I will confirm what Gene is saying about the BX towers who's cross members are
riveted, not welded. In my youthful radio career, I put up 104 ft of HDBX
tower. It was 4-way guyed at several levels. I installed a 6 foot torque bar at
the top guy point to help reduce the twisting movement in the wind. Periodic
inspection revealed rivets that had popped on this tower. As these were
discovered I would install 1/4-20 bolts in their place. This tower survived 4
or 5 seasons with a 6-element 15m homebrew yagi on top. Oh, the boom was 2" OD
irrigation tubing and was 40' long. (Did I mention I did this installation in
my youthful radio career?)
As an aside, our local radio community was fortunate to have a ham operator
(WA0MGY - sk) who was our source for inexpensive undisturbed earth anchors.
These were really slick... you auger an 8" hole at the proper angle about 8
feet deep, drop the anchor into the hole, use a special tool to expand the head
of the anchor into undisturbed earth, and backfill the hole. Later when I
removed the tower, I simply unscrewed the above ground head of the anchor and
pulled the rod out. You would never know there had been an anchor in that
location. I wanted to install elevated guy posts but was (fortunately) elmered
away from this type of installation.
Trylon towers are also riveted and I have found rivets that have popped on
these towers as well.
I conclude these mechanical failures are a consequence of torsion forces
created by the long booms on the antennas we use today.
73 de Bob - K0RC
* * * * * * * * * * * *
Date: Tue, 24 Jan 2006 14:27:12 +0000
Subject: Re: [TowerTalk] tower specs/not wind loads
You're correct - the BX-series of antennas has a torque limit on them.
This is because they are assembled with rivets, not welds, between the cross
pieces and between the cross pieces and the legs. The rule of thumb with these
towers is that a ten-foot long boom would impart just about the maximum safe
wind-induced torque on the tower. A longer boom might twist the thing apart.
Gene Smar AD3F
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