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Registered Member #2941
Joined: Fri Jun 25 2010, 08:08AM
Location:
Posts: 143
hello everybody,
I want to couple a magnetron with a high voltage high power 75 ohm coax for some experiment, but I can not get any information about the output impedance of a standard magnetron. the 1/8 wave antenna of the devise suggest a low impedance source (but I am not sure), impedance transformer are not an easy think to build unless I have some idea of the output impedance of the magnetron itself, please can some one help me ?
ps:I am a high voltage ingenieur and I have some understanding about microwave energy ,so please, spare yourself the traditional warning paragraph .
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The antenna should be 1/4 wave not 1/8, as a 1/4 wave probe is a standard way of coupling to waveguide. The tip is a high impedance voltage node, which is why it has the large metal end, to stop breakout. And being a 1/4 wave vertical, the impedance at the base ought to be about 50 ohms.
It could be done a number of ways, but if it were me, I'd attach another 1/4 wave line to the tip of the antenna to extend it to 1/2 wave. This will transform the impedance back to what it was at the magnetron. Then attach a coax connector (N type?) to the end. Enclose the resulting line in a copper shield that attaches to the magnetron body at one end, and the outer part of the coax connector at the other. Make sure the connection is really good, the shield is a good Faraday cage, and there is a decent amount of clearance around the line. (ideally the shield is a tube of a size that would be 75 ohm coaxial with the diameter of the antenna tip)
You could also add a tuning screw that allows you to vary capacitance to ground around the 1/4 wave node.
Disclaimer: never tried it. Seen others do it a different way, using a short piece of rectangular waveguide with another 1/4 wave probe in the other end.
Registered Member #2941
Joined: Fri Jun 25 2010, 08:08AM
Location:
Posts: 143
thanks for the answer, I realize my mistake about 1/8 antenna, I didn't consider the ceramic part of it witch of course radiate as well, now the funny thing is that with the ceramic part the antenna is 25.75 mm long, witch is not exactly 4/1 wave , but maybe they use trick to tune it with the internal feed line...50 ohm to 75 ohm is (sort of) of acceptable to me as I don't intend to run the magnetron at full power.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
It's the easiest way to understand, as it's just the magnetron antenna in reverse.
Doug Coulter's ion source shows the technique I tried to explain. I think you can just put a coax connector on the end of the hardline where his resonant cavity is.
Registered Member #1667
Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 373
might be interesting for you. The link above shows the coupling of an antenna to a rectangular waveguide / cavity. I would also consider the stud coming out of the magnetron as an antenna, the cylinder shape effectively shortens the geometric antenna length and surpresses arcing.
You would start calculating the geometry based on the cut-off frequency of the most suitable cavity mode, then place the magnetron and coupling antenna at the electric field extrema.You could then also make one wall movable to adjust the power output.
Literature suggestion: J.D. Jackson, classical electrodynamics (a very thorough introduction).
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magnetron coupling.
