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