Kurt did you do an analysis of the 40m beams put up there at Comstock this
last summer?
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
-----Original Message-----
From: Kurt Andress
Sent: Wednesday, November 01, 2017 11:47 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Subject: Wind survival + load ratings... vs,
Jim Thompson, Jim Lux et al......
Your comments on this subject are appropriate and appreciated, but there
is more that needs to be explained....this is not a simple matter and is
most likely why few have ever gotten any of it anywhere close to right......
Both Jim's are right that the members of a yagi antenna deflect when
exposed to the wind! This causes them to experience lower loads as they
become inclined to the wind as explained by Dick Weber, K5IU in his
papers from ~20 years ago as he explained his "crosswind principle".
There is absolutely no way to easily account for this principle in
analyses without spending exorbitant amounts of time to manually
calculate the deflections of each of the portions of each of the
sections in a member at each wind speed increment! This is a non-linear
phenomenon, and no one has created software to be able to automatically
do it! So, linear Static Analyses will be conservative by the amount of
deflections the members see. It is important to note that properly
designed ~100 mph 20M thru 10M antennas do not experience huge
deflections, like 40M & 80M antennas do, so the intrinsic safety margins
they will get from becoming unloaded by deflections will be much lower
than for the larger antennas. I think we should accept static linear
analyses for them as they are, with their unknown safety margins that
come from their deflections. The relative comparisons of the safe wind
speeds of the different antennas are always valid, as long as they are
analyzed with the same methods. TIA/EIA 222 has yet to require the
complexities of appurtenance member (I.E. antennas) deflections into the
standard, towers on the other hand are analyzed with non-linear methods
that are accurate for structures with relatively low deflections. I.E.
nobody designs towers with huge deflections.....my website
K7NV.com\notebook shows what happens with that stuff....
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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