On 12/10/2019 2:41 PM, Bob Shohet, KQ2M wrote:
My non-engineering understanding of wires and stretch is that a given wire after being
subject to variable strength forces over a given period of time, will stretch only so far
before it breaks. Assuming that that is conceptually correct, let’s say that we have
two wires – Wire A and Wire B. Wire A has been pre-stretched before being put up and
is now compared against Wire B which has just been put up without being pre-stretched
Now we subject both wires to the same forces over the same period of time. I would
expect that going forward Wire A will not stretch as far as Wire B, but is more likely
to break and break sooner under significant force than Wire B because it has already
been pre-stretched. Wouldn’t this be correct?
If this is not correct, can you please explain why?
It all depends on what you define as a significant force. I use 100#
weights at ground level on the ropes on one end of my high (125 ft)
dipoles to maintain tension and allow for "give" with tree sway. That's
a LOT less than the pulling tension applied by W6GJB's pickup to stretch
and break them.
Hard drawing copper strengthens it significantly; NEC requirements for
wire antennas call for either CCS or hard drawn copper for strength (so
they won't break and create a potential shock or fire hazard). The
stretching we're doing is a first approximation of hard drawing.
73, Jim K9YC
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