Back in the 70's we were building what we thought were heavy duty
yagi's. Constant tapers with 1/2" or 5/8" tips. They buzzed like crazy
in light winds and shed pieces of the tips on a regular basis.
Someone suggested the rope-in-element trick, which cured the problem
absolutely. Every element got the rope, which I believe was usually 3/8"
poly - the cheapest rope we could find. I've always believed the
mechanism was simply the rope slapping the tube if the flutter tried to
start. If that is it, then it would make sense to use something that has
some minimum amount of mass, but not so thick that there is no room to
move.
Lately I've been buying commercial OWA Yagi's; they seem to have solved
the issue with judicious taper scheduling.
-Steve K8LX
On 8/29/2022 7:25 AM, Paul N1BUG wrote:
I'm trying to get a better sense of how the rope in elements to stop
vibration thing works. I've read that it works because as the element
swings in one direction, the rope slams against the opposite side,
providing a counter-force. What confuses me is that I don't understand
how the rope is free to move around. Nylon or polypropylene seem to be
recommended but those ropes come coiled or folded and they have a memory
effect, so they don't lay flat. When put inside an element, they are
going to be pressing against it in multiple places and directions. It
seems like that would limit the rope's ability to move to do its job.
What am I missing? I have seen braided rope with extremely fine strands
that is soft and doesn't have the memory effect. Is that what I should
be using?
My 6 meter yagi was having a problem. The element tips were oscillating
at about a ~200 Hz rate, and the tips were moving at least two inches!
It took four trips up the tower to rig and get it down. It will take
another four climbs to put it back up, and more if it still has a
problem and needs to come down again. I'd like to try to get this right
on the first try to avoid all that extra climbing. The four foot center
section of each element is .750" OD, with .625" tips. Any practical tips
or experiences with similar elements would be appreciated.
Paul N1BUG
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