[TowerTalk] Adding Expanding Foam inside boom

[Grant Saviers]

The use of fast spiral insulator wraps for wire catenary spans is 
interesting.

The principle of slow spiral wraps on cylinders is about reducing vortex 
shedding.  The build up and separation of a vortex is what causes the 
cylinder to vibrate.  Spiral wraps are done on all sorts of cylinders 
including tall cylindrical buildings.  It doesn't take much of a spiral 
to reduce the vortex creation.  OTOH, a larger element diameter means 
more wind load so for yagi antennas it may not be a good tradeoff.

https://en.wikipedia.org/wiki/Vortex_shedding

Telrex put the rope inside for mechanical dampening and it was a 3 
strand polypro in the TH7's I built.  In boat anchoring, 3 strand is 
preferred for the anchor rode over braid since it stretches more and 
dissipates more energy in that process.  That may apply to yagi element 
dampening but I think it is more the mechanical side to side motion that 
absorbs energy.

Parachute cord seems a bit small in diameter, I think what Telrex 
supplied was 1/4 or 3/8" and per the original manual were 2 ft long in 
the element ends and captivated by a cap plug over the fanned out rope 
fibers.

Grant KZ1W

On 4/26/2018 14:52 PM, jimlux wrote:
> On 4/26/18 1:31 PM, Kimberly Elmore wrote:
>> In a similar vein, I'm helpingto rect an antenna in a relatively 
>> inaccessible location and want to minimize element vibration. I have 
>> no intention of tryingto fill each element with foam, but I've read 
>> that securing a rope inside the element will prevent resonant 
>> vibration from occurring. If this is true, will something like 
>> "parachute cord" do the trick? It won't be exposed to eather, so UV 
>> deteriorartion isn't an issue.
>>
>
> An even better way is to wrap the rope in a spiral on the outside.  
> That kills the wind induced vibration very effectively. (you see this 
> on car antennas).
>
> doesn't even have to be the full length.
>
>
> http://preformed.com/energy/distribution/motion-control/spiral-vibration-damper 
>
>
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