The idea of stringing rope inside of tubular elements or tieing
something loosely to a vibrating object to lessen Aeolian vibrations
isn't to change the resonant frequency. The point is to insert a
lossy element into the system and absorb the energy. Rope placed
loosely inside an element will move against the inner walls as the
element vibrates, causing frictional losses. Same thing with a twig
leaning against the poly rope.
Apparently, Dacron rope has more frictional losses and so cannot
maintain the resonance.
Kim Elmore N5OP
At 12:20 AM 1/11/2011, Neal Laugman wrote:
> > No doubt your poly rope was trying to shrink with the cold too and
> > had plenty of tension.
>
>That was exactly it.
>
> > So you left a twig leaning on the poly to upset that resonance?
>
>It was holding up the end of an 80m inverted vee and I tried to bungee
>the line but it just found new resonance. I finally just dropped the
>line that night. I really needed to get some sleep. I used some
>dacron after that. Learned a lesson in false economy, and that's what I
>got for being cheap <g>.
>
>--
>Neal, NL7VL
>
>-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
>On Mon, 10 Jan 2011 23:19:42 -0600
>"Dr. Gerald N. Johnson" <geraldj@weather.net> wrote:
>
> > Power and open phone wires (not used much anymore) sound the loudest
> > with just a 2 or 3 mph wind. Any stronger wind and the wind noise
> > hides the sound on the ground. Those wires are generally strung at a
> > fairly high tension to minimize the need for tall poles. One way of
> > checking the tension is to strike the line and measure the time it
> > takes that wave to propagate the span out and back.
> >
> > No doubt your poly rope was trying to shrink with the cold too and
> > had plenty of tension.
> >
> > So you left a twig leaning on the poly to upset that resonance?
> >
> > 73, Jerry, K0CQ
> >
> > 73, Jerry, K0CQ
> >
> > On 1/10/2011 10:03 PM, Neal Laugman wrote:
> > > Jerry and the group:
> > >
> > >> The wire elements in the Hex Beam and Spideream surely vibrate in
> > >> the wind, on power and phone wires its called Aeolian vibrations,
> > >> most prominent at light winds.
> > >> http://www.uh.edu/engines/epi1653.htm describes it in more detail.
> > >
> > > I wondered if there was a name for that effect. Polyrope was indeed
> > > my enemy one 30 below moonlit night out in the Alaskan Bush. I had a
> > > 20-foot spruce pole mast attached to a cabin's perlin with a
> > > polyrope halyard running the length of the mast. I thought I was
> > > loosing it as I the entire cabin seemed to resonate with this
> > > chilling sound. After I had been outside a couple of times checking
> > > for flying saucers, I finally figured out the mystery whilst
> > > reclining in my bunk. I went back outside, put the flashlight to
> > > the eighth-inch line, touched it - and the sound stopped! It was
> > > the temperature and an ever-so-slight breeze off the river.
> > >
> > _______________________________________________
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> > TenTec@contesting.com
> > http://lists.contesting.com/mailman/listinfo/tentec
>
>-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
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