On 9/27/22 4:04 PM, Steve Maki wrote:
Thanks Jim.
I know this is a scenario of diminishing returns...how many skin
depths does one shoot for?
That's an interesting question. It has to do with the fraction of
current in the "skin". If you've got 5 skin depths, then almost all of
the current is in the copper. The current density goes as
exp(-d/skindepth) and exp(-5) is 0.007 - so 99.3% is flowing in the top
5 skin depths.
But as microwave people plating gold over nickel have found, if you're
at the 1 skin depth, then you're only down to 37% at that level, so a
fair amount of the current is flowing in whatever is underneath. And if
"underneath" is ferromagnetic, then the skin depth in *that* is really
tiny, so all the current is flowing in a tiny area, so the loss is high.
For 2 skin depths, you're down to 86.5%, and 13.5% is in the
underneath. That's like 0.6 dB, if you assume all the 13.5 is loss.
Note: this doesn't even get into "round" conductors. As long as skin
depth << diameter, then you can treat it as a flat plate.
On 9/27/2022 5:31 PM, Lux, Jim wrote:
On 9/27/22 2:22 PM, Steve Maki wrote:
I seem to recall ordering Copperweld wire, but it was a long time ago.
Just a quick note that after replacing the (insulated) #12 THHN with
the same length insulated #12 "copper clad steel tracer wire", we
could see no change in resonant frequency, so there seems to be
enough copper cladding for my purpose at 3.5 MHZ.
There will only be an inch or so of wire exposed to the air at the
center insulators, so that will be easy to monitor for corrosion
over time (such as it is :-) Or I may coat the exposed wire with
liquid tape.
73, Steve K8LX
Skin depth in copper at 4 MHz is 1.3 mils, 32 microns.
Copperweld
https://www.copperweld.com/application/files/4915/3833/2604/Welded_Copper-covered_Steel_CCS_21.pdf
21% CCS AWG 12 has 0.0808" diamter (2.05mm) with a nominal thickness
of 0.0616 mm ( so about 2 skin depths at 4 MHz)
https://www.copperweld.com/application/files/7115/3833/2604/Welded_Copper-covered_Steel_CCS_40.pdf
40% CCS has copper that is 0.2052 mm thick ( 6 skin depths)
On 9/21/2022 2:07 PM, Donald Chester wrote:
Steve Maki lists at oakcom.org Tue Sep 20 15:19:37 EDT 2022 wrote:
I had sworn off copperweld many years ago after noting that the
copper
plating did not last forever.
Was that real copperweld, or was it copper-plated steel with only a
minuscule layer of copper deposited on the surface, like the stuff
used for electric fence wire?
Real copperweld has a thick copper jacket that is practically
indestructible. I use #10, salvaged from an abandoned railway
telegraph line of yesteryear, scored free for the taking many
decades ago. You can beat it with a hammer or leave it buried in
the soil (as ground radials) for years and the jacket won't come
off unless it's deliberately removed with a file or grinding wheel.
That copper-plated junk begins to rust through in only a couple of
years. The copper is so thin that it probably has negligible
benefit from the skin effect; you are effectively using plain old
steel wire.
Don k4kyv
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