This is great a discussion on how to sheild a vessel containing amagnetron from leeking dangerous levels of microwaves is discussed. Thisis abot shielding outside a vessel with a magnetron inside.
For us this would be the opposite, shielding the inside from RF orMicrowave from the outside. A ti called me years ago telling me he made asteal and copper coffin. the copper worked much better but only when itwas sealed shut, then he couldn't breath. Here they discuss a spiraltube that could allow an opening while stopping microwave leakage.Another problem is having an electrical wire that passes through theshield acting as an antenna that totally bypasses the shielding makingit useless. Filters on theinside as well as the outside is the solution to this problem. The newprinter I bought came with a magnetic filter to be put on the powercable just before the current enters the printer through the powercable. An electric cable has with it a magnetic shield surrounding the currentflow.
The shape of the enclosure doesn't seem to be a factor butthe door must seal. Something called a 1/4 wave is used around the edgesof your microwave oventhat seals with an almost magical effect. This was very good readingand answered a lot ofquestions.
read on: Peter Rosenholm
These are some key questions, statementsand answers.
http://tinyurl.com/26pkrzw
subject:I am trying to coat a glass vesselwitha microwave paint that will contain the microwaves within the glassvessel.
Solicitaion for employment for anyone capable ofanswering this question: thanks foryour answer, and if you havea proprietary answer, then maybe we might have a employment situationor consulting arrangement. We are in need of a top notch microwaveengineer with thinking out of the box ideas.
Answer #1: are you completely enclosingthevessel? If not there will be significant leakage. For example, if thevessel is a flask, you would need a metal stopper in the top of it too.
Answer#2: But if you have a crack anywherewherethe vessel metal does not contact the metal stopper, there will be a lotof RF leakage.
Answer #3: (breathing holes while stillshielding) BTW, if you had to have anopen top ofthe vessel, you could make a vessel that had a long circular tube out ofthe top that was also conductively coated. If the diameter of the tubewas very small compared to the RF wavelength, then the tube would be a"cutt off waveguide" that will not let much of the RF leak out of thevessel. You would want the tube to be perhaps 2 to 4 wavelengths inlenght (or in I would imagine neccessary to breath)
Answer#4: Multiple layers separated by say1mmwill give a dramatic improvement in shielding. The layers do not need tobe earthed and are probably best left isolated. The biggest problem isjoining the shield to make it continuous. This is made worse when youenter the shield. If you enter the shield with say one power wire andyou fail to use a filtered feethru, the shielding will be defeated (willnot work). This would be the prime reason why your shielding, whilstlooking complete, will not work.
Answer #5: Copper, silver, gold orany metal isgoing to be good to any frequency. I work at mmwave frequencies, amongstothers, and beyond 100GHz. Metals contain and shield just fine at allfrequencies of interest to both you and me. The company may bespecifying the paint up to 1GHz because of their measurement ability.Alternatively the filler may be making the paint so poorly conductingthat it needs to be thicker than a normal paint thickness to beeffective.(Give us a link to the material you are using and we cancomment further)
Statementof difficulties: My firstvessel I just wrapped with athin copper metal and it worked very well.
That was before I lookedup the specs for copper, and could not find where it was good for 2.45GHZ.
Sometimes this microwave stuff is justmagic to me. I am a physicist and electronics engineer, but I reallysalute you microwave guys. Sometimes things work, and the next timethey don't.
Statement of dificulties 2: I havetried coating it (painting it)with silver, then copper, then silver, and it worked pretty good thefirst time. The next time I tried it, it did not work very well. Maybemy technique and not paying as much detail to it as the first time.The two cylinders around the vessels were a failure. Maybe I need toput filters on the power lines to the transformer, as I am using the oldtransformer, capacitor, diode circuit to feed the magnetron.
Answer#5: Copper, silver, gold or anymetal isgoing to be good to any frequency. I work at mmwave frequencies, amongstothers, and beyond 100GHz. Metals contain and shield just fine at allfrequencies of interest to both you and me. The company may bespecifying the paint up to 1GHz because of their measurement ability.Alternatively the filler may be making the paint so poorly conductingthat it needs to be thicker than a normal paint thickness to beeffective.(Give us a link to the material you are using and we cancomment further)
If you use vacuum deposition of the metal thenyou will not have a problem. If you use foil the critical part is howwell you join the sheets. Both faces needs to be really clean and youwill want to overlap the sheets by say 2cm to get a low impedance joint.
Answer#6: If you use vacuum depositionof themetal then you will not have a problem. If you use foil the criticalpart is how well you join the sheets. Both faces needs to be reallyclean and you will want to overlap the sheets by say 2cm to get a lowimpedance joint.
Qestion #2: >Maybe Ineed to put filters on thepower lines to the transformer, as I am using the old transformer,capacitor, diode circuit to feed the magnetron.
Answer #7: Yes!Youabsolutely need filters on ANY wire going through the shield. Even ifthe wire is not connected to anything at all, say 1 inch outside thescreen and 1 inch inside, the shielding will appear to be completelyuseless. What you have is a receive antenna on the inside and a transmitantenna on the outside. The shield might just as well not be there inthis case.
Statement # 3: thank you again. thatexplains why wethought we had a "ground leak". We used a spectruum analyzer with a rfprobe and found that the ground from the power plug was emitting astrong rf signal all the way back to the ground, but when we measuredthe continuity, it was what it was theoretically was supposed to be forthe guage of wire we were using. We also found it on the power leads.It was really baffeling us.
the link to the paint is:http://www.lessemf.com/paint.html
Thesilver paint that I used:
http://www.2spi.com/catalog/spec_prep/silver-paint.shtml
Answer #8: Changing units givesa lot of scope forerrors to creep in. However I will give it a go and you can check thesums.
Copper has a resistivity of 1.7E-8 ohm.metres
Thecopper paint is 0.3 ohm/sq for 1 mil (=0.001") thickness. This is0.00762 ohm.mm or 7.62E-6 ohm.metres.
Theresistivity of thepaint is therefore 448x higher than the pure metal. This means that theskin depth issqrt(448) times higher, ie 21x higher. Hence the skindepth at 2.45GHz is actually 27µm rather than 1.3µm. So theminimummaterial thickness needs to be at least 100µm.
It looks like youshould try at least four coats to build up the thickness to a suitablelevel.
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Answer #9: RF leaks easily through any smallcrack opening. Example, if in your microwave oven, the rf source and theoven cabinet had a 0.001" gap for even one inch length, you'd havearcing and high rf leakage. The door on a microwave oven does have asmall gap with a magic length of 1/4 wave which helps, seemingly bymagic (though not really) to keep rf in.
Answer #10: It sounds as if themagnetron is notcoupling into the load (water) effectively enough to preventinstability. If the load is inadequate the reflected power will be toogreat. It sounds as if the magnetron is squegging.
Answer #11: The loss in a rectangular cavity shouldbe similar to the loss in a cylindrical cavity
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