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

On 17 Oct 2006 03:01:52 -0700, 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
It sounds like a really stable mechanical design.

The first thing that comes to mind to me is to damp the vibrations. A
spiral wrap of something soft like rubber tubing on the boom and the
elements. Unfortunately it would add both weight and wind resistance.
Also it would contribute to maintenance. I doubt it would weather
well.

The next thought is to try some soft sleeves randomly placed on the
elements and boom. I would be inclined to use chunks of tubing about
75mm (3 inches?) long and secured with cable ties protected with black
tape. Splitting the damping tubing should make it easy to install.

End caps on the elements and boom are a given but if drains are not
provided for each instance water will collect and burst the tubing by
freezing.

I hope you have an easy way to get at the antenna for experimenting!
John Ferrell W8CCW

Lots of commercial antennas come with rope inside the elements for
mechanical damping. Just find some dense and preferably synthetic rope
that just fits inside the elements. If it’s a vertical antenna, cap the
top elements to keep rain from entering, otherwise leave the elements
open so any moisture will bake out in the sun.
Bob


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

Take a sheet of rubber (say 3-5mm thick) maybe 150mm long and the
circumference of the element wide. Make 3 equally spaced cuts 70% of the
distance of the long dimension. It will now look like a 4 fingered hand
shape!

Using a hose clamp tie this rubber sheet around the uncut end around the
elements at some convenient place. The idea being to damp mech
oscillation. Maybe 1/3 the way down each element but I haven't exactly
thought about where the best place would be.

It may slightly detune the antenna but I haven't thought that through
either!

Cheers Bob VK2YQA


wrote:

Hello all,
I build one year ago an 8 element 50 MHz yagi. Boom is made with 50 mm
square aluminum tubing with

Fill it with expanding foam used for heat insulation.
Art




Bob wrote:
Lots of commercial antennas come with rope inside the elements for
mechanical damping. Just find some dense and preferably synthetic rope
that just fits inside the elements. If it's a vertical antenna, cap the
top elements to keep rain from entering, otherwise leave the elements
open so any moisture will bake out in the sun.
Bob


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

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


Hi Francesco,

You did not describe the vibrations, so this may be off base. If it is a
"sound", then perhaps the tubing is acting like a flute. Plugging the
ends of the open tubes would stop the sound. It may be necessary to
leave a small hole for drainage.

73,
Gene
W4SZ

Also, be aware that you MUST eliminate that
resonance ! Many years ago, myself (and
another ham) obtained the 6 meter moon bounce
array from a well known ham (he built it, got
echos, than abandoned it- reason, at that time,
he was the ONLY op capible of 6 meter moonbounce!
helped the owner of the first half put it up
on 5 inch irrigation pipe! The array worked well-
but the wind started oscillations in the elements.
The elements began to shear off from metal
fatigue, at the point where they went thru the
boom. Same happened to mine, even as stand alone
yagis! These had elements of 6061-t , 5/16 inch
dia. tubeing, if memory serves.
Translation: IF allowed
to continue, look for the elements to start
breaking off, at some point in time! Would agree
that the insulating foam might be enough to
stop the oscillations in the elements. Good luck
Jim NN7K

Gene Fuller wrote:

Hi Francesco,

You did not describe the vibrations, so this may be off base. If it is a
"sound", then perhaps the tubing is acting like a flute. Plugging the
ends of the open tubes would stop the sound. It may be necessary to
leave a small hole for drainage.

73,
Gene
W4SZ

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


If these are hollow elements simply put small rope inside of them and
then seal the ends. This will dampen the sound, and have the added
benefit of possibly reducing work hardening of the elements.

tom
K0TAR

Hi Gene,


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


Hi Francesco,

You did not describe the vibrations, so this may be off base. If it is a
"sound", then perhaps the tubing is acting like a flute. Plugging the
ends of the open tubes would stop the sound. It may be necessary to
leave a small hole for drainage.


The sound is low frequency, I'd say less than 100 Hz with amplitude
modulation of 1-2 Hz probably
due to boom vibration. I thought about flute effect but I think it
would be an higher pitch in that
case. I read many interesting suggestions though, I hope to be able to
try some of them in the next
week end.

73

Francesco IS0FKQ/I8

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