Re: Glass loss question

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

> I know glass is a lousy dielectric at tesla frequency, I know it
> from you and by experimentation. Could someone explain me why
> glass is so little heated by microwave. May be get disturb by an
> electric field between 2 conductor plate and be cross by
> electromagnetic field have not the same effect on dielectric ????
> or the loss are there only in a range of frequency????
> 
> Cheers,
> 
> Luc Benard

	The answer is that glass is not necessarily a lousy or lossy
insulator!  There are glasses and glasses and glasses.  The reason the
glass doesn't get very hot in the "microwave" is because it isn't that
lossy.  Loss goes up with frequency, so the loss will be much worse at
2450 MHz than at say 250 kHz.  By the way, in the early days of
"wireless" telegraphy it was very common to use banks of "Leyden jars"
for the main capacitor.  These were glass plated with copper on both the
inside and outside.  There are still plenty of these oldies in existence
and it would be interesting to measure the dissipation factor of one.

	One of the problems with using window glass for high-voltage capacitors
is the non-uniformity of it, which can lead to puncture at much lower
voltage than expected.

	By the way, Dow Corning used to make (and maybe still does for all I
know) a line of monolythic glass capacitors for electronic use,
including RF.  I still have a lot of them, and they are not a bit lossy,
although I haven't measured the loss tangent of any of them.

Ed