I own several smaller 40m OptiBeam antennas which have held up fine and
from an RF viewpoint they are fantastic however I have heard the horror
stories about their 80m beams from other tower installers. OptiBeam also
makes a massive full sized 4el 40m which may have the same issues. I asked
Tom once what aluminum he uses and I seem recall a non specific answer
like aircraft aluminum or something like that. It is interesting that high
strength tubing may not be available in EU. Also metric tubing does not
telescope nicely, I wonder how that effects the strength of the joint.
Imperial sized tubing with a .058 wall fits together perfectly making it
easy to homebrew. For my homebrew antennas I only use 6061-t6, I see most
ham suppliers sell 6063-t8. What is the difference?
John KK9A
To: towertalk@contesting.com
Subject: [TowerTalk] Subject: Wind survival + load ratings... vs,
From: Kurt Andress <andresskurt@gmail.com>
Reply-to: kurt@k7nv.com
Date: Wed, 1 Nov 2017 21:47:45 -0700
Jim Thompson, I think your evaluation of the Optibeam 80 is lacking some
important knowledge. I learned more than I ever want to know about that,
by being the mechanical design engineer, and lead for all tower work for
the W7RN station. We tried twice to redesign, reinforce, and do everything
possible to make two of those antennas survive up there, with no success.
After a few years of effort, and constant search & inquiries, I learned
that many EU antenna builders do not have ready access to high strength
alloy tubes, like we do here. They can get ~40 ksi equivalent tubes in the
small tube sizes (maybe up to around 1" diameters), but not in the larger
ones.....the larger tubes are rolled and welded, with visible seams, not
drawn, with a yield strength of about 1/2 of what we can get here with
6063-T832 drawn tubing. I heard some of the tubes were made in Turkey.....
So, plug 23,200 psi yield into those analyses and see what it does. In the
UK, they can get real 6061 alloy tubes, but they are 1/16th wall and don't
telescope, so they have to grind tubes to get them to fit together. So, to
solve the W7RN problem, I designed from scratch, new elements for one
antenna with 6063-T832 American sourced tubing. The static YS analyses say
they are safe @ 130 mph. My Linear Finite Element analyses show that the
tips of the elements are deflected far enough to be parallel with the wind
at that speed, so they are capable of well over that wind speed. Emperical
experience says those elements survived the worst winter in over 10 years
up on the Comstock, with some significant icing....they are the only
elements that have survived up there for longer than a few weeks ;-)
YMMV......etc,...... back to net......
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