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Dan Andersson · October 18th 06, 07:54 PM posted to rec.radio.amateur.antenna

wrote:

Hello all,
I build one year ago an 8 element 50 MHz yagi. Boom is made with 50 mm
square aluminum tubing with
a second square tube on the center of the antenna to strenghten the
boom. Elements are made with 20 mm
round aluminum tube in electrical contact with the boom. The antenna is
a good performer but it has one
problem: it sounds even in light wind and vibrations come down the mast
and are very annoying in the whole
house. The worst problem is that resonance succeeded in unscrewing the
rotor bolts. I'm going to use a
special for bolts and screws but I'd like to find a method to avoid
mechanical resonance of the antenna.
Does anyone know a good method? Would it be better to add two insulated
pulling wires at both ends of the boom (even if probably also elements
produce udible resonance)?
Thanks in advance

Francesco IS0FKQ/I8

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! 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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