> Hi chaps.
>
> I recently built two five eles for 28 MHz from the ARRL antenna book
> design. This is 24 foot boom. Element center pieces are 1/2" for the
> first three feet on each side. Then we taper to 5/8" to the tip. This is
> the "light duty design", but follows my own usage of 2" square boom and
> flat plates on top of the boom for securing the eles. This keeps the
> eles in line much more readily than round boom, and makes boom to mast
> plate mounting very accurate and direct.
>
> All was well for about four weeks, and then about one foot of the outer
> 5/8" tubing of D2 of the upper antenna of the stack vibrated off. It
> looked almost like it had been cut with a tube cutter. I noted when I
> climbed the tower that other eles were vibrating even in light wind. It
> looked like the element tip was vibrating at many c/s back to a point
> about a foot from the tip, where it appeared to be stationary. Then the
> vibrations started up again and reached another maximum about 18" in
> from that point. the flexion was about one inch max travel. Bad news.
> The element was singing with the effort and no wonder D2 didn't last
> long.
>
> I have put rope in the tubing at the tips. As much nylon rope as I can
> stuff in. The rope fills about 5/8 of the inner diameter of the tubing,
> and is pushed back to the 1/2" dia section where pop rivets get in the
> way of any further pushing. Then I have put plastic end-caps on the tip.
>
> Chaps. Have I done it right, and what else should I be doing to overcome
> this little problem please ? So far it looks OK, but it has been fairly
> windy, and this problem looked at it's worst in light winds and
> particularly on REF, D2 and D3.
>
> Comments based on real experience will cut more ice than theorising
> about it thanks !
>
> Ron Stone,
> Vine Antenna Products,
> The Vine
> Llandrinio
> POWYS SY22 6SH
> UK
>
> Tel - +44 (0) 1691 831111
> Fax - +44 (0) 1691 831386
>
> Our WEBSITE is at http://www.gw3ydx.demon.co.uk
> __________________________________________________
This is very common in a vibration situation. I was chief
engineer for a company which made grided parabolic antennas
(4ft-15ft). Every once in awhile, I would have to go out to
investigate situations where the tubing which was welded into
this parabolic shape would just fall out of the reflector. We
usually had these analyzed in a metalurgical lab. They appeared
as you described. The lab results always pinpointed the failure
as 'work hardening'. The material (aluminum) was very hard and
brittle at the failure becoming cystalized. Once they reached
this state, they would just fracture and fall out. This was in
spite of being welded at either end and appearing rigid. The
unique thing was they were all a certain length for a particular
tower, and each tower was different.
The common cause was tremendous vibration on the tower usually
caused by wind.
Realize that there were literally tens of thousands of these
antennas in the field since the product was made over a 35 year
period.
Our solution was to add vibration dampners by welding additonal
straps across the tubing lengths to avoid that length which
failed.
The tower appeared to generate a particular frequency that found
the right length of tubing to vibrate and fail. So it was either
to change the tower design to eliminate the frequency (not too
likely) or eliminate that reasonant frequency length from the
antenna.
In your case, you can either find a way to break up the vibration
by some sort of strap (insulator material) or changing the mass
of the material to change the reasonace of the tips. Thicker or
thinner tubing.
A final observation was that the worst problems of this sort
seemed to be where there was plastic guylines. Steel guylines
have a lot of mass which acts as vibration dampners on a tower.
gil, w9bub
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