Skin Depth & Coated Wire

It is not uncommon to use plated wire in spacecraft. The plating is (usually) either silver or nickel – or unplated copper. Of interest is the effect of this plating on signals. I am not considering coax cable here ; simply versions of “hook-up wire.

As frequency increases, skin depth decreases. While lower frequency signals propagate within the wire – primarily the copper section – a greater proportion of the signal propagates through the coating at higher frequencies. Silver has a slightly higher conductivity than copper; nickel’s conductivity is significantly lower. Just to add interest, nickel is also ferromagnetic with a permeability >> 1.

Assume the current return path is “far” away. The wire is infinitely long. Current in this wire creates a magnetic field both inside and outside the conductor. The total wire inductance is the sum of the two components. While the internal inductance is due to the magnetic field within the conductor and independent of the current at DC, the external inductance is dependent on both the current and the resulting external field 9which is affected by geometry, hence – an infinite straight cylinder for analysis.

As frequency increases, the skin effect confines the magnetic field to a smaller cross-sectional area of ring-shape which reduces the effective internal inductance. At very high frequencies, the current travels essentially on the surface of the conductor and the total inductance is almost completely due to the external field.

What happens when a copper wire is plated with a material of differing material properties?

I didn’t start out intending to get this deep into the weeds … but I’m curious …

The interest here is the effect of the coated wire – especially magnetic nickel – on signals with frequencies where the skin depth is such that significant portions of a digital signal straddle the coating. The impedances are different; is the group delay significantly affected when the lower harmonics are primarily in one material while the higher frequencies in the other. Maybe – maybe not. I’ve never seen any discussion on the topic. perhaps I’m doing nothing more than proving wheels are round.

A work still in progress – more to come.