[TowerTalk] overhead truss for 80M rotary dipole

[Markku Oksanen]

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
> To: cgerarden at atomix.com; towertalk at 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
> 
> 
> 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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