On Thu, 19 May 2016 21:29:06 +0100, Chris Hogg wrote:

On Thu, 19 May 2016 20:57:19 +0100, Graham.
wrote:

On Thu, 19 May 2016 18:56:44 +0100, Chris Hogg wrote:

On Thu, 19 May 2016 18:24:00 +0100, Graham.
wrote:

On Thu, 19 May 2016 10:04:14 -0700 (PDT), wrote:

On Thursday, 19 May 2016 17:44:04 UTC+1, John A Knight wrote:
replying to MM, John A Knight wrote:
http://www.scienceabc.com/pure-scien...appen-if-your-
microwave-oven-develops-a-hole-in-it-will-the-waves-leak.html
snip

That site's info re hole size & leakage is complete ballcocks.


https://groups.google.com/d/msg/uk.d...I/WSBDBrNF0GkJ

Clearly the OP needed to buy a new microwave, but from an RF
perspective the information on that scienceabc.com page is actually
quite accurate.


I thought it was complete ******** too; it treats microwaves as if
they're tennis balls, but I'm no microwave expert. If it's not
********, 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?

A narrow slot several centimetres in length as in an ill fitting door
would leak energy of the appropriate polarisation.
https://en.wikipedia.org/wiki/Slot_antenna


OK, that makes sense. Now what about the mesh in the door?

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.

--
Johnny B Good