[TowerTalk] Adding Expanding Foam inside boom
grants2 at pacbell.net
Fri Apr 27 01:24:44 EDT 2018
The use of fast spiral insulator wraps for wire catenary spans is
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.
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
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
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.
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