Roy said:
I'd be a little suspicious of el cheapo cable with copper plated steel
center conductor. I'd be surprised if it was Copperweld, which has a
very thick copper coating. Copper plating thickness would be a logical
place for cost-conscious vendors to skimp.
An extra thin layer of copper wouldn't hurt at VHF and above, but it
could really be a killer at HF, where significant current could flow in
the steel.
skin depth in copper at 1 MHz is about 2.6 mils( thousandths of an
inch). At 10 MHz, 0.8 mils (goes as the square root of frequency)
AWG18 is about 40 mils in diameter. If the copper thickness were, say,
3 or 4 skin depths.. call it 4-10 mils, almost all the current is
carried in copper.
Copper clad steel is usually specified as a percentage of conductivity
of pure copper (in AC powerline applications, for instance) as say, 40%.
If we make the assumption that steel is an insulator, 40% CCS would
have 40% of the cross sectional area.. that would mean the cladding is
about 11% of the overall diameter.
Using AWG18 as an example, 11% of 40 mils is about 4 mils, so certainly
at higher HF frequencies, 40% CCS would be pretty close to pure copper.
even 20% CCS (which would have cladding about half the thickness of 40%)
wouldn't be all that lossy.
At the VHF and higher frequencies typically used for 75 ohm coax, a very
thin cladding would be as good as solid copper.
(I note that there is coax with silver plated stainless steel as the
center conductor and shield for microwaves in cryogenic applications)
(Steel is extra lossy at RF because its permeability further
reduces the skin depth by a large factor.) I'd definitely measure the
loss if I intended to use it at HF.
Always a wise idea, of course...
