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#14
October 21st 06, 12:03 AM
posted to rec.radio.amateur.antenna
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how to eliminate mechanical resonance?
Dan Andersson wrote: wrote: snip Don't make it complicated! Complicated equals heavy! You most probably suffer from the effects of turbulence combined with harmonics from the aerial design itself. The immediate solution is to do what you do with high chimneys and car radio aerials - you wind a thin spiral round the full ( most? ) length of the aerial rods. This will break most of the turbulence. You choose lightweight and thin material for this. Try a couple of materials until you are happy. Also, bear in mind that it might only be one part of the aerial that generates the problem! By adding ropes etc, you just contain the result of the turbulence and the harmonics! But David isn't that the object of the exercise? The object is to dampen the occillation such that metal fatigue does not take place You can remove or redirect the causes of occillations but energy cannot be created or destroyed. Recognising this some manufactures place rope inside the elements and it works even tho it costs the manufacturer. To change the surface of the object receiving these stress reversals only hurts what you are trying to achieve unless what you are adding delays the effect of stress. With foam insulation I only followed the path that is generally used except I also wanted to isolate the inside from environmental effects such winds from other directions as well as providing stiffness. The bottom line is to make the elements as aerodynamic as possible and dampen the occillation that are left. Making things less aerodynamic only serves to worsen things. Are you sure that these additions to chimneys is to dampen occillations as opposed to strengthening the structure? Cheers Art By adding the spiral around the antenna rods, you have a good chance of eliminating the source of the problem which is the rods being susceptive to air turbulence, created by the environment as well as the aerial construction itself. The reasons for adding spirals on high rise chimneys are twofold, first to create a controlled turbulence as an added up draft as well as eliminating the possibility of creating a self oscillation due to the air pressure/turbulence and the harmonics generated in the design. By adding these spirals, your aerial will be susceptive to icing at cold weather! Check so that you don't get a crashing aerial because of heavy ice coating! Cheers Dan / M0DFI |
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