[TowerTalk] overhead truss for 80M rotary dipole

[Grant Saviers]

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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