magnetron microwave launcher design
magnetron microwave launcher design
(OP)
I plan to direct the output from an oven magnetron from a launcher/horn combination to dry wet soil as efficiently as possible. I have played with a couple of launchers, but despite an auto tuner showing no reflections, the (aircooled) magnetron overheats (over 100 C in a couple of minutes). When mounted back into the oven, it levels at 90 C. I think airflow is not an issue in my configurations. I noted two launchers have a grid of small holes and a dimple opposite the emitter on the sidewall of the waveguide..for what benefit? I will make another launcher and want to know besides the 1/4 wavelength back standoff, is there a trick I am missing that prevents overheating? Should the exit flange be placed at 1/2 wave from the emitter or is 1 lambda better? or is some other fraction better? All hardware is W284 size and I wonder if it shouldn't be bigger?





RE: magnetron microwave launcher design
How are you trying to launch the energy into the soil? Water has a dielectric constant of 80, dry soil has an Er of around 5, air is 1. So if you are trying to go from the waveguide to air above the ground with some sort of antenna, when it hits the wet earth you will have nearly total reflection from the Er of 80. If you eliminated the air layer, and went from waveguide into some sort of dielectric (maybe an Er=9.9 alumina sheet 0.4 inches thick--a quarter wavelength) you could get better penetration into wet soil.
RE: magnetron microwave launcher design
RE: magnetron microwave launcher design
RE: magnetron microwave launcher design
One question you have to ask is "does the waveguide horn have a low reflection coeeficient at 2.46 GHz"? If it is a standard WR284 horn, it is probably tuned for the best radiation efficiency (ie lowest reflection coefficient) in the band center, which is 3.275 GHz. It may not work well at 2.45 GHz, causing a big reflection of power to the magie.
Tell us if the magie works fine when you are pointing the horn into the sky.
RE: magnetron microwave launcher design
RE: magnetron microwave launcher design
RE: magnetron microwave launcher design
I suggest that the situation is logically "baffling" because one of the presented "facts" is false.
RE: magnetron microwave launcher design
If you leave the magnetron in the oven, cheat the 3 door switches and connect waveguide over the rf output, then to your horn (or wg-coax. transition), life may be better.
I've done similar before without problems.
kch
RE: magnetron microwave launcher design
Higgler, I was considering your idea but thought a 'proper' launcher might do better. I still need advice (?) for that. Can you list the causes for the 'center conductor heat build-up' if other than reflected energy? Am I correct to assume an auto-tuner can't fix the short backspace of the launcher?
RE: magnetron microwave launcher design
RE: magnetron microwave launcher design
Are you certain your voltage is the same inside the oven as outside. Have you checked the current draw on the unit in both conditions, maybe it's suppose to be higher heat due to higher current draw.
kch
RE: magnetron microwave launcher design
RE: magnetron microwave launcher design
If I'm not picturing the setup, please excuse the error.
Maybe describe what's removed from the oven, is it the magnetron with integral launch probe and without ground cup shaped launcher??
kch
RE: magnetron microwave launcher design
I am still looking for guidelines to make a proper launcher (even though I am making a couple of them right now as I wait for ideas from the experts). I don't have the funds right now to buy another commercial one.
RE: magnetron microwave launcher design
I am looking at my 1947 edition of Very High Frequency Techniques. Magnetrons were new and novel at the time; flushed with success during the war. Several of the drawings show "coupling loops" inside the magnetron cavity. The output is then on a coaxial line. This makes sense now that I think about it; the magnetron is a vacuum cavity device and that doesn’t sit well with a waveguide output.
I just pulled the cover off the magnetron output on my microwave oven. It has what looks like an E-field launcher in the waveguide, albeit with jumbo proportions. It is very simple. I think you should stick with the standard terminology of "launcher" for the coaxial to waveguide transition and "horn antenna" for the waveguide to free space transition.
I think you will find the E-field launcher is around one quarter lambda from the short-circuited end of the waveguide. The "back-short" is used to tune the launcher. If you dismantle the magnetron and use your own waveguide then the position of the back-short is going to be crucial. How you can adjust it at full power and not blow the f*ck out of it I don’t know. I would think the original designers set it up with directional couplers and a tuneable back-short and only applied full power after it was tuned.
Some pictures of the before and after assemblies would be very nice to see. If your microwave oven is like mine you would have to cut up the original waveguide to see how the launcher is positioned in the guide relative to the back-short and indeed if it is an E-field or an H-field launcher.
RE: magnetron microwave launcher design
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RE: magnetron microwave launcher design
RE: magnetron microwave launcher design
Ramo, Whinnery and van Duzer (Fields and Waves in Communication Electronics, 3rd ed) fig 8.11g shows an example of the matching but refers to Collin (Field Theory of Guided Waves) which I don’t have, but which I have now ordered!
RE: magnetron microwave launcher design
http://www.hi-techtermitecontrol.com/
kch
RE: magnetron microwave launcher design