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I am trying to building a magnetron plasma using microwave magnetron.

I now need to build magnetron to antenna link using rectangular waveguide, WR-340.

I then found out that it needs to be tuned precisely to move all power from magnetron to antenna. I also found out that waveguide can be tuned with short and two stub tuner. There are also auto tuner available that can tune waveguide automatically depending upon load, continuously.

I search over internet and found this video which explains most of the thing about what happen inside the waveguides:

http://www.youtube.com/watch?v=DovunOxlY1k

Now my questions is:

How do I measure amplitude of microwaves inside the waveguide to know the % of reflectance? Once I know it, by what amount should I change the stub height of two stub to make a match?

I am complete newbie in waves, but I am good at mechanical as well as mechatronics/automation stuffs.

enter image description here

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2 Answers 2

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You can use a directional coupler to measure VSWR (reflected power). There are many app notes online for this: http://www.minicircuits.com/app/COUP7-2.pdf here is an example.

However, the fact is that waveguides need to be tuned because no two are exactly the same when you put multiple components together. This means you won't be able to just change the height a set amount to 'fix' the match. You could characterize each component and get a set of "heights" to adjust, but I would recommend trying to have a control loop where you are constantly measuring the reflected wave and tuning to minimize power.

And again, the VSWR is kind of a derived term and it will be best thought of as "tuning for minimal reflected power" when setting your system up.

And wowzers, I just saw that you have a WR340. Those are gonna be some hefty stubs!

edit: Do you really expect your match to be that dynamic? If not, you can just tune your waveguide piece on a network analyzer, epoxy/lock washer the screws and you'll have yourself a well matched piece.

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  • \$\begingroup\$ Somewhere I read that plasma is unstable load which will need continuous tuning. \$\endgroup\$
    – iamgopal
    Feb 22, 2014 at 9:55
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    \$\begingroup\$ Very possible. I have not dealt with dynamic tuning in the past but I have made static tuning with tuning screws many times. If you put three screws, each a quarter wave apart, you should have no problem tuning for a good match. If you need precise filtering, you may need more screws. Usually, the screws will only be a fraction of the way into the cavity of the part. \$\endgroup\$
    – scld
    Feb 24, 2014 at 14:58
  • \$\begingroup\$ should screw electrical be connected to waveguide ? or should they separated by insulating material from waveguide ? , also , external part of the screw can be expose to environment ? ( i.e. will it work as a antenna too ? ) or we need to create a compartment to cover the screw ? I am asking to get physical/mechanical sense of the theory. \$\endgroup\$
    – iamgopal
    Feb 25, 2014 at 4:26
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    \$\begingroup\$ The screw should be a part of the waveguide electrically. The screw won't act like an antenna however perfect isolation is impossible, of course. Your need to cover the screw holes will be dictated by your sensitivity. \$\endgroup\$
    – scld
    Feb 25, 2014 at 5:58
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    \$\begingroup\$ No, the antenna needs to be isolated from the waveguide ground. If the antenna was touching the side of the waveguide, it would just act like a large stub. It may be easiest for you to purchase a WR340 to N-Type adapter and then purchase an N-Type antenna. Or, you could use a horn antenna. If you still want to make your own antenna, you'll need some sort of feedthru that is isolated from the wall. If you purchase the Waveguide to Coaxial adapter, they basically are little antennas that you can base your design on. \$\endgroup\$
    – scld
    Feb 26, 2014 at 13:09
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look after surfatron

university of montreal, Prof. Michel Moisan

called surface wave plasma..

good luck

be careful with the microwave radiation

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    \$\begingroup\$ Do you have some sort of reference / links to his work relating to the field? It would make the answer a lot more useful rather than having to go searching for it. \$\endgroup\$
    – PeterJ
    Mar 26, 2014 at 5:37
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    \$\begingroup\$ Has nothing to do with the question. \$\endgroup\$ Sep 2, 2015 at 17:59

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