Hi Ron,
String, nylon cord, or anything non-conductive, wound helically around the
element or boom at a very low pitch - 1 turn per 5-6ft perhaps, and just taped
at intervals. This works on VHF yagi booms so I presume it works on individual
elements. From an aerodynamic point of view, this will break up the laminar
airflow over the round front section of the element - ie. make the air
turbulent before it normally breaks into turbulence behind the element, and
thereby prevent resonances from building up. I should practice what I preach. I
lost two 9 element OZ5HF's last week for exactly this reason! They just split
in half.
73 & Merry Christmas/HNY
Stewart GM4AFF
-----Original Message-----
From: Ron Stone, Vine Antennas [SMTP:ron@gw3ydx.demon.co.uk]
Sent: 30 December 1998 20:41
To: Towertalk@contesting.com
Subject: [TowerTalk] Element vibration
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
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