The value of dielectric constant for most plastics is well known and not
necessary to an approximation of the physical quarter wave length of a piece
of tubing. The DE will only shift the resonance somewhat of a thin hollow
cylinder, but with the Microwave oven somewhat broad anyway, a tube that
looks like a quarter wave cavity would be likely to overly react to the RF.
What you are wanting is to test a small sample piece that is non resonant
and see if it heats up within the time it takes RF to boil water in a cup.
The loss in plastic or other insulation is directly proportional to
frequency, thus the use of microwave to characterize in a rough way, a
material for any RF frequency use.
My caution was because although small insulators of plastic are is what is
usually tested by oven, a coil form of the size for the antenna tuner might
easily approach a resonant state, as some odd multiple of quarter
wavelengths.
-Stuart
K5KVH
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