[TowerTalk] overhead truss for 80M rotary dipole

Thu Jul 10 19:33:12 EDT 2014

 Markku,
Thanks for all the great information. After reading responses I think
my tubing is too small.I will also raise my truss height as suggested.
Chuck
W0DLE

----- Original Message -----
From: Markku Oksanen 
To:"Grant Saviers" , "Chuck Gerarden" , "towertalk at contesting.com" 
Cc:
Sent:Thu, 10 Jul 2014 07:00:43 +0000
Subject:RE: [TowerTalk] overhead truss for 80M rotary dipole

Hi
Here at south OH-land (not Ohio) a dipole like this needs to start
with 80 mm / 3 inch tubing with 3 mm walls. As tubing resistance to
bending goes with 4th power of the outer radius, any extra mm is good.
 The tube at the truss attachment is 60 mm.
Secondly, the compression (bending) caused by the truss wire
increases proportional to 1/Sin(a) (or Tan(a), for small angles these
are close) where the a is the angle between the element and the truss.
This gets very quickly very large as we move to small (Sin(a) becomes
small) angles between the element and the truss.  So, moving the
truss higher on the tower will help.  
I have a 100 foot home brew 80 m rotatable dipole and the truss is 3
m/10 feet above.  Truss goes to the 40% part and the ends are
designed to stand the elements free of support. So, 40 feet out, 10
feet above means a compression force  4 x weight seen at the truss
attachment point.
I don't know what the accepted rule of thumb would be but if a
truss-element angle of less than 15 degrees causes the weight of the
element seen by the truss to be multiplied to a compression force of
4x the weight. This would be OK if the tube would be supported so that
it can't bend under the pulling force.  With no wind this is the
case.
For these reasons I believe there are three options: Increase of tube
stiffness (diameter) so that the tube will be straight under all wind
conditions (the part between the truss and the tower), move the truss
higher to lower truss induced compression force or more difficult, add
2 more trusses so that they are 120 degrees around the element and
prevent this bending and add more failure modes.
I would move the truss higher, there is very little reason not to
have it as high as you can (easier than changing anything else). The
truss just about only is there the support the weight of the element
and doesn't do much for sideways dynamic forces.

MarkkuWW1C/OH2RA/OG2A

> Date: Wed, 9 Jul 2014 22:36:15 -0700
> From: grants2 at pacbell.net [1]
> To: cgerarden at atomix.com [2]; towertalk at contesting.com [3]
> 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 < 13:1. 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
> 
> 
> On 7/9/2014 5:50 PM, Chuck Gerarden wrote:
> > I have had several 80 meter rotary dipoles over the years and
they
> > have failed in the same manner due to very high windsthey get
bent but
> > never actually break. I wonder if the placement of the overhead
> > element truss may cause this effect as thewind blows. The truss
is
> > pulling up on the element but as the wind blows harder, the truss
is
> > actually pulling on the element
> > at an angle other than "up" due the the wind deforming the
element.
> > The harder the wind blows, the truss pulls the element harder
into the
> > mast.
> > I am thinking the truss is too far out on the element and maybe
it
> > should be moved in closer. This changes the "pivot point"as the
wind
> > is hitting the element and the outer element area is moving more
and
> > the inner area is more stable.
> > Is there a formula or does anyone have empirical knowledge on
where
> > the best place on an element or boom the truss should attach?
Each
> > element is 41' long for a total length of 82'. The antenna is
center
> > coll loaded for resonance and fed with a25 ohm balun.
> > I of course could have 1 overhead truss and a side truss to
resist
> > horizontal forces, but I would prefer to keep it simple ifthe
> > engineering allows it. This entire problem may be the aluminum
tubing
> > is not big enough or thick enough to beginwith. The elements
start at
> > 2 1/2 inches and taper to 1/2 inch.
> > My latest solution is to use tapered 40' fiberglass poles as the
> > elements with a wires inside. With big antennas I have often had
> > better results after a wind storm since they return to their
original
> > position.
> > Anyone out there have some good engineering advice on building 80
> > meter rotary dipoles?
> > ThanksChuckW0DLE
> >
> >
> > _______________________________________________
> >
> >
> >
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> 
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> 
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> 
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