At 06:20 PM 1/15/00 +0000, Ian White, G3SEK wrote:
>There's a lot of variation between different grades of the same plastic
>- for example, "PVC" can range between very soft and quite rigid, with a
>wide range of filler materials and plasticizers. However, many grades of
>PVC perform quite well in the microwave oven test, relative to PTFE
>which is definitely the best.
Since they are already tubular, I'm most familiar with plumbing PVC's. In
the 30 years or so that these materials have been readily available, I have
yet to find any that has relatively poor RF performance. I keep expecting
to find some, but so far have been lucky I guess!
>
>It's also interesting to note that some grades of PVC get less warm than
>some ceramics.
>
>The main difference is that most plastics tend to soften on heating, and
>their RF absorption then increases significantly (greater molecular
>mobility) and that can lead to a runaway situation.
It is interesting to note that CPVC (clorinated polyvinylcloride?) is rated
for hot water service and shows little or no softening at water
temperatures in excess of 150 deg F. This stuff is also a bit stiffer to
start with than regular grade.
At least one major antenna manufafturer uses schedule 40 sunlight resistant
PVC electrical conduit for elememt insulators. His antennas are rated for 5KW.
>The other problem is that RF chokes and loading coils can be subject to
>direct arcing between turns. This isn't the plastic's fault, but once
>the arc begins to burn the plastic - and even PTFE can suffer this -
>then the losses increase dramatically and the former ends up just as
>dead.
I'd bet this scenario is responsible for the poor RF reputation of plastics
in general. As with any thing else, you can't hold the component
responsible for poor design or operator error.
Just for the record, I've blown more steatite insulators than plastic ones,
go figure! Now after having said all of the above, my preference is still
ceramic, teflon, G10, in that order.
73,
Larry - W7IUV
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