Larry Molitor wrote:
>So, my question is: If PVC doesn't heat up in the
>microwave, and doesn't burst into flame when used as a loading coil, why is it
>so problematic when used as an anode choke? Rich, Ian, anybody?
Because the RF stresses in anode chokes seem to be higher than almost
any other application where these materials might be used.
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.
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. This doesn't happen
with ceramics - they may get warmer, but they remain stable.
PTFE is the best because it has very low losses, which don't get
significantly worse at high operating temperatures. On the other hand,
it's very easy to be sold an inferior "near-PTFE" material by mistake.
It also seems that quite innocent-looking differences in construction of
RF chokes and loading coils can make big differences in RF stresses, so
you get conflicting stories where the same material is OK for one person
but goes up in smoke for another.
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.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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