On 4/24/2013 5:15 AM, Bob McGraw - K4TAX wrote:
Comment.....unproven.....leave the insulation on the wire. It serves
to prevent oxidation of the copper, to which oxidation provides a
higher skin resistance than bare clean copper.
The insulation also lowers the resonant frequency slightly (1-2%)
I use #12 AWG solid for my antennas and #10 AWG solid would likely be
better, although heavier and a bit more costly, but being less likely
to stretch with snow and ice.
Excellent choices. My high dipoles are under much more tension than is
typical of most ham installations because they are up 110 ft, suspended
between redwood trees that sway in the wind, and are fed with RG11. Over
the years, I've used both #12 and #10 THHN, but also #8 bare copper that
I've stretched to "hard draw" it, which makes it about #9. I think that
hard drawn is the most mechanically robust. The THHN, which is not hard
drawn, stretches over time, and I have to drop it every few years and
shorten it to bring the antenna back to resonance. My wires are all
suspended using pulleys on both ends, with one end tied off and the
other with a 100# weight (a big water jug filled with dry sand).
As to "THHN" -- the designation is simply for the voltage and
temperature rating of the wire insulation, the definition of which is
within the National Electrical Code, the standard code of practice that
is adopted by nearly all local governments to be "the law" for all
installations. In the US, if you go to any hardware store or electrical
supply store to buy wire to use in wiring your home, it will be THHN, or
multiple conductors of something close to it within some sort of a jacket.
Jerry Sevick, W2FMI, wound some chokes like I've described with THHN,
and others with enameled wire, and published a bit about them in his
last book. I've not seen anything to suggest that he understood the
importance of a high RESISTIVE common mode impedance with respect to
power handling nor to the suppression of noise, but he did note that
THHN yielded Zo on the order of 100 ohms and enameled wire on the order
of 50 ohms, both of which are in agreement with my observations. .It's
also important to realize that while he had access to #43 (lossy) and
#61 (far less lossy) materials, the much newer #31 material was
developed by Fair-Rite after he was active, so was not available to him.
73, Jim K9YC
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