[TowerTalk] T-bars (capacity hats) was: Re: Aluminum tubing

[Jim Thomson]

Date: Sat, 17 Sep 2011 07:41:46 -0400
From: Pete Smith <n4zr at 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