[TowerTalk] overhead truss for 80M rotary dipole
Grant Saviers
grants2 at pacbell.net
Sat Jul 19 15:09:50 EDT 2014
On 7/19/2014 5:49 AM, Jim Thomson wrote:
> Date: Wed, 09 Jul 2014 22:36:15 -0700
> From: Grant Saviers <grants2 at pacbell.net>
> To: Chuck Gerarden <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
>
>
> ### 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.
I think the guy and element compression loads decrease with larger
included angles to the boom of the guys. Or conversely, lower guy
tension results for a given side or down restraining force. So, longer
cross arms and higher mast offsets are a net positive. Leeson in
Physical Design of Yagi Antennas covers how to calculate all of the
bending moments and loads for a guyed system. However, I copied the
factory LL wire configuration rather than doing my homework on that,
assuming (maybe incorrectly) that the original LL design had some valid
structural design in it. I did internally sleeve a the crossarm and the
elements in several places which YagiMech showed to be relatively weak.
I did misquote Leeson in one regard, his design is two guys down to a
bent crossarm and one up to the mast for more fully constraining the
movement of the boom. I also don't think more than three are advisable
- no structural gain and more windload.
As for more than one set of guys, the moment calcs in Leeson would see
if they were necessary. Considering that any guy has stretch, probably
a finite element analysis is necessary to get meaningful results with
multiple guys. As with any slender structure the objective is to keep
it "in column".
Finally, as my tests of Phillystran secured with cable clamps (clips)
showed, this system is poor (unacceptable to me) compared to the specs
for Philly guy grips. W8JI reported that 1200# with clamps slipped
under ice loading on one of his antennas. So, either use the next
larger Philly with grips or some better 1200# clamping method. I have
some Nicopress sleeve terminations on the 1200# Philly guys for booms on
large antennas made by K7ZSD which I know were up 7 years in a tough
environment. They show no signs of slipping and the booms for three 20m
5L OWAs that were heavily ice loaded show no sign of bending (42' x 3"
booms). I intend to do some tension tests on Nicopress sleeves for
1200# Philly vs cable clamps.
>
> ## 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.
Agreed, more guy angle to the element/boom the better, but not sure this
is a necessary improvement at my site.
>
>
> ## 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.
With the longer element, I don't need capacitive loading, so don't have
that weight and windload to contend with.
>
> ## 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
>
>
> _______________________________________________
Grant KZ1W
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