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CCS ladder line suitability at lower HF

A very popular form of commercial ladder line is that using #18 wire, comprised of 19 strands of #31 30% IACS conductivity copper clad steel. The copper cladding on such a conductor is about 14µm in thickness.

At frequencies where the skin depth is less than about one third of the copper cladding thickness, the effective RF resistance is determined almost solely by the copper cladding (Terman, 1955, 21). For conductors with 14µm copper cladding, the effective RF resistance above about 200MHz is almost identical to copper, fine for their original purpose, VHF TV feed line.

At frequencies lower than 200MHz, the effective RF resistance will be progressively affected by the core, in this case steel with higher resistivity and more importantly, higher permeability, both leading to increased effective RF resistance. Increased effective RF resistance increases line loss.

The effective RF resistance of single core #18 30% IACS CCS conductor is as good as solid copper above about 10MHz.

These transmission lines are usually promoted without measured lower frequency loss data, and widely regarded as having insignificant loss.

If the loss had not been quantified, be cautious about assertions that it is insignificant.

References

Terman 1955. Electronic and Radio Engineering: McGraw-Hill New York.


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