I have certainly used Andy's technique with the microwave oven to test some
materials. Obviously any 'resistive' losses with generate heat ( see the
effect on metallised glaze crockery !!) but also these units cook by
utilising and exciting the water-molecule resonance in food. I wonder if it
possible that some materials containing small amounts of bound water (like
wood) might get hot in a microwave, but not show significant effects at LF??
OR do I have to rethink my new LF loading coil fomer built on plywood
cable-drum cheeks!!

I do remember a number of years ago diathermy used to be a popular therapy
for arthritus and for anyone living within about a mile of a hospital the
'office hours' qrm levels on HF could be quite high. I believe the
'dielectric heating'  was conducted in the 400-600kHz range. I seem to
remember this technique was also used in plastic fabrication for welding PVC
and such materials, but I have no idea whether the same frequency range was
used. One never hears anything of RFdiathermy now so perhaps this therapy
has died out. I believe the machines were in the 250W  to 1kW range, like
power multivibrators and fed from raw rectified 50Hz power. It must have
been in use before WWII as I believe many of  these units were pressed into
service by RV Jones and his team. I wonder whether there is any information
in this are that would be of use in selecting LF low loss materials.

Cheers de Alan G3NYK
Alan [email protected]