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[TowerTalk] overhead truss for 80M rotary dipole

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Subject: [TowerTalk] overhead truss for 80M rotary dipole
From: "Jim Thomson" <jim.thom@telus.net>
Date: Sat, 19 Jul 2014 05:49:29 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Date: Wed, 09 Jul 2014 22:36:15 -0700
From: Grant Saviers <grants2@pacbell.net>
To: Chuck Gerarden <cgerarden@atomix.com>, towertalk@contesting.com
Subject: Re: [TowerTalk] overhead truss for 80M rotary dipole

I rebuilt a tired EF180C (no longer sold) which is 86' long.  Many of 
the rivets were loose and were drilled out and replaced with cross 
bolts.  I particularly like W6NL's advice on connecting elements - two 
bolts at 90 degrees which does reduce the movement in 2 planes.  
Additionally he advises two guys above, so I use Phillystran to about 
20' out on each element.  Mine are attached to a cross arm 4' above the 
boom and about 30" each side, about what the linear loading wires were 
originally. It is now tuned with a Tornado variable inductor and 25 ohm 
balun for full 80m coverage < 1.3:1.  He also recommends a down guy to 
the mast, then the antenna is totally constrained (I don't have one).  
W6NL's view is that updrafts are likely and wind induced oscillations 
can also move the elements vertically as you note, so a down guy is 
important. This antenna might move to a higher more exposed tower 
position and then I will add a down guy.

I also ran the original (unguyed) design through YagiMech from DX 
Engineering and that verified why there were some small bends.  The wind 
survival barely was above 70mph.  With internal sleeving I was able to 
improve that to almost 90mph.  So far so good after 3 years, although my 
wind conditions are very benign even at the 100' element height.  btw 
the tip elements are 1/4" diameter but the wind load is so small on them 
they are not the weakest link.

I think it is unlikely that an element will fail in column buckling 
before failing in bending, and you are correct that guys load the 
element in compression.

Grant KZ1W


### I wonder if the two guys, and in your case,  on the ends of a 5 ft cross 
arm, are actually
doing anything?   I can see why F-12 did it that way, since they used LL  
wires.   The longer the
cross arm, the bigger the load on the guys.  IMO,  a single guy, mounted to the 
mast, would do the
trick, and fully support the dead weight of the ele halves.   Even with a 5-10 
ft long cross arm,  with cross
arm mounted  4-8 feet above the ele, they wont provide any lateral support. 

##  What might work better  would be to use two guys, and re-configure so that 
the ele is guyed in two places.
IE:  say  10 ft and 20 ft out from the mast...with all 4 x guys,  2 per ele 
half,  all converging to the same point
on the mast..say  4-8 ft above the ele.   In the case of the fellow from OH 
land, with his 100 ft dipole, he has it
guyed  40 ft out from the mast.  Putting a 2nd guy out to the 20 ft point, 
would support the dipole better, esp with a 
snowload etc. 

##  The ultimate setup, as he mentioned would be to use 3-6 guys, and guy every 
120 degs, like the m2 long booms.
Then you would definitely support any lateral side loading.   It also becomes a 
3 dimensional nightmare. 

##  Note to grant.  If your guys are out 20 ft on each side,  you really want 
the cross  boom to be 5+ ft above the ele, and not 4ft.  

##  Now this assumes any F-12 LL  has been removed, and replaced with coils, or 
coils and T bars... aka capacity hats, which resemble
10m eles, and mounted  50-60%  out each side.    Since I have removed the LL 
wires on my F-12 EF-180B.... which is 68 ft long...and also
no longer made,  and replaced  with a combo of capacity hats + seco tornado 
drive, the oem cross bar is no longer required. 

##  Be careful with using too big a tubing OD.  Sure its stronger, but it also 
increase windload.  Another method would be to use doublers
to increase wall thickness.    You can use any of the yagi software, like Yagi 
stress   to figure it out.   IE:  treat the dipole like it was the 
boom of a yagi...with no eles of  course. 

##  DX eng now supplies 6061-T8  alloy  tubing  in .120 wall etc.  The T8 alloy 
is a full 25 %  stronger than the T6.   50 ksi  vs  40 ksi. 
The beauty of the .120 wall is that each piece just slides into the next one !  
 With .125 wall  6061-T6,  it has to be machined in a lathe, so that
the pieces slide into each other.....PITA !   So this new T8  kills two birds 
with one stone, 25%  stronger, and no machining to fit  pieces. 
IE:  1.75 inch slides into 2 inch..which slides into  2.25 inch, which slides 
into 2.5 inch etc, etc.    For the outer, smaller diam sections,  you could use
either .058 wall, or  double that up with doublers, and .116 wall.    Or use 
.083 wall  in some places. 

##  Either way, the 80m dipole could be mech designed  using any of the popular 
 yagi  mech stress programs available.  Just model the 80m dipole
as  the boom of a conventional yag..sans eles.   BTW, the T8 material is great 
to build yagi booms with.  Then use a simple doubler to splice the
boom halves together in the middle.   Or use a single piece in the middle, and 
use a doubler inside of it. 

Jim  VE7RF


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