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## [TowerTalk] antenna element vibration damping - how

 To: [TowerTalk] antenna element vibration damping - how on4kj@skynet.be (on4kj) Mon Aug 11 17:09:52 2003
 ```Hello Hank, Thank you very much for the content of this message. I suppose their will be no more bad formulated questions neither answers, with this message in mind. I told Jim SMxxxx, If one ask you how paradise looks like, Just tell him how to get there, he will find out most of it himself. If you have'nt been there, dont make them believe you have been there.. As far as I am concerned there are no stupid questions, just stupid answers. That was also the mean raison I started joking on the subject..... Time to dig -up the calculators Guys Just remember Ham is much more then just playing " Chicken Bander ". Thanks again Hank, at least this is a good start for some, and a good reminder for other ( included myself ). Take care mate. Jos on4kj. ----- Original Message ----- From: To: Sent: Monday, August 11, 2003 10:05 PM Subject: [TowerTalk] antenna element vibration damping - how > List members: > > The phenomina you are witnessing is called vortex shedding or oscillation. It > is a function of natural frequency of the element, in this case a cylindar with > l/d >25, and the weight per unit length plus the wind velocity. > > For those who wish to see some illustrations go to: > > http://www.itsc.com/movvkv.htm > http://www.mecaconsulting.com/vortex_shedding.htm > > For a ton of sites use google and type in "vortex shedding" . > > For those interested further now come some formulas: > > for straight cylinder nat freq (f)= C/2*Pi*L Sqrt(E*I*g/w) > > C=3.515 for 1st mode > L= length of element > E=Modulus of elasticity of element material > I=Moment of Inertia of element cross section > g= acceleration of gravity > w= element weight per unit length > > IF the element is tapered then: > > f = 3.52*De/48*Le Sqrt(E*g/2*w) > > De= equivalent dia of tapered element > Le= equivalent length of tapered element > > De=D+Dend/2 > Le= L*Sqrt(2*De/De+D) > > The critical velocity to induce oscillation is: > > v= f*D/S S=Strouhal number =0.2 > D= D or De > > As can be seen as the natural frequency goes up the critical velocity goes up > > When you add a rope inside an element then you have increased the mass which > increases the unit weight without modifying any other component which will > reduce the natural frequency and there by decreasing the critical velocity. > This helps explain why these things occur at lower velocities. > > IF you taper the element then you reduce both the diameter, reduce the I and > the w and reduce the length. The upshot is that the natural frequency normally > increases thus the critical velocity increases. > > This occurs again at the next harmoinc natural frequency. The force generated > at natural or resonant frequencies of structures increase/decrease very quickly > at the resonant frequencies with very little change in velocity. So as the wind > starts and if the critical velocity is reached vortex shedding and its > accompaning oscillations occur. As you pass through the critical velocity point > then the vibration will damp out. If the wind speed continues to rise it is > likely that it will approach the 2nd harmonic frequency velocity and the > oscillations will occur with additional node and so on. > > Which is better as far as mass increase via rope in the element or tapered > elements ? Who knows. > > Which ever mechanism you choose it would be advantageous to have a fundamental > natural frequency that would be high enough to exceed normal wind velocities at > a given location. However this on a commercial basis would be very difficult. > > In general if you use straight non-tapered elements use rope inside if tapered > then I would think that the taper would be very dramatic as to lessend Le and > De but not to the detriment of the I or stiffness of the section to withstand > design wind and ice loadings. See not an easy answer with all kinds of > comprimises. > > This is my 1 euro input but I hope it expands the understanding behind this > type of vibration. > > 73 > Hank Lonberg, P.E.,S.E. / KR7X > Lonberg Design Group, Ltd. > > > > > > > > > _______________________________________________ > > See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. > > _______________________________________________ > TowerTalk mailing list > TowerTalk@contesting.com > http://lists.contesting.com/mailman/listinfo/towertalk > > ```
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