[TowerTalk] Fwd: which rope for element anti-vibration

[Lux, Jim]

    How odd, it went to just Paul, not back to the list..

Here it is



-------- Forwarded Message --------
Subject: 	Re: [TowerTalk] which rope for element anti-vibration
Date: 	Mon, 29 Aug 2022 07:11:43 -0700
From: 	Lux, Jim <jim at luxfamily.com>
To: 	Paul N1BUG <paul at n1bug.com>



On 8/29/22 4:25 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. 


It increases the damping (reduces the Q) of the resonance, the mass also 
will reduce the resonant frequency (proportional to sqrt(k/m)), but it's 
the damping that is the big factor.

rigid metal tubes have remarkably low damping (0.0025 is a typical 
value, equivalent to a Q of 200) - It's a resonator, and the amplitude 
is Q*(excitation amplitude), so if you can get damping bigger, by say, a 
factor of 20, then the amplitude of the oscillation (for the same 
aeolian drive) will drop by 20.

A well known, effective way of killing the aeolian forcing (rather than 
trying to damp the resonant rise) is to wrap a spiral around the rod.  
You've seen it on car antennas. It's somewhat empirical (I'll think I've 
seen some design equations, but can't remember where), but a spiral that 
is 10% of the diameter of your element should work.  So you have 0.75" 
OD, that would be a 0.075" OD "thing" that you need to wrap.  It can be 
anything, but you probably want to pick something that is UV resistant, 
not too stretchy, etc.

The helix pitch should probably be on the order of 6" (just looking at a 
picture of a car antenna with the helical overwrap)


FWIW, you'll see the same technique used to kill aeolian vibrations on 
tall smokestacks and flare towers and the like.