# [TowerTalk] Response to R for math

Michael Tope W4EF@pacbell.net
Tue, 13 Oct 1998 16:15:57 +-100

```David,

This question is not as absurd as you might think. First of all, I think
people are confusing two issues, bending strength and damping. Filling
an antenna element with some "stuff" to dampen a mechanical resonance is
probably a useful technique which may prevent metal fatigue. If I recall
correctly one technique that people used is to put ropes inside hollow tube
elements to dampen vibration. Epoxy would probably add too much weight to the
element, so I suspect that this is not a good idea - foam on the hand might work well.
Unfortunately, my knowledge of all this fall into the category of acquired folk
wisdon. I have never really studied it rigoursly, so I may be all wet here.

In terms of bending strength you are probably correct that adding epoxy to the
inside of a mast would not contribute much to the overall strength of the
installation. The idea does beg an interesting question though, how does one
calculate the bending strength of a mast made of two different materials? For
example, what if I have an aluminum mast with a steel insert inside of it? How
does the bending moment distribute itself between the two materials. In this
case I suspect it depends on the Youngs modulus of the two materials (stress/strain).
The stiffer material (not necessarily the stronger material) will take most of the
load (kind of like two concentric springs with different Hook's constants) when subject
to a bending moment.

I recall getting a pretty good grade in my statics course in college, but thats about all I
remember. I will definetly have to get out a book to figure out this one. Any other
mechanical mavens out there who know the answer?

73 de Mike, W4EF........

----------
From: 	DavidC[SMTP:davidc@bit-net.com]
Sent: 	Tuesday, October 13, 1998 2:08 PM
To: 	CQK8DO@aol.com; towertalk@contesting.com
Subject: 	Re: [TowerTalk]   Response to R for math

> << Question then...how much additional strength, if any, is added to a 2"
>  mast that is filled with  epoxy cement or similar substance that is
>  poured in and becomes solid or semi-solid  >>
>
> ZIPPO!  ...
> Denny

This response is absurd.  Even if the mast were to be filled with popcorn
it would receive a minuscule increase in strength.
Can we please stick with meaningful evaluations and avoid absurd hyperbole,
some of us are here to learn and could be making choices that involve
serious money and safety!
73, DavidC  AA1FA

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```