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Re: Glass loss question



Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

Not very lossy at 2.5 GHz maybe..
In a capacitor, though, a loss factor of 0.001 will be intolerable, but in a
1 kW microwave, it might only correspond to 1 W (if the whole 1kW goes
through the glass...) which might barely make it warm...


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, November 27, 2001 5:04 PM
Subject: Re: Glass loss question


> 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
>
>
>