Re: Glass loss question

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

Original poster: "R Heidlebaugh by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rheidlebaugh-at-zialink-dot-com>

on 11/27/01 6:04 PM, Tesla list at tesla-at-pupman-dot-com wrote:

> 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
> 
> 
> 
I am glad to see something about glass capacitors with inteligence. Glass
and mica have been the best capacitors we could get in the past. We expected
them to last 60 years or more. The problem with them is they are not good
for mass production. They break when given rough treatmemt. Thet dont like
thermal shock, but they are the best for long time service. Plastic is good
and rugged, but they age and dont stand up to long service in remote
locations like mountain top radio relay stations.
   Robert  H