Rudy N6LF researched a number of issues that relate to these questions =
http://rudys.typepad.com/files/insulated-wire-and-antennas.pdf
Andrew published some research that shows the PIM (passive intermod)
differences between braid and solid copper shields for coax. I couldn't
find that pdf on the web, but it shows how braid generates more IM
products than solid Cu tubing. I think its pretty insignificant for HF
ham radio.
Wire diameter/length ratio significantly affects SWR bandwidth (or Q if
you like), the formulas are in the handbooks. One of my more useful 80m
dipoles was made from scrap 7/8" bare Al CATV hardline, using the shield
as the conductor.
My preference for wire antennas is to use insulated Davis RF copperweld
stranded wire to increase the resistance to corrosion and reduce the
visibility. It's strong and slippery.
Grant KZ1W
On 4/24/2018 12:05 PM, Shawn Donley wrote:
Some recent posts on grounding reminded me of something I've always wondered about. How
is the radiation efficiency of a copper wire HF antenna affected by oxidation of the
copper over time? Empirical evidence is that any effects are small/negligible,
otherwise the dipole you put up last year would not work so well this year. My limited
understanding is that the two oxides of copper, Cuo and Cu2O, are semiconductors. So
after a while, the outside of the wire is covered by something approaching an insulator
(relative to clean copper conductivity). The depth of the oxide, as far as I could
research, is on the order of 100 nano-meters. OK...so the RF current is forced under the
oxide and follows the skin depth with frequency relationship. Not much effect on the
current or the "RF resistance" of the wire, if I can be forgiven for using that
term. But what about stranded copper wire? That's where things might get interesting.
Does the skin effect with clean copper wi
re
cause the RF to stay on the outside of the overall collection of strands, all of which
have good contact with their adjacent strands? If so, what happens when all the
individual strands are oxidized and not in low resistance contact with their partners?
Anyone know of actual measurements of the effects of oxidation or how such a measurement
would be done? Short of measuring the Q of a tuned circuit built with "clean"
and oxidized wire inductors, I'm not sure how you could measure the effect and even less
sure of how those measurements would translate to the original question...effects on the
radiation efficiency of an antenna.
N3AE
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