[TowerTalk] which rope for element anti-vibration

[Wes]

Leeson in "Physical Design of Yagi Antennas" says on page 4-39, "A common 
failure is in the tip section.  This process can be prevented by adding damping 
to the resonance of the tip section, typically by inserting a piece of 
waterproof rope inside the length of the tip or by weighting the tip to reduce 
the mechanical resonant frequency below the vortex shedding frequency shape.  
Knot the rope or glue it in place with silicon caulk at the inner end so it 
can't slide out if the tips sags under heavy ice."

He says nothing about the size of the rope relative to the element ID but I 
would suspect bigger is better.

Wes  N7WS


On 8/29/2022 7:48 AM, Lux, Jim wrote:
> 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
>>> _______________________________________________