Topband: Pulleys for antennas in trees
kj9c at iquest.net
Tue Jan 27 12:50:57 EST 2015
Speaking as an engineer, the owner of an oak tree farm, and a ham with
antennas in trees, please use some caution when using ropes in trees:
1. An antenna tied to a tree branch exerts some force W (lets say it's a
pulling force or hanging weight) on the branch.
2. If instead of tying to the branch, you tie off the other end of the rope
on the tree trunk or use a counterweight, the force on the branch doubles
(to 2W).... there's still the weight of the antenna, but you add the weight
of the counterweight (or anchoring force die to tying off at the trunk)
3 If instead of the antenna rope, you tie a pulley to the branch, then the
pulley sees 2W force (the antenna plus the counterweight) and that force is
transferred to the branch.
4.But....iIf the pulley's support rope is tied off at the trunk or
counterweighted, then the force the branch sees doubles (just as in example
2 above) exerting 4W force on the branch !!!! So there's 4 times as much
stress on the branch vs merely tying the antenna rope to the branch.
However, most of us can't get high enough in a tree to tie off the antenna,
so we sling ropes over branches.
So if using a pulley, please be sure the supporting branch is stout.... I
had to replace the windshield of my truck to prove this point to me. I find
it best to install inverted vee type (center supported) dipoles or very
light inverted L's to minimize the sideways stress on the branch (sideways
stress intensifies when the wind blows hard). And yes I have pulleys high in
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