coaxial cable vs waveguide for 2.4ghz magnetron project? - Page 1
Forum section: electronics / rf-microwave

author: Nitrous
author_rank: Newbie
timestamp: March 27, 2023, 02:26:22 pm
content: Hi folks,I don't think this question has been asked (I hope not, anyway) but I'm trying to work out a microwave plasma torch based on a 2.4ghz magnetron (salvaged microwave oven).I did s-band dielectric studies way, way back when (i was a phys chem grad student) but wondered if there was a practical way to couple a simple coaxial plasma torch to a magnetron. ie with coax rather than waveguides.I'm not much of a sheet metal fabricator - but have basic electronic skills that have managed to outlive my anesthesia career...Any suggestions/direction/links would be greatly appreciated.ThanksDoug

author: stephen_g
author_rank: Newbie
timestamp: March 28, 2023, 03:41:10 am
content: First of all,be incredibly careful. The transformer driving the magnetron produces extremely high voltage,andits output is isolated so you lose any protection from an RCD/GFCI.Wikipedia cites at least 33 accidental deaths from electrocution of hobbyists trying to use microwave oven transformers for fractal wood burning.So as far as I can tell, the output of the magnetron is basically a transition that goes into a rectangular waveguide -http://ffden-2.phys.uaf.edu/631fall2008_web.dir/ness_S/works2.html.You could basically create a bit of waveguide with a hole for the magnetron, make sure it's well gasketed and bolted on tight, and then you could get an off the shelf transition like this -https://www.stepelectronics.com.au/product/s-band-wr284-waveguide-to-coax-adapter-with-n-type-connector/. We usually CNC our waveguide adaptor sections in two halves to have a good mating surface to things like this, but it could be done perhaps with a milled end piece and then the waveguide made of sheet metal and welded on.Of course, off the shelf transitions are usually fairly expensive (a few hundred dollars). Since you don't care too much about the RF performance, you could probably make your own. Basically it's just a little antenna at the end of a waveguide, but the dimensions of everything is extremely important.Be warned though, it's a very large amount of RF power, so it will probably start to melt things if you, say, run it while the transition is un-terminated, or if you design one and it's not very well matched... And the cable is going to have to be pretty beefy to handle the current, and you want to check that all your connectors and everything can handle the power.Sounds like a very dangerous project all in all.

author: jonpaul
author_rank: Super Contributor
timestamp: March 28, 2023, 04:13:31 am
content: waveguide rated for the power levelsBeware of the many hazardous issues with plasma torchEMI émissions, eg 2.4 ghz can cook the eyeballs and cause cataract or blindnessTorch plasma émission of powerful UV light...burns of cornea, retina or skin.2000 V 300.1000 ma AC from transformers,Magnetron filament at -2400V, LETHAL !first Take out good liability, health and fire insurance.Get approved goggles, face shield, gloves and wear rubber soled shoes.PLEASE DO NOT ATTEMPT THIS VERY DANGEROUS PROJECT as a learning experience!Just the ramblings of an old retired EE( I had designed and consulted on power supplies and high voltage since 1970s...up to 12kW and 75 KV)bon chanceJonresearch the historic and accidents with microwave ovens, and plasmas

author: coppercone2
author_rank: Super Contributor
timestamp: March 28, 2023, 06:30:25 am
content: I strongly suggest that you just buy yourself a cheap plasma cutter and forget about itIf you want to do it right, get yourself a proper lower power transmitter (say 5 watts) meant for radar or something, and then learn on that. Its not easy, and IIRC I even saw the magnetron burn through a wave guide in a microwave oven that went brezerk while making chili (I have the whole circuit hooked up to a light switch, so I just turned it off and opened it up the next day to do a visual inspection, iirc there was a hole by the waveguide magnetron interface inside of the unit, and the electrode of the magnetron was melted. No clue, the microwave was about 2 years old and SHARP brand too. Thats a pretty well engineered damn old design, and it managed to breach containment, even with all the data a company like sharp generated for however many years they are selling those little ovens. IIRC it looked kinda like someone hit it with a arc welder set to kill.If you are a chemist your question is like someone going strait for a 50 gallon reactor before learning small glassware chemistry.Based on what I saw, I would not even recommend experimenting with high power wave guide unless the whole circuit is inside a grounded metal box. Aviation has GOOD insurance for those people and alot of red tape and bureaucracy (why they might have cool looking out in the open waveguides, because of the 6 hour class on tightening bolts and 3 safety managers).And Uh... don't trust the chinese researchers methods, they look fucking nuts in a few of those magnetron papers. glad we have osha

author: 0culus
author_rank: Super Contributor
timestamp: March 29, 2023, 02:46:21 am
content: As everyone said, just be really careful. It sounds like you have some experience, but high power and high voltages require a certain respect that you may not get a second chance on.

author: CaptDon
author_rank: Super Contributor
timestamp: March 30, 2023, 03:31:01 pm
content: I think one of the posters missed a word and meant to say "The high voltage is not isolated". One side of the winding is in fact grounded to the transformer frame and the anode of the magnetron is its body. As someone mentioned the 2.5vac filament winding is also connected to a voltage doubling circuit (the magnetron itself being one of the two diodes) and sees up to -4kv of pulsating D.C. as measured to chassis ground. The capacitor normally includes an internal discharge resistor. An R.F. burn to your skin will be very deep and very painful should you somehow come in contact with the 'stinger', even briefly!

author: stephen_g
author_rank: Newbie
timestamp: March 31, 2023, 04:24:53 am
content: Quote from: CaptDon on March 30, 2023, 03:31:01 pmHmmm, looking at a diagram of a microwave oven that does seem to be the case, yes. In that case, perhaps the information I had looked at may have been talking about when people remove the transformers out of the microwave (to use for dangerous things like fractal wood burning) they inadvertently remove this ground. Of course the RCD being able to function may not make much difference at the voltages and currents we're talking about...