On Feb 7, 2016, Ed Huntress wrote
(in ):

On Sun, 07 Feb 2016 18:13:06 -0500, Joseph Gwinn
wrote:

On Feb 7, 2016, Ed Huntress wrote
(in ):

On Sun, 7 Feb 2016 15:58:29 -0500, "Jim Wilkins"
wrote:

"Ed wrote in message
...
On Sun, 7 Feb 2016 13:14:22 -0500, "Jim Wilkins"
wrote:


These insulators support the two sections of the dipoles
without
letting them short to each other or the metal supporting
structure.
The radio waves picked up from the air appear as a tiny voltage
between the inner ends.
http://www.hottconsultants.com/pdf_files/dipoles-1.pdf

-jsw

I haven't heard of anyone using wood insulators boiled in
paraffin
since I had my first ham radio license, in 1960. You do drag out
some
old technology, Jim. g

I'm sure it worked well. In fact, I had a ladder line feeding my
first
40m dipole with paraffin-soaked wooden insulators, but that was a
gift
from an old ham who was helping me get started. I never measured
its
performance but I worked Australia and Norway with 50 W using
that
antenna.

Anyway, if you're looking for modern materials that are excellent
insultators, have low dielectric, and stand up to sunlight, you
have
a
lot to choose from. A QRP fanatic/friend I know, who is a
plastics
engineer, uses FEP. I don't know where you get it, but it's
supposed
to be good stuff.

My old end-fed wire (which is now down) has ceramic egg
insulators,
which I've use for most of the past 50 years.

--
Ed Huntress
KC2NZT

The two local station I use that antenna for are near 200 MHz, and
I
haven't found loss tangent data on thoroughly dried wood above 50
MHz.
Immersing wood in 280F molten wax certainly forces a lot of water
out
the end grain, and seals the wood to keep it out. I made a waxed
pulley from scrap casket mahogany that lasted unchanged outdoors
for
many years until a glue joint failed.

Ned Simmons sent me some cutoff ends of 1" Teflon rod to try. I
couldn't copy the existing insulators while they were 50' up in the
air, but now I can experiment with either machining the rod or
compressing it into a hot mold. It doesn't matter if the cycle time
is
4 hours per part or if I have to weigh out the quantity of plastic
that will just fill the mold.

-jsw

There are several factors involved, most of which I forgot over 20
years ago. The varieties of modified or alloyed FEP (including
Teflon)
are used for all sorts of RF insulation, and terminal insulators
should be easy, but be aware that PTFE has half the dielectric
constant of Teflon. And stay away from PVC. It's dismal for
high-frequency insulation.

Don't ask me why. Remember, I forgot that stuff a long time ago.g

--
Ed Huntress

I absorbed as much of it as I could as a Mitre lab tech and circuit
board designer. This is what we used for GHz circuit boards.
https://www.rogerscorp.com/acs/produ...Laminates.aspx
--jsw

Yeah, I guess that reinforces (forgive the pun) the idea that PTFE is
the right material for high frequencies -- low and consistent
dielectric constant, combined with extremely high resistance.

Low moisture absorption is also essential - water has a high dielectric
constant, and is also quite lossy.

Joe Gwinn

I should know enough to stay out of plastics discussions -- the
chemistry drives me nuts -- but I see that PTFE has extremely low
absorbtion, as does FEP.

I also see that I have been mislead, or I misread something years ago:
Teflon is the name for two products. The common one is PTFE. But there
also is an FEP version, which, I see now, DuPont calls "Teflon FEP."
Finally, the data on dielectric constant is somewhat contradictory for
FEP versus PTFE from different sources. Some say they're the same,
others say that of FEP is twice as high.

According to Wikipedia, both have a dielectric constant of 2.1 at 1 MHz.


Finally, the dielectric *strength* of FEP versus PTFE (which probably
is irrelevant here) is three times higher for FEP.

Yes, dielectric strength is rarely a big issue. Surface leakage is usually
the limiting item.


polymer chemistry torture off

How about some electronic materials torture? Waxy plastics (and paraffin
waxes for that matter) have low dielectric constants because the molecules
are non-polar, and the molecules are large and get in each others way.

Unlike water, where the molecules are polar and will turn to align themselves
to any ambient electric field. Liquid water has a dielectric constant of 80,
but ice is lower, because freezing retards molecular rotation. Test frequency
has a very large effect on the dielectric constant of water, but not so much
on waxy plastics.

Joe Gwinn