depends on what you define as being a magnetron. Common cavity magnetrons are limited by the dimensions of the resonator cavities to a diameter of about half a wavelength. There are alternative constructions that could have smaller folded cavities or slower material resonators. You must decide why you need to make a smaller magnetron, when a gunn diode will do the job in a smaller space. If you want more power from less volume you should consider a gyrotron.
 
  • Like
Likes   Reactions: Guineafowl
Guineafowl said:
I was asked the other day about a miniature magnetron for a ‘thought-invention’ by a friend. Putting aside (for now) safety and power supply considerations, can they even be miniaturised? Say you wanted a 50- 100W one instead of the usual 1000W.

My intial thoughts, unqualified, are that much of the physical size is related to heat sinking, bulky magents and the resonant cavities, whose size are dictated by the wavelength of microwaves. So no, in other words.

A search online revealed many normal-size magnetrons and an awful lot of unrelated devices called ‘magnetron’ because it sounds cool. Exercise bikes, for example.
To go back to the invention of the magnetron, when people needed very short wavelengths, they had tried making smaller and smaller triodes etc.but it was of limited success. But the magnetron enabled a large structure to generate short waves, so it could handle a lot of power and the construction was relatively simple and rugged. So to pursue miniaturised magnetrons seems the wrong path to follow.
 
Baluncore said:
Common cavity magnetrons are limited by the dimensions of the resonator cavities to a diameter of about half a wavelength.
I would have thought that internal EM fields would not scale nicely and that could limit the possible power at higher frequencies due to arcing.
 
sophiecentaur said:
I would have thought that internal EM fields would not scale nicely and that could limit the possible power at higher frequencies due to arcing.
The fields scale as expected. Since the magnetron is a high vacuum device the breakdown voltage is going to be decided by electron emission and anode current. Smaller devices will have lower voltages for the same current. Magnetrons are very hard to tame because they have the negative-resistance needed to make them oscillate. Magnetrons have unstable anode currents, even when in series with their specially inductive power transformers. Magnetrons thrive on internal voltage breakdown.

The only way to control the generated frequency is by tuning the cavities. Each time anode current starts to flow in a microwave oven magnetron, thermal heating expands the metal anode cavities which typically results in a down chirp of about 5% of operating frequency. The down chirps can be seen with an S-band analyser, especially at meal times when they sweep across that ISM band, annoying the little blue-toothed hoppers that graze the spectrum grass. It is a jungle out there.
 
Last edited:
  • Like
Likes   Reactions: Nik_2213, Paul Colby, sophiecentaur and 1 other person
Many thanks all. It doesn’t sound like a viable option, then.