[TowerTalk] which rope for element anti-vibration

[Grant Saviers]

The TH7DX used what looks like polypropylene rope inside elements. The 
rope in my NOS TH7X kit is about 1/8 to 3/16" diameter fine braided. I 
don't think there is magic in polypropylene other than it is cheap, 
doesn't absorb water, and rot proof (but not UV resistant).

Solid woven (not straight core paracord) nylon or polyester might work 
just as well.  Strands rubbing against each other might be part of the 
energy loss mechanism.

 From the TH7 manual:  cut 6 equal lengths of rope 2ft long, insert into 
the end of the elements, fold back the ends 1/2" spread out the fibers, 
fold them back, and cover with a cap plug.

It worked in the two TH7s I had up, no buzzing.  I've seen speculation 
that because of the nearby drivens spacing vibration was increased. 
That makes sense if one is in the shedding vortex of the other and has 
the same primary mechanical resonance frequency.

OTOH, the dynamics of oscillation and coupling in antenna 
arrays/towers/guys is mind boggling complex.  So I doubt there are taper 
techniques that guarantee no buzz in what is always pretty turbulent air 
flow.  Some cheap light no-maintenance internal damping seems like a 
good idea with no increase in wind loading.

If the elements buzz, remember that 6061 has no stress limit for 
infinite fatigue failure life, contrary to steel.  Even at very low 
cycling stress levels Al will eventually fail.  Higher stress and faster 
to fail.  I've proven that with my buzzing 80m vertical.  That's why 
tips fall off yagi designs even with tips with adequate wind load strength.

Grant KZ1W

On 8/29/2022 04:25, 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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