> >That might be true in some HF applications but certainly not for coil
forms!
> You're saying there is a material which passes the "microwave oven"
> test but is *not* suitable for HF coil forms?
Yes, that is what I am saying. It is HOW we use the material that is
important!
Pure Teflon will significantly de-Q a coil if placed in an area of strong
electric fields because the dielectric will increase stray capacitance.
Because a material gets hot, it does not mean the material will add
significant loss. All you have to do is use the material where the
displacement current or electric field is low.
I think the microwave folklore comes from the idea that the dielectric must
dissipate power to add loss to a system, or that one that does get hot will
always add loss.
Neither idea is true. Anything that increases capacitance in the coil
increases loss, even lossless dielectrics will increase a coil's loss
(sometimes significantly). A material with high dissipation factor has no
effect as long as we are careful to not place it in a critical area.
73 Tom
|