One thing that would affect RF conductor resistance is the paramagnetic
properties of the conductor. If, for example, it was bare iron or magnetic
steel wire, then we would have eddy current losses. And those losses could
very well be much larger than the ohmic losses measured at DC.
Copper has eddy currents, too. So does aluminum. Any conductor in a
time-varying magnetic field can have eddy currents.
The issue with iron is permeability, which can concentrate flux and decrease
skin depth. The end effect of any of that is really just what the RF
resistance is. Steel hardware in antennas and amplifiers can be perfectly
fine, depending on current and surface area and other things. Steel towers
work just as well as copper towers as a radiator. Steel chassis are fine.
Carbon conductors, even lacking any iron, would not.
Stick the end of a tank coil against a copper wall and the effect is nearly
the same as steel. Either one reduces Q, and reduces the inductance. Eddy
currents in the copper or steel cause a countering magnetic field that
reduces inductance. This is why iron RF cores are powdered. If they were
solid, inductance would decrease as they were threaded onto an RF coil.
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