Date: Sat, 17 Sep 2011 07:41:46 -0400
From: Pete Smith <n4zr@contesting.com>
Subject: [TowerTalk] T-bars (capacity hats) was: Re: Aluminum tubing
insert overlap...
About T-bars -- does anyone else remember pictures of Sigma 80 yagis
broken at the junction between the T-bars and the elements? I think I
recall one station where all 4 broke (on a 2-element).
73, Pete N4ZR
## This is new to me. What actually broke ? The T bars are nothing more than
a 10 M ele on the shortest Sigma. EF-180S, 54' long els.
was in wind damage, or ice loading, or both ?
## I recently rebuilt my F-12 EF-180B... [ 68' long and alumo weld LL
wires]... and tossed the Al LL wires.
They were replaced with 17.5' long T bars... that are triple walled in the
exact middle..then double walled beyond that.
It uses 3/8" tips , then 1/2", then 5/8",then 3/4" in the middle of each T
bar. To attach them to the 2" main trunk section,
I used some old Wilson boom to ele clamps from my old 5-el 10M yagi... from
1977. These are NOT the clamshell types, virtually identical to the hy-gain
ones.
These are the original types, that are cast in one piece. They completely
wrap around the 2" boom..and are pinched tight
with a pair of 1/4-20 SS bolts. The top of the circumference has a cast
flat plate on top. The 10m ele drops right down on top
and held on with a pair of U bolts [ no saddles]. I simply used new tubing
to make each T bar... and also used DX eng
saddle clamps and SS U bolts to attach each T bar to the flat plate. The
original wilson 10m els were 3/4" OD in the middle..so to get
everything to fit right, I triple walled the exact center with a short length
of 3/4" tubing.
## The Old Wilson clamps can be mounted right side up..or upside down.
Sure, a flat plate + 4 x u bolts could have been used, but the
original Wilson 'far out clamps' will no way slip on the 2" boom, not in a
million years. You won't break one either. It's abt the only thing they
ever
did build correctly..then like bozo's.... switched to the ill conceived hy
-gain style clam shell boom to ele clamp assy's. Another option is to use the
F-12 ele to boom style brackets, that in this case are riveted to the 2" main
trunk section..then hang the T bar from it.
## Since the main trunk section of the F-12 EF-180B is 2" OD.... the Wilson
clamp was the cheap and easy way to quickly mount the
T bars. Just beyond the T bars is the solid fiberglass insulators... which
are 1". That's all heavy wall tubing on both sides of the insulator's too.
Either a shorting strap, made from AL strap is placed across each
insulator..OR the oem insulators are removed..and replaced with 1" heavy wall
AL tubing.
A single 5/16" dacron rope supports each ele half. The remaining loading is
done via the compressible coils from Seco systems. Coils are 1/4"
CU tubing..and motor driven..and located right at the feedpoint. A 5 turn
helical hair pin does the matching.
I have seen pix of the F-12 Sigma 180-B T bar to ele assy, and it looks very
robust to me. In fact, it looks extremely strong. How the heck can it break ?
I'm talking abt the 54' long sigma..which came in either a 80m rotary dipole
or 2 -el version. They also made a longer one..using 76' els..and even longer
T bars.
I hate using just U bolts and no mating saddles...so opt for the DX eng style
saddle clamps..which consist of a SS u bolts with a longer thread, plus mating
cast AL saddles.
later... Jim VE7RF
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