It is a fact of nature that a tube in moving air will generate vortices
which will induce vibrations. Whether or not the antenna vibrates under that
is a matter of element rigidity (material), not element design (to a large
extend, that is true, see note below). If the element is rigid (i.e. made of
material with high tensile strength, strong material if you will), it will
vibrate.
To eliminate the vibration, either use soft, ductile material (such as lead,
that would work great to dampen vibrations :-), or install a damping
mechanism. That's what the rope is for. The rope inside the tube does the
same thing as if it were wound around a bell: stop the vibration.
Note: in the US, many cars come with a straight piece of stiff wire as an
antenna. These "sing" when the car is at speed. This is the same phenomenon
as the beam antenna elements vibrating, because the wire is much smaller,
the vibrations are at a much higher frequency. To eliminate the singing,
someone patented an idea: wrap a wire in loose turns around the antenna wire
(like 10 to 15 cm pitch). That kills the vibration. Now, all the US
manufacturers have patented that design. This is not very practical for a
beam antenna, as the "wire" that would be used would have to be almost as
large as the element tube itself..
73,
Didier/KO4BB
----- Original Message -----
From: "Jan Erik Holm" <sm2ekm@telia.com>
To: <towertalk@contesting.com>
Sent: Saturday, August 09, 2003 10:33 AM
Subject: [allowed] [TowerTalk] Re: Antenna element vibration damping - How
> Why use poorly designed antennas in the first place,
> ropes in the elements just fixes a poor design. Sure
> if you get those poorly designed antennas for free
> but I wouldn?t pay money for them. Proberly designed
> elements doesn?t need ropes in them.
> Also with rivets it makes it difficult to dismantle
> elements if you ever need to.
>
> 73 gl Jim SM2EKM
> ----------------------------------------------------------
>
> David Robbins K1TTT wrote:
>
> >>...with riveted elements I have never had a tip fall out! My HyGain
> >
> > and
> >
> >>Cushcraft beams have all been modified to riveted element
> >
> > construction,
> >
> >>which also do NOT snag on the guy wires on the way up!
> >
> >
> > What is really fun with bad vibration cases is that the element tips
> > fatigue and break off. I have had that happen with both the newer
> > telrex designs and cushcraft... no double hoseclamping or riveting will
> > ever stop that!
> >
> > David Robbins K1TTT
>
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
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