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Electronics Forum / Basics / May 2007Shielding from microwaves
Hi, I am having a little microwave experiment, and I want to shield some cracks from leaking microwaves. A friend said to me that a paper-type called "mica" is used for such reasons in microwave ovens. Actually what he told me is that it allows microwaves to pass through, but it blurs them so as they are not focused as before. Is it true, or does anyone know any better way to make leaking microwaves less harmful (apart from Faraday cage, this is too hard for me to make, I suppose). > Hi, I am having a little microwave experiment, and I want to shield some > cracks from leaking microwaves. A friend said to me that a paper-type [quoted text clipped - 5 lines] > microwaves less harmful (apart from Faraday cage, this is too hard for > me to make, I suppose). Mica is a mineral that naturally forms thin sheets. It used to be called "isinglass." Mica is an insulator, so it's transparent to RF; in thin-enough sheets, it's even transparent to light. In a microwave oven, what defocuses the microwaves is the fan in the cavity. As the blades rotate, they change the geometry of the cavity, so it flips between various resonant modes. A Faraday cage is just a continuous conductive shell. It can have holes in it, as long as the holes are small relative to the wavelength of interest. The earliest microwave ovens had contact fingers around the door, to complete the cage. Current ones use a trick: the door overlaps the edge of the cavity and is _insulated_ from it. The length of the overlap is half of the operating wavelength, so the radiation encountering the crack sees a half-wave open transmission line. That looks like a dead short. > Mica is a mineral that naturally forms thin sheets. It used to be called > "isinglass." Mica is an insulator, so it's transparent to RF; in [quoted text clipped - 11 lines] > the crack sees a half-wave open transmission line. That looks like a dead > short. The thing is a classic experiment that proves that there is no ether. For various reasons we do not want to have microwaves leaking behind the surface they hit. I think the ideal for it would be some kind of material that "absorbs" the microwaves, as lead does for radiation. If you know such materials or have better ideas, it would be great to let me know. > The thing is a classic experiment that proves that there is no ether. > For various reasons we do not want to have microwaves leaking behind the > surface they hit. I think the ideal for it would be some kind of > material that "absorbs" the microwaves, as lead does for radiation. If > you know such materials or have better ideas, it would be great to let > me know. At microwave (and lower) frequencies, the way to prevent EM radiation from penetrating a surface is to make the surface very conductive. Copper works; silver-plated copper works better, partly because of the lower resistivity of silver and partly because silver oxidizes slowly, and silver oxide is conductive. To make a microwave-proof enclosure, just make a Faraday cage and make sure that there is no crack that might act as a slot antenna. The "tuned seal" strategy used in microwave ovens works for a single frequency, but if you need to stop broadband radiation, there is no substitute for a continuous conductive shell. Yes, there are surface treatments that absorb microwave energy instead of reflecting it. The best ones are military secrets, because their main use is to make things hard to detect by radar. BTW, you do realize that "radiation" means more here than it does in the National Inquirer? > The thing is a classic experiment that proves that there is no ether. > For various reasons we do not want to have microwaves leaking behind the > surface they hit. I think the ideal for it would be some kind of > material that "absorbs" the microwaves, as lead does for radiation. If > you know such materials or have better ideas, it would be great to let > me know. charcoal is also black at microwave wavelengths, it could take fairly thick panels, but it's cheap. The stuff will heat up as it absorbs energy.  SignatureBye. Jasen >> The thing is a classic experiment that proves that there is no ether. >> For various reasons we do not want to have microwaves leaking behind the [quoted text clipped - 5 lines] > charcoal is also black at microwave wavelengths, it could take fairly > thick panels, but it's cheap. The stuff will heat up as it absorbs energy. On the web I came across references to Velostat. Does anyone know if it absorbs microwaves and where may one get it from? > On the web I came across references to Velostat. Does anyone know if it > absorbs microwaves and where may one get it from? That's for guarding devices against ESD. > >> The thing is a classic experiment that proves that there is no ether. > >> For various reasons we do not want to have microwaves leaking behind the [quoted text clipped - 8 lines] > On the web I came across references to Velostat. Does anyone know if it > absorbs microwaves and where may one get it from? It is a 3M trademark for their antistaic products. <http://www.google.com/search?num=100&hl=en&newwindow=1&rls=GWYA%2CGWYA%3A2006-31 %2CGWYA%3Aen&q=Velostat+3m+foam&btnG=Search> SignatureService to my country? Been there, Done that, and I've got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida >> Mica is a mineral that naturally forms thin sheets. It used to be called >> "isinglass." Mica is an insulator, so it's transparent to RF; in [quoted text clipped - 20 lines] > you know such materials or have better ideas, it would be great to let > me know. Approximately what frequency? I'm not sure that this is the best solution, but there are materials specially made for absorbing microwaves. For example, see: http://www.eccosorb.com/pages/63/ECCOSORB%C2%AE If you made the surface they hit a very large sheet of metal, most of the microwaves would be reflected and very little would leak behind the surface they hit. Is there a good reason why you want to use microwaves rather than visible light? Signature----------------------------------------------------------------------- To reply to me directly: Replace privacy.net with: totalise DOT co DOT uk and replace me with gareth.harris > Approximately what frequency? Close to the frequency of the cooking microwaves. > I'm not sure that this is the best solution, but there are materials > specially made for absorbing microwaves. [quoted text clipped - 6 lines] > the microwaves would be reflected and very little would leak behind the > surface they hit. I am interested in such microwave absorption materials. > Is there a good reason why you want to use microwaves rather than > visible light? Yes, the experiment that proves the non-existence of ether uses microwaves. >> Approximately what frequency? > [quoted text clipped - 17 lines] > > Yes, the experiment that proves the non-existence of ether uses microwaves. All that that proves the non-existence of is sufficient subtlety in the experimental protocol(s) to verify its existence. Cheers! Rich >> Hi, I am having a little microwave experiment, and I want to shield some >> cracks from leaking microwaves. A friend said to me that a paper-type [quoted text clipped - 21 lines] > the crack sees a half-wave open transmission line. That looks like a dead > short. Good explanation, but one small error - it's a quarter wave line, not half wave. See: http://www.microwaves101.com/encyclopedia/quarterwave.cfm Signature----------------------------------------------------------------------- To reply to me directly: Replace privacy.net with: totalise DOT co DOT uk and replace me with gareth.harris > Good explanation, but one small error - it's a quarter wave line, not > half wave. > > See: > > http://www.microwaves101.com/encyclopedia/quarterwave.cfm Oops. I _knew_ that. I plead temporary insanity due to caffeine deficiency. > Hi, I am having a little microwave experiment, and I want to shield some > cracks from leaking microwaves. A friend said to me that a paper-type Are you experimenting with a microwave OVEN?!!! If so, I hope you know that ovens are extremely dangerous, since any significant microwave leakage will cause permanent blindness. It's called "RF cataracts." If not an oven, then what is your microwave source? What wattage? And why do you think there might be leakage? Do you have a microwave meter to tell you the milliwatts of leakage? When it comes to high power microwave radiation (watts and higher,) I would say that you MUST build a faraday cage. If you can't, then it's stupid to experiment with it. Here's another way to say it: playing around with leaky microwave ovens is deeply stupid; much more stupid that playing with shotgun shells and a hammer. > called "mica" is used for such reasons in microwave ovens. Mica is transparent to microwave radiation. It's not a shield, it's a window. And no, mica does not "blur" microwaves. Your friend doesn't know what he's talking about. It's just there to keep the exploding spaghetti sauce from getting on the magnetron antenna and causing a short. |
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