[Antennas] Re: Antenna Vibration (perhaps tower vibration)

2003年1月22日 01:30:25 -0500

Jim, kb9vtm, noticed his tower vibrating when the wind is 
blowing:
> Has anybody had this problem before?? This year I've really 
> noticed my tower vibrating during the wind blowing. I'm not 
> talking about wind storms. But just any breeze. Its 
> getting so bad that you can hear it in the house. I've 
> tried turning the beams to see if that will stop it, but
> only for awhile. My tower is only 40 feet tall. I only 
> have 2 small beams on top of the one thats next to the 
> house. But I'm wondering if the vibration could even go 
> down into the concrete and crake it? When you put your hand 
> on the tower, this puppy is really dancing to a tune. 
> So the question being, can anything be done to slow the tone 
> down. I don't know what musical note my tower is playing, 
> but it must be around low "C". Any help will be really 
> appreciated.

With a tower this large, an audible vibration is unlikely if 
the entire tower is vibrating. With that much mass, you would 
need an exceptionally rigid structure to get resonance at that 
high a frequency. And just because an object vibrates does 
not mean it is going to be destroyed by metal fatigue. As 
long as the vibrations are within the elastic limits of the 
vibrating components, fatigue does not occur. But if the 
vibrations exceed these limits, the damage is cumulative. In 
any event, I would not worry about a "galloping Gertie" 
(Tacoma Narrows) failure yet.
The key to understanding the problem is to estimate (or 
preferably measure) what frequencies are present in the 
vibration. Then determine which parts of the antenna system 
can resonate at these frequencies. Look at what the antenna 
system does in a high wind. The ends of a Yagi might flop 
around at most a few times a second for a very small beam, 
with lower frequencies as the length of the elements is 
increased. The boom of the Yagi will move at even lower 
frequencies, perhaps taking a second or longer to complete a 
cycle. Now think of the natural frequency of the tower itself 
which should be rather low.
To get a vibration in the audible range, you need some pretty 
small parts vibrating, or else something with a very low 
mass. He did say a tone like "low C".
Low mass? Think of air...
The first thing to look for is a weep hole or bolt hole with a 
missing bolt. When the wind blows across such a hole, it can 
make a dandy flute. Here you are developing a resonant air 
column inside the section of closed tubing. Think about 
playing a jug in an old-time band, or blowing across the 
narrow opening of a Coke bottle. The usual solution is to 
place a piece of tape across the hole to cover it, and then 
punch a hole in the tape for the drain. That little ring of 
tape is usually enough to dampen the acoustic vibration. Or 
you could stuff a piece of porous foam or rope to fill the 
hole. [One of the most annoying howls I have ever heard was 
from a weep hole in the loading coil of a mobile whip 
antenna. Drive along the Interstate and the antenna would 
scream - but the whip was dancing back and forth at less than 
a Hertz or two.]
Of course, you can still see lots of antenna vibration - but 
at lower frequencies. In Yagi elements, a suggested technique 
is to string a rope inside each element moderately tightly.
The rope provides damping as it stretches when the antenna 
elements flex.
For the boom, guying can help. But note that very tight guy 
wires can "sing" too. Placing the boom-to-mast support ever
so slightly away from the balancing point can help prevent
resonant vibrations too.
On the tower, one very important point that most hams forget 
is to not place the guys symmetrically. For example, on a 
tower with two sets of guys with one set at top, do not place 
the second set of guys exactly half-way up. Instead place 
that set up or down a foot or two from there. This prevents 
the sections from both vibrating together at the same 
frequency. You should do similar things if additional sets of 
guys are used.
Breaking up the lengths of guys with insulators can prevent 
them from being electrically resonant interfering with the 
desired antenna pattern. But likewise, those insulators break 
up mechanical resonances too - especially if the lengths are 
slightly randomized.
If you want to dig deeper into mechanical vibration analysis, 
Agilent (formerly H-P) publishes some great application notes 
to go along with their low frequency spectrum analyzers. You 
can do much of this at home (but not nearly as easily as with 
their equipment and software). All it takes is a vibration 
pickup and an analog-to-digital converter and your computer to 
capture some time period of the vibration and use software 
like a fast Fourier transform to determine the frequency 
content. Some good sound cards are suitable but most do not 
record very low frequency signals.
Vibration analysis is a fascinating subject. While my 
professional career has been in instrumentation, most of my 
work was with chemical instrumentation. But because I had 
experience with electrical instruments in hazardous (flammable 
and explosive atmosphere) locations, I was once called in to 
help a team of mechanical engineers doing vibration analysis.
This gave me the opportunity to learn about the subject (and 
take some excellent H-P training courses). I have used the 
knowledge on quite a few projects since. Vibration analysis 
makes a great tool to predict when many machines are going to 
have bearing failures and such - long before the bearings have 
actually failed.
 73, Dr. Barry L. Ornitz WA4VZQ [email protected]