Fuck off homeownershub.com
Future generations will wonder in bemused amazement that the early twenty-first century?s developed world went into hysterical panic over a globally average temperature increase of a few tenths of a degree, and, on the basis of gross exaggerations of highly uncertain computer projections combined into implausible chains of inference, proceeded to contemplate a rollback of the industrial age.
Richard Lindzen
I thought it was complete bollocks too; it treats microwaves as if they're tennis balls, but I'm no microwave expert. If it's not bollocks, and if microwaves can't leak out of a hole smaller than 12 cm. why all the fuss in the past about microwaves leaking around ill-fitting doors, and why the need for a fine mesh across the window in the door? If the article is OK, you barely need to shut the door, let alone worry about leaks around the edge, and wide-mesh netting would do in the window allowing a much better view of the contents. What am I not understanding?
This issue is nothing specifically to do with microwaves. It relates to
*all* sorts of waves and their behaviour. Look up diffraction. I'd do it meself, but SWMBO has just announced that the scoff is ready.
The notion that there will be no leakage if a hole is 1% less than the wavelength is quite accurate?
NT
OK. I know a little about diffraction (A-level physics circa 1962 but not a lot since), but I looked it up on Wiki anyway. It says, amongst other things, that diffraction is most pronounced when the width of the slit or aperture matches the wavelength of the radiation, which is the situation in this case. But the aperture doesn't block the radiation completely. You just get a pattern beyond the aperture of alternating destructive and constructive interference, either side of the main beam. I wouldn't want my kidneys positioned either in the main beam or at a point of constructive interference, while watching my supper cooking. And it doesn't answer my point about gaps around the door, or the screen mesh across the window.
A narrow slot several centimetres in length as in an ill fitting door would leak energy of the appropriate polarisation.
OK, that makes sense. Now what about the mesh in the door?
I found this:
I guess the implication is that although the wavelength is a few cm, the mesh has to have small holes to sufficiently reduce the radiation that would diffract through the holes.
As anyone who has ever tried to make an RF-tight screening box will know, the problem of totally confining the RF energy bouncing around the innards of such an enclosure so as to eliminate unwanted interference to other kit or other modules within the apparatus is not quite so simple to solve as it may appear to the uninitiated (I had quite a laugh at the EMC 'measures' being introduced in the construction of personal desktop computer cases nearly two decades ago).
Whilst a screening mesh with a hole (or a few holes) less than a wavelength in diameter makes a reasonably effective screen, it's not perfect, merely 'perfect enough as makes no difference' in practice.
Such a screen won't totally stop such radiation passing through, it merely attenuates it to a much lower level (maybe 40 to 60db per sub- wavelength hole?). This is usually more than enough when the waves only get one chance to pass through the screening mesh. However, in the case of the cooking cavity, they'll get hundreds, if not thousands of attempts at leaking out as they bounce around the low loss cavity.
The food will absorb most of this energy... eventually but there will be eigentone pathways not obstructed by the food where the levels of radiation will be magnified hundreds to thousands of times the average level in the cavity. Quarter wavelength diameter holes in the viewing screen mesh won't offer sufficient attenuation, halving the diameter of these holes will require a quadrupling of said holes which counters the improvement in screening to some extent, hence the *very* small size compared to the wavelength involved to sufficiently attenuate radiation leakage to below a level deemed to be acceptably safe.
The smaller the holes, the better and the most likely reason for their extremely small size is probably more to do with the lowest size possible without offering too much obstruction to viewing the contents rather than a maximum allowable size to contain the microwave leakage to an acceptable minimum.
How stunningly Christian.
Maybe I should join.
Ahole, how did that happen anyway? Basically, you need to use screws and a bit of metal. The piece has got to be seen as part of the whole. Most ovens interior size is cavity which loads the Magnetron to some extent, but there are always areas that are more concentrated where standing waves and cancellations occur, hence the need for a turntable to move the food around through these variable areas. actually the leakage from a small hole is not as great as you might think. Often it depends on where the whole is and the design of the cavity. A small patch should be tolerated as long as it is seen as part of the cavity wall.
Brian
The issue of holes in Microwaves does beg the question how did the hole get there in the first place of course. I stand by my method. I assume one can still buy those microwave leak detectors. The most likely place for the microwaves to leak is at the door bottom which tends to rust away.
Short term, ie ten seconds or less exposure is not going to do you much harm as you measure the leakage. Remember its a party trick in radar labs to heat up your sandwhiches using the open end of a waveguide when it is connected to a magnetron. :-) Brian
The issue us not diffraction, but reflection.
You don't need a perfect surface to reflect.
Its the same principle as using a yagi array or similar. The presence of the metal bits affects propagation around a far greater volume than they physically occupy, because a field is induced in the nearby areas.
What? Christianity or home-owners hub.
YOu are too stupid to even be a Christian, but I suspect you'd fit right in at homeowners hub.
Chris was asking why you couldn't just use wide-mesh netting. I'm saying that considerable energy can diffract through that (apparently), even if the mesh size is less than the wavelength.
Its the same issue as a half silvered mirror.
It doesn't reflect 100%. Diffraction is not really relevant.
If considerable energy can diffract or otherwise pass through wide mesh netting, and I don't doubt it for a minute, how can a bloody great hole 12cm across not be a major hazard, as the article linked to at the top of this thread said wasn't? (hope I've not got double negatives there, but YSWIM!)
well 2GHZ is around 3.75cm for a quarter wave..
so its of the order of a wavelength anyway.
I am fairly sure that diffraction is a bit of a red herring though.
Its one of those lectures I probably didnt pay much attention to, but I think its a matter of reflection.
You can build a radar reflector adequately out of chicken wire..
replying to MM, Alien wrote: I bought a Sainburys own brand stainless steel microwave, the clock never kept time within the first month. The base floor under the rotory plate had a hole in it within three months. I would love to show the photos, but that nasty company has more money than I have!
replying to Fash, Michigan wrote: Hmmm.. why fix i perfectly functional machine when you can add it to the rubbish tip and just frivolously buy another.. repeatedly.. year after year after year ???
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