[TowerTalk] Adding Expanding Foam inside boom

[Grant Saviers]

I don't think there is any magic.

The mechanical properties of household construction foam can't add much 
strength or dampening at frequencies that might matter.

Then it has to stick permanently onto the id and have sufficient 
strength to not shear from bending.

And not absorb moisture, ever or degrade from heat/cold cycling. And 
weigh very little.

Sounds to me like a project for Boeing Research.

Internal sleeving is the way to strengthen elements/booms as Leeson 
points out in his book.  No more wind resistance and the mechanical 
properties are the same as the thicker wall solid tube.  He also 
advocates triple boom guys (one above, two down to a spreader) to 
prevent uplift failures which for ridge/hill top locations such as his 
QTH are a real problem.

Coupled structural resonances are numerous in tower + yagi structures, 
but that is not addressed by any ham radio treatise I've read.  Modeling 
same would be prohibitively complex.   Aluminum has no fatigue infinite 
life threshold as does steel, so it is possible for elements to 
eventually break from fatigue at low stress levels and many cycles.  The 
limit of cycles vs stress is in charts for many aluminum alloys/heat 
treats.  That happened to a vertical I had that vibrated strongly.  
Telrex put polypro rope inside TH7DX element ends to absorb vibration 
energy, presumably elements were subject to fatigue.  A simple, durable, 
& cheap fix if you have a such a problem .

Grant KZ1W


On 4/25/2018 19:49 PM, Charles Morrison wrote:
> Has anyone tried this ?
> Can it improve survivability of very large yagis?
> It won’t prevent a boom failure, but could it help last a little longer?
> -Charlie N1RR
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