The photos of the PVC chimney with the burnt 'blisters' from Tony
King show typical overheating from the inside out, via dielectric
loss effects. The bubbles and blisters are what appears as the
material softens and pockets of either solvent, moisture or
inhomogenities in the plastic first melt inside and pucker and
sometimes explode. The inside overheats before the exterior as there
is no way to for the temperature rise inside to stabilize, it runs
away. I have seen material which is excellent insulation (Nomex)
rupture like a mini-shield volcano eruption like the ones on Hawaii
(big island). Goop oozes out, and once it runs away like that,
usually it arcs and burns up also.
The evidence there is conclusive that PVC is not a great RF
insulator. Moving it to a lower field area or away from the wall is a
work-a-round, but think about the RF loss still there, and the
gradual heating between the anode itself and the deck below the
anode. Its not really a great way to use expensive RF power, to warm
the chimneys.
Teflon sheet can be made into a simple chimney by cutting it in the
pattern desired to make a cone, and at the overlap, punch a few holes
and use nylon screws. These are more lossy, but they are tiny and as
you only need a few small diameter to hold it as a cone. Holding it
to the deck requires a sort of tab sticking out on 2 or 3 sides of
the bigger edge of the cone, which get bolted to the deck with normal
brass hardware.
I agree that microwave ovens serve a limited purpose in doing tests
of dielectrics for relative loss effects. I have been spoiled since I
found a VHF preheater made by WT LaRose that uses a triode at about a
kW at 90 MHz. That thing cost me less than $50 at a junkyard here,
and after cleaning it up, it can put out about 15 kV of RF voltage
between two plates. I test all hoses, insulating material, tube deck
material, etc, that will be exposed to big RF voltages, in that
machine at home. It often surprises people what that thing will do to
their perfect water hose for tubes. One of the things that always is
a give away is if a hose has a black jacket or black core in it. That
usually contains lots of carbon, and it only lasts a minute or so
before flame out.
73
K5PRO
John
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