Date: Thu, 15 Nov 2018 10:42:45 -0800
From: JVarney <jvarn359@gmail.com>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Optibeam OBLY14-5
<I bought a used OptiBeam 11-3. Your post probably explains why the
<previous owner replaced all of the original elements with DX
<Engineering tubing.
<Kurt has noted -- and I've seen -- some wear issues where the
<square U-bolts for the elements meet the square boom. The sharp
<corner of the boom causes a stress concentration at that point.
<There are some places where the U-bolts have worn a groove into the
<boom corners. I could see where in the long run this could initiate
<a stress crack in the boom and could eventually lead to boom failure.
<I'm not too concerned about it, but then I have the luxury of a tower
<friendly location where high winds are very rare and there's no ice.
<73 Jim K6OK
## I dont see any advantages to the use of square booms. Optibeam uses
round
booms on a lot of their other yagis, go figure.
## My problem with the tubing optibeam uses for eles is, its all metric
tubing,
unobtainium anywhere in NA. And none of it neatly telescopes.
## 6063-T832 is 1/2 the price of 6061-T6, and is nearly the same yield
strength,
39 ksi vs 40 ksi, big deal. 6063-T832 in any diam, is readily available
from DX eng.
My understanding is the metric tubing used in EU, is a lower ksi rating than
the more common
39 / 40 ksi material used in NA.
## For boom material, 6061-T8, available from DX ENG is superb material.
Comes
in .120 wall, which means it all telescopes nicely. 6061-T6 boom material on
the other hand,
comes in .125 wall, which does NOT telescope, pita, and requires .005 to be
turned down
on a lathe.. more pita.
## The newer 6061-T8 boom material, in .120 wall, is also readily available
from DX eng, along
with every type of SS u bolt and solid saddle combo you can think of.
## If you really want to go crazy, and HD, channel aluminum in various
widths is also readily available
from any local metal shop. Channel aluminum is ideal for mounting els to a
round boom, bomb proof.
Back in the late 70s, I saw a fellow use channel Aluminum for mounting els to
a round boom.... but in a
unique fashion. Half round holes were cut on both sides of the vert walls
of the channel Aluminum.
The half round holes, were of course the same diam as the diam of the boom.
The half round holes
on both vert walls of the channel aluminum were dropped over the boom, then
welded. This process was
done on a dead flat table..and repeated for each element location. The
actual ele is then simply
U bolted to the channel Aluminum flat portion, at each end.
## End result is the channel aluminum boom to ele assys are bullet proof.
They cant and wont break.
And no need to spend a lot of time trying to align brackets to a boom. Draw
back is, the ele positions
are now permanently fixed.
## Channel aluminum comes in various widths and also various vert wall
heights. The last stuff I bought
was 4 inches wide x .250 inches thick. The pair of vert walls were 2.0
inchs tall x .281 inch thick..
and made from 6061-T6. You can also get the vert walls in 2 inches tall
x .25 inches thick.
Several versions are readily available for the 4 inch wide channel Al, like
4 x .187 with a pair of 1.625 tall x .25 thick vert portions.
## You can also get channel aluminum in 2 and 3 inch widths..and also 6 and
8 inch widths. They typ come in 10 and 20 foot long lengths . I just
get em to cut to the desired length I want. Typ 4 inch wide x 8-10 inch
long for 20m els. And a lot longer for 52-59 foot long
40m shortened els, like 16-24 inches long. I used 16 inch long pieces to
retrofit onto my F12 340N yagi. The oem F12 flat plates
on mine were all slightly bent in the middle. F12 had used a flat plate
bolted to the underside of their ....wrap around bracket.
Part of the problem was the flat plate they used, ends up with a slight gap
between top side of plate and 6 oclock position
on the boom, hence the flat plate bending at the same point. Flat plates
tossed and replaced with channel Al..and a lot longer,
problem solved.
## I bought a lot of 4 x 2 channel aluminum from the metal scrap dealer, in
pristine shape.... in 6-10 ft long
lengths. I then just cut it up to desired lengths. They were used on a
long boom 20m yagi back in the late 70s,
and also a long boom 10m yagi in the early 80s. Those designs back then,
I just used a pair of U bolts to bolt
the channel al to the boom.... NO mouse holes or welding.
Jim VE7RF
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