In 10.33 21/01/01 , Andy wrote:

 It is very straightforward to find if any material intended for an
insulator or for placing near loading coils is itself lossey.   Just put it
in a microwave cooker for a few minutes.  If it gets hot or catches fire it
may absorb something at 137k.   I've yet to find ANY plastic that gets warm.
Plywood is terrible, it got too hot to touch after 20 seconds

Some PVC tubing gets warm. And about the plywood, it is glued curing
the glue with RF (hundred of kW per m2, frequencies around 1-3 MHz).

If anyone is interested I have many used triodes for that purpose ... up to
500 kW RF out ...

73, Marco IK1ODO