Gary,
I ask because I made a few measurements with a thermal probe on the side of a
single core FT-240-31 with 8T RG-11 that is a choke I have for my 10m antenna.
What I discovered is the the temperature rises very quickly on the core
surface when you turn on the power and then more slowly diffuses into the
center of the core.
Strange. Flux density, and thus heat production, happens in the bulk of the ferrite, not just its surface. They are strongest where the magnetic path is shortest, and that's along the inside surface of a toroid, while along its outside surface flux density and heat production are minimal. In between there is close-to-linear distribution through the bulk of the core.
Do you happen to know how much end-to-end voltage you had on that choke, when it got warm? According to my measurements and calculations, with 8 turns on that core, at 28MHz, it takes 194V end-to-end to cause a volumetric loss of 300mW/cm?, which equates to 8W total loss in that core, enough to heat it up reasonably fast. So that could be seen as a not-to-exceed value for ICAS service. In a 50? system where antenna balance is such that half of the signal voltage appears end-to-end, that's enough to handle legal limit power. But if the choke is used in a high impedance system, its power limit will be much lower.
Jim,
Skin depth is a thing, too. And since it's proportional to 1/sqrt(mu), higher mu makes the skin depth smaller. Fortunately it's proportional to sqrt(resistivity),
and the resisitivity of ferrites is very high
Let's not forget that skin effect is a phenomenom that affects conductors. There is no such thing as skin effect in ferrites, which are essentially insulators. The reduction of skin effect with ? happens when the electric conductor is also a magnetic material, such as steel wire. I hope nobody here is using steel wire to wind CMCs! But copper-clad steel wire is OK, as long as the copper layer is plenty thick enough to accomodate the skin depth at the frequency of operation.
About the dielectric loss in plastics:
There are two families of plastics: Polar and non-polar ones. In polar plastics each molecule is electrically asymmetric, making it react strongly to electric fields and thus absorbing a lot of energy at RF, turning it into heath. In non-polar plastics each molecule is electrically balanced, drastically reducing the absorption of energy from RF electric fields. PVC and nylon are polar plastics, polyethylene and teflon are non-polar ones.
Water is a polar molecule, and that's why water absorption in plastics increases their dielectric loss. But polar plastics have such high dielectric loss anyway that water absorption in them is probably of pretty low importance on their total dielectric loss.
