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ProfDrK (Materials) (OP)
21 Feb 08 1:08
I have a regular 1.3 kW magenetron/launcher/stub tuner/waveguide to coax adapter in series and was dissatisfied with losses. (tuner worked perfectly) I am using the exposed (1/4 wave) coax core as a radiator/antenna for heating within a hole. Can a coax center-lead be connected to the magnetron stub, insert some coax inline tuner and incur minimal losses to deliver the most heat to my material?
Right now, if it makes a difference, all work is in air but the goal is to run the coax into a vacuum chamber eventually. (I know about physics of plasma at 1 Torr)This eliminates the very cumbersome and expensive waveguide/vacuum-window/waveguide interface.
biff44 (Electrical)
21 Feb 08 12:25
Well, how DO you know the tuner worked perfectly?  Were the walls of the metal components very hot to the touch?  If not, then they were not absorbing the energy, so you must have had mismatch loss (ie much of the power was reflected back into the magnetron).   Another possibility was that the components WOULD HAVE WORKED fine, except that something arced over in the waveguide and reflected all the power back to the magnetron.
VE1BLL (Military)
21 Feb 08 12:33
The words magnetron (1.3kW) and coax raises the obvious question: Is the coax inline tuner and associated coaxial cable capable of handling the output from your 1.3 kW magnetron?

Even the cheapest $59 600w microwave oven uses waveguide internally, as opposed to probably-cheaper coaxial cable, for a reason - the smell of molten coax would affect the flavour of the food.
ProfDrK (Materials) (OP)
22 Feb 08 3:03
The tuner is automatic and digital, and it reports forward and reflected power and stub lengths in real-time. Tuner was working perfectly like I said.
I wanted an answer about direct magnetron to coax connection, not critique of my experiment that I know already worked. My connectors are 7/16 DIN style which are rated 2 kW.  The 3/4 inch coax I used was not fried or arced, just a little warmer than room temperature at the working end. So I don't appreciate jokes about burning smells and why coax isn't used in ovens. I see no electrical reason why I can't run the magnetron stub right into the coax core. The unknown is the shield and transition from air gap to dielectric coax cladding. My guess is nobody sells such a device because coax is not best for heating purposes and that is why I need to make one myself.
VE1BLL (Military)
22 Feb 08 8:45
The extra details about your coax (which were obviously not included in your first post) help to clarify that the coaxial components are perhaps not an issue.

You have to remember that we can't see your coaxial components through the Internet. If you don't provide the details up front, then we must ask the obvious questions. We can't tell if you're a wingnut using RG-58 (or not)without asking. The humour was simply a very concise way of making the technical point about power and loss. I'm sorry you don't like the humour, but I'm afraid you'll simply have to suck it up, Sunshine.


To offer another suggestion:

Although your inline coax tuner may provide a match along the coax; what about the match at the output of the tube? Is the tube designed to feed the characteristic impedance of coaxial cables? How do you prevent the stub from radiating? ...isn't it designed to radiate? Can you make that tube-coax junction without creating an impedance bump? How would you confirm that you've done a good job with that junction? Can you sacrifice a tube to measure what the tube sees from inside?

biff44 (Electrical)
22 Feb 08 17:13
I do not think he was joking with you.  Because burning teflon is very poisonous, he was just trying to save your life.  Your use of non-eningeering terms, such as "magnetron stub", lead one to assume you did not understand the basics.

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