Automatic impedance matching in microwave waveguides

controlled-impedanceimpedanceimpedance-matchingmicrowavewaveguide

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.

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Best Answer

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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