At Simsmith bimetal line type – a comparison around the first MLL minimum I reported calculated matched line loss vs cladding depth for a single core copper clad steel conductor in a feed line such as Wireman 551.
The common assumption is that as frequency is reduced, so is loss, and at low frequencies loss is roughly proportional to square root of frequency.
That model is for homogenous conductors with well developed skin effect and is not applicable to the CCS line under discussion.
Above is a plot for various cladding depth on a 1.024mm (#18) 30% IACS (67µm cladding) CCS conductor at 1.8MHz where skin depth δ is 49µm. MLL is minimum around cladding depth 100µm or 2δ.
It is often stated that a conductor with outer layer of at least 3δ gives copper like performance, but in fact 2δ cladding is sufficient in this case (and slightly lower than a full copper conductor).
W551 specs state that it uses 30% IACS CCS which in turn is specified to have a 67µm cladding. So, it should be suitable for frequencies where the skin depth in copper is less than 33.5µm which is 3.8MHz. The curve is fairly low gradient at that point, and you could take that operation above 3.5MHz was quite fine.
At 1.8MHz we can see from the graph that loss is very slightly worse, insignificant really, and so whilst not optimal, it is still quite good.
Above is a plot of the current distribution on a 1.024mm (#18) CCS conductor with 17.8µm cladding at 25MHz where δ is 13µm. The W553 product with 19 strands (#30 / 0.25mm??) of 30% IACS CCS probably behaves similarly to this. At 30MHz MLL is close to copper and the current distribution explains why, the current density in the copper cladding is high falling to about 30% at the inner extremity of the copper. An alternative view is that at 1.8MHz, it is probably towards three times the MLL as the W551 line.
- Duffy, O. Apr 2017. A model of current distribution in copper clad steel conductors at RF.