On 8/29/22 7:36 AM, Steve Maki wrote:
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.
Pretty much - you're looking for an increase in damping - You want
something that will turn motion into heat, i.e. frictional losses,
whether from the rope flexing or the rope sliding on the internal surfaces.
And since it's inside a pipe, it's not subject to UV, nor other
weathering forces, so cheap is the name of the game - and besides if it
did get all ratty, it wouldn't make any difference as long as it stayed
in roughly the same places.
I note that people have done damping by things like sand and rubber, or
things like automobile undercoat. There's definitely sticky paper based
damping, but there could well be "rope and glue" products too.
One could probably fill the tube with some sort of foam - although one
would need to think about what properties of the cured foam you'd want.
Probably not something super rigid.
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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|