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

On 18 Oct 2006 01:46:45 -0700, wrote:


Francesco

Do you receive my email?

Wes N7WS

Hello Wes,

didn't receive anything, I sent an email to you.

73

Francesco IS0FKQ

Wes Stewart ha scritto:

On 18 Oct 2006 01:46:45 -0700, wrote:


Francesco

Do you receive my email?

Wes N7WS

wrote in message
ups.com...

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.

those vibrations are quite common in 10m and 6m yagis where the elements are
made of aluminum tubing. they are from the elements fluttering in the wind
(usually just in light wind).

to put a few of the misconceptions to rest...
First this is not a 'wind over the bottle' or flute effect. those are much
higher frequency and are easily stopped with caps on the ends of booms and
elements. so forget the solutions about plugging the ends of elements and
booms for this one.

This problem is caused by 2 properties of antennas about that size coming
into phase with each other. the first is the vortex shedding around round
objects like tubing or wire. this is an aerodynamics/fluid flow problem
that is caused by air flowing around the tubing creating a low pressure area
behind the tubing, this sets up two vortices behind the element. at a
critical wind speed these vortices start to slip away from the element, and
when they do they create an unbalanced force... i.e. if the top vortex sheds
first the element gets pushed down, this often disrupts the flow causing the
bottom one to shed which then pushes the element back up, which causes the
top one to shed again, etc, etc, etc,... the effect alternately pushes the
element up and down at a rate mostly determined by the wind speed. the cure
for this is the spiral wrappings that you often see on chimneys and power
lines. the spiral wrap is a way of breaking up the smooth flow around the
object so that the vortices don't have a chance to form in the first place.
they have nothing to do with strengthening the object, in fact they add to
the wind load a bit, but the breaking up of the vortices is their main
purpose.

This vortex shedding problem is there all the time on all types and sizes of
antennas, but on most antennas it does not cause the damaging type of
vibration you see on some of them. why is this??? as long as the shedding
frequency is not close to the element's or boom's mechanical resonance there
is no problem, the forces occur but they are out of step with the natural
response of the element so the vibration can't build. however in 6m and 10m
yagis with commonly used tubing the combination of tubing weight and
stiffness happens to have a mechanical resonance about the same frequency as
the vortex shedding frequency at low wind speeds around that size tubing...
everything is wrong. to make it worse, the added motion from the mechanical
resonance makes the vortex shedding worse since the element moves more,
causing it to move even more, and eventually fatigue and break in extreme
cases. note, that the combination of very stiff larger diameter tubing used
in later design telrex yagis down to 15m and 20m also had this problem, i
found this when i started picking up pieces of elements and eventually whole
elements from the ground under the towers from this effect.

if you don't break up the vortices the other option is to change the
resonant frequency. there are 2 ways to go, you can increase the frequency,
which is very hard... to do that you have to make the element stiffer or
lighter. you can't get much stiffer than the tubing being used without
extreme measures, and likewise lighter is generally not an option. so the
answer is heavier or less stiff. its easier to go heavier, just fill the
element up with something heavy. This can also effect the 'stiffness' of
the element, if the weight you put in the tubing is free to move it acts
like a floppy weight and reduces the effective stiffness of the element.
this is the combination that led to putting rope into elements. a good
heavy hunk of rope (this is not the time for polypropelene) smaller than the
inside diameter of the tubing adds weight and flops around effectively
absorbing the vibration before it gets going. this gets harder on small
diameter tubing usually seen on 6m yagis... on those try either aircraft
cable or the plastic coated wire clothes line, or even heavy guage insulated
wire.

there are also other ways to stop vibrations like this. i have some 20m
yagis now that have a long gap between the 1st and 2nd director, this gap
was fluttering in certain winds so i snubbed it by adding a second boom
brace rope in the middle of the section with just a little bit of tension to
keep the boom from vibrating. electric utilities also use dumbell like
spring devices that change the resonant frequency of large cables, these are
often seen in pairs on either side of the insulators on towers in flat wind
prone areas.