[TowerTalk] Adding Expanding Foam inside boom

[Kimberly Elmore]

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.
Kim N5OP

      From: Bob Matthews <kt3rr at verizon.net>
 To: charlie at thegallos.com 
Cc: towertalk at contesting.com; jimlux <jimlux at earthlink.net>; "Bob Shohet, KQ2M" <kq2m at kq2m.com>
 Sent: Thursday, April 26, 2018 2:20 PM
 Subject: Re: [TowerTalk] Adding Expanding Foam inside boom
   
I used a 2 part foam many years ago on a 30 ft 10 meter Yagi boom. It made the boom quite rigid and strong. The materials I used were industrial marine grade so they would not absorb water. I’ve seen similar products on Amazon  https://www.amazon.com/TotalBoat-Urethane-Density-Flotation-Reinforcement/dp/B01AAWRYZG/ref=mp_s_a_1_1?ie=UTF8&qid=1524769730&sr=1-1π=AC_SX236_SY340_QL65&keywords=two+part+foam&dpPl=1&dpID=51TCGebM86L&ref=plSrch
Or you can just search 2 part foam marine to see the various products available. I can say that the particular product I used held up very well. But it was so long ago I have no idea what manufacturer I used. 

Bob  KT3RR

Sent from my iPhone

> On Apr 26, 2018, at 11:42, Charles Gallo <charlie at thegallos.com> wrote:
> 
>> On Thu, April 26, 2018 9:20 am, Bob Shohet, KQ2M wrote:
>> I would also expect that the rate and quality of the “curing” would be
>> impacted by the ambient temperatures and humidity levels/climate at a
>> given location.  In much of New England the temperature can easily vary
>> by more than 120 degrees in the course of a year – and significantly more
>> in the great plains states.  And the climate can very enormously from the
>> generally wet/humid East to the desert SW. This has to have an impact.
>> 
>> 73
>> 
>> 
>> Bob  KQ2M
> 
> Of course there are foams (2 part) that mix on spray, and don't require
> water vapor to cure
> 
> That said, what is the problem you are trying to solve?  Usually foams are
> used to stop vibration/change resonance frequency.  Here is the question
> (and I don't know the answer) - how often do we see fatigue
> cracking/failure on the BOOM of an antenna?  Not the elements - heck, they
> crack on some, but I don't think I've heard of booms failing except during
> extreme events like ice/wind storms.
> 
> The foam might not give you ANY extra strength, and even you look at
> strength/weight, I'm going to guess it'll be a net negative.  Now if you
> know your boom has a vibration problem, we are talking another story. 
> Even then, there are probably better ways to make the boom stiffer (put an
> end cap on both ends, attach a cable that runs inside the boom from end
> cap to end cap, and come up with a way to tension the cable - aka put the
> boom under compression, or put a metal ROD in, and put the boom under
> tension - both will change resonance)
> 
> The material that interests me.  I've heard that they make tube that is
> similar to ACM sheet - basically a pair of Al tubes, with a composite in
> between (ACM siding is polystyrene or similar)
> 
> I'm wondering if these materials might be the way to go in new designs -
> they are about 90% as strong, but also less than half the weight.  I
> wonder if we could go to some of the carbon fiber/aluminum composite
> tubes, and get MORE strength, for LESS weight (for the same size), or
> downsize, to go SAME strength, but less area and load.. (of course at
> greater expense)
> 
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