On Tue, 10 Feb 2009 21:00:32 -0500, "Dr. Barry L. Ornitz"
wrote:

I have recently been working on an introductory paper for electrical
engineers on the mechanical and electrical properties of polymeric
plastics.
(...)

"The electrical properties of a polymer are due to the structure of the
polymer both microscopically and macroscopically. Most polymers are
dielectrics (insulators) as opposed to metals where electrons can move
freely.
(...)

Argh. Is this really for an introductory (beginning) publication? I'm
either fatally obsolete (the most likely possibility) or materials
have progressed well beyond my 25 year old chemistry experience. Looks
like I'll be doing some more reading to decode the technical terms.
Don't change, as I prefer accurate and complete explanations even if I
don't initially understand them.

I'm partial to seat-of-the-pants testing for RF loss using a microwave
oven. In general, if it gets hot in a microwave oven, it's going to
be lossy. I haven't tried various coil coating formulations and
tapes. However, I have tried various common hardware store plastic
and fiberglass products to find something suitable for a 2.4GHz
antenna radome. The problem was that it was impossible to assign a
numerical value to the RF losses using the microwave oven test. Some
would be hotter or less hot depending on the color (doping). My IR
optical thermometer was also rather sensitive to surface reflectivity,
resulting in additional errors. Still, the stuff that didn't work,
was fairly obvious by the deformation, smell, and sometimes smoke.
Incidentally, I've lost count of how many microwave ovens and toaster
ovens (for glue curing) that I've destroyed.

Thanks much for taking the time to supply the technical detail on
various materials and techniques. I've learned more from your
postings than from the usual uninformed speculative rubbish (such as
what I tend to post).


--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com
# http://www.LearnByDestroying.com AE6KS

Jeff Liebermann wrote:
. . .
I'm partial to seat-of-the-pants testing for RF loss using a microwave
oven. In general, if it gets hot in a microwave oven, it's going to
be lossy. . .

If it gets hot in a microwave oven, it's going to be lossy at 2.4 GHz,
but it isn't necessarily going to be lossy at HF or even VHF. However,
if it *doesn't* get hot in a microwave oven, it's probably pretty low
loss at any frequency up to 2.4 GHz.

There is, of course, a problem with reducing loss to a binary quantity
of "lossy" or "not lossy". A relatively high amount of loss can easily
be tolerated at points of low electric field strength, such as an
insulator at the feedpoint of a half wavelength dipole. On the other
hand, you need very low loss for some other applications like potting
high Q inductors or for feedline insulators when the feedline has a very
high SWR.

Roy Lewallen, W7EL