On 2/8/21 1:51 PM, Manfred Mornhinweg wrote:
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.
Indeed - so it would be an issue in laminated steel cores, but nobody is going to be using that for RF.
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.
This is all true, but in practice, we rarely see "pure" plastics, except in applications like clear windows, or high end applications like spaceflight, where they want "traceability to sand". They're formed in various ways, with fillers and additives, as well as scrap from earlier batches or recycling.? So you can have a plastic that is normally quite low loss, in the pure form, but quite high loss in the form used. This is especially true in price sensitive applications - plumbing and wire insulation strike me as in that bucket.? I would think that coax has fairly well controlled properties, and the inner dielectric is quite pure. But hookup wire, machine tool wire, THHN (which is PVC, with a nylon coating on the outside), and plastic pipe are not so well controlled, except for manufacturing ease and price. As long as the THHN passes the voltage breakdown test, it will pass, lossy or not.? Even wire procured to MIL standards might not be particularly well controlled - they test to the spec, and if the spec doesn't call out RF properties, it might be variable.
