From: "Dr. Barry L. Ornitz" <firstname.lastname@example.org>
Subject: Re: Raising coil Q in regen RX- Glass Properties
Date: Thu, 1 Feb 2001 03:54:56 -0500
"Dick Carroll" <email@example.com> wrote in message
> firstname.lastname@example.org wrote:
> > Although in a great many applications, the former
> > material isn't all that important (...yeah, Reg, I use
> > cardboard tubes and PVC pipe sections too!...), be
> > careful about glass. There are LOTS of different kinds
> > of glass. Some glasses are fairly high loss. To make
> > matters worse, if you use glass as an insulator in a
> > high power system and it starts to heat, its dissipation
> > factor typically increases rather rapidly, possibly
> > leading to thermal runaway. We had a serious materials
> > selection problem in one of our products because of
> > exactly this phenomenon. If you stay with fused quartz,
> > you should be fine! :-)
> Every "Large lab glass vial" that I ever saw was Pyrex®
> glass, which is what is (was) used by Eimac for the
> envelope on their high power amplifier tubes. I'd expect,
> then, that Pyrex should be OK at RF, right?
> For sure you wouldn't want to use some leaded glass for
> a former.
Most large pieces of laboratory glassware are borosilicate
glass (Pyrex is one trade name). It can take much higher
temperature than "soft" glass. Receiving tubes generally
did not use the "hard" glass of transmitting tubes. Glass
has a fairly large dielectric constant, and its loss tangent
is nothing to brag about. As glass heats, it can become
quite conductive. Microwave printed circuit boards are made
with quartz fibers in Teflon rather than glass to keep down
losses and lower the dielectric constant.
If you have any Aquadag (colloidal graphite) like that used
to coat picture tubes, you can easily demonstrate that glass
is an ionic conductor. Get a piece of glass rod a few
inches long. Wrap two pieces of bare copper wire around it
separated about an inch. Coat the wire and glass with
Aquadag to hold the wire in place. Connect the wires to an
ohmmeter and heat the glass between the wires with a propane
torch. Long before the glass softens, you will see its
resistance drop. By the time it is at "red heat", it is
conductive enough that you can sustain its heating by
passing current through it. Many glass furnaces are started
with gas heating, but switch to electrical Joule heating one
the glass becomes conductive enough. The relationship
between temperature and resistivity of glass is known as the
Rasch and Hinrichsen law.
Often in high power, high voltage tubes, you can notice a
gradual coloration (usually brown or blue) with use around
some of the leads coming through the glass. This is usually
not de-vitrification but electrolysis and ion migration in
the hot glass.
However, going back to regenerative receivers at HF, I would
not worry too much about glass, ceramic, or most plastics.
Medicine bottles make excellent coil forms.
73, Barry WA4VZQ email@example.com