For insulation in amplifiers, I suggest Teflon. It's soft -- not
strong -- but is easy to cut, drill, and tap. It has fairly good
properties as an insulator, especially looking good at VHF
and higher. It's somewhat out of vogue now, and certainly
wouldn't be a commercial choice, but all of the Teflon that
would be required for an amplifier project will usually not
cost very much compared to the time it would take you to
find an acceptable alternative. Teflon is slightly abrasive,
but usually drill bits and taps will not be damaged too
much by an amplifier project or two. Overcome the
problems with the physical properties of Teflon by using
fairly massive pieces.
Oh, I admit that Teflon is not going to be the lowest cost
material, nor is it going to have the very best properties,
but it is widely available, and it is a fairly safe choice.
Honestly, though, I'm better at antennas than at amplifiers.
If you're looking for a dipole end insulator, 15 feet or so of
one of the synthetic rope types will usually satisfy, even when
soaked in rain water, if the tie-down point is non-flammable
and preferably grounded. This means that it is smart to
provide a ground wire from the tie point to the nearest
possible ground if the tie point is, for instance, a screw eye
into a wooden fence. I've used polypropylene rope for
center insulators on Field Day with no signs of trouble,
but that was when running barefoot (and was not tested
when wet).
Another few possible sources of end insulators:
I was never in a position to see how this one survived over
the years, but back in the 1970's decade when I worked
for W6HX putting up ham radio towers and antennas,
several of the installations included owner-supplied
end insulators that were apparently fashioned after the
recommendations of members of the Southern Calif. DX
Club. They took long pieces of clear Plexiglas rod and
drilled a hole in each end to pass wire through. Usually the
rod was 3/4" or 1" thick. Clearly this is not the best
choice of material because it is ever so slightly conductive
and somewhat vulnerable to U.V. damage. All the
same, it's so much lighter and impact resistant compared
to porcelain that it made sense. All kinds of Plexiglas
seems to be available on the internet. The antennas that
I put up had Plexiglas insulators around 18" to 2' long.
I knew someone who bought fiberglass quad spreaders
that were too long -- used for 10/15 meter directors
that were inserted in between the triband elements --
and used the extra lengths of fiberglass as end insulators.
They didn't last forever -- the wires cut slots into the
fiberglass and needed to be cut short and re-drilled
every year or so.
PVC pipe can make a nice end insulator. If there is any
question about it's properties as an insulator, then
make it very long.
Any material such as PVC pipe can be tested for its
performance as an insulator by putting a foot long piece
into a microwave oven and trying to heat it up. If it
stays cold then it's going to do a good job. If it
proves to be a poor insulator, then use a 2 foot length
or so as the end insulator and you will usually be
satisfied with the results.
----- Original Message -----
From: "John T. M. Lyles" <jtml@lanl.gov>
To: <amps@contesting.com>; <r@somis.org>
Sent: Wednesday, March 24, 2004 9:37 AM
Subject: [Amps] insulation
G10 will melt down into an awful stinkin' and
burnin' mess when heated in high Rf fields. I
rarely use it anymore, except for PW
applications. It is certainly a big step above
Delrin acetals or nylons. Have settled on several
wonder materials like:
Rexolite (crosslinked polystyrene) rod and sheet
Polyetherimide (ULTEM* 2300) with 30% glass, also known as Tempalux*
Polysulfone (UDEL*), also known as Thermalux*
G7 (silicone resin reinforced glass)
All more expensive, but if you want the highest Q
and no heating, they are worth it. These are all
high temperature engineered plastics. If you are
concentrating E field flux in the dielectric,
even with a kW, G10 will heat up. Good old UHMW
(ulta high molecular weight) polypropylene and
polyethyline made good insulators but have low
glass transisition temperatures and will soften
and dimensionally change with heat.
Your local plastics supplier has them. Pricewise,
the first two are about $1000 for a square foot
of 1 inch thick material! The rod stock is much
cheaper, and for smaller coils it is more so. The
G7 is more difficult to machine or turn on a
lathe than G10 due to the lamination layers. But
it is excellent material structurally as well as
RF'ly.
Cheers
John
K5PRO
Rich AG6K said:
> I bought 50-lbs of porcelain clay and I tried making porcelain
>pottery. The shrinkage factor is about double ordinary stoneware
clay,
>and porcelain clay is hellish to work with. My advice is use G-10
>fiberglass-epoxy for insulating. It's good around RF, strong, and
easily
>drilled/machined.
>
>>My YL has a ceramics business, and I started thinking about trying
to
> >manufacture my own ceramic (porcelain) insulators. Has anyone
tried this?
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