I have W6NL Moxons so am interested in this analysis.
If I understand Kurt's well written post correctly, the only forces the
Moxon tips (or other cylinders) can generate are perpendicular to those
cylinders. Thus one side of the DE and Reflector is in tension and the
other is in compression from the forces on the "T" tips. Assuming the
boom, DE, Ref and tips stay perpendicular, those forces are added to the
ends of the boom. Or 2x projected area of one T is like a single
projected area where the DE and Ref attach to the boom. A small torque
is placed on the boom if the areas of the DE T and Ref T are different.
The difference is 4" length x 0.375" dia in my Moxon or 4x = 0.04 sq ft
so can be ignored.
For my "W6NL 120 mph" version I calculate the rectangular area as 122.3
sq in for the reflector T, so two of these is 244 in sq. The boom
diameter for me is 2" at the DE and Ref mounting points.
If that 2x tip area is added by lengthening the 2" boom 244/2 or 122"
(10' !) then the result will be a much larger moment on the boom than
reality since the center of that area is 5' too far out. The DE and Ref
positions don't change, there is 10' of boom sticking out beyond them.
That would be a very conservative modeling approach.
Depending on the modeling program, there may be other ways to
approximate placement of the tip area at the DE and Ref mounting
points. If a larger diameter, shorter, but same 244 sq in area boom
section could be added centered at the DE and Ref mounting points then
the modeling will be more accurate.
It is a surprising result to me the amount of boom load the T tips can
create. I should note that one of my Moxons was acquired and has a 10
degree downward bent (kinked at the boom plate) Ref in the center 1.25"
sch 40 + 1.5" sch10 pipe section on one side from ice loading at its
prior QTH, so that is another consideration about loading from the T
tips. This antenna is going to become a plain 30m 2L and does have a 3"
boom.
Grant KZ1W
On 4/13/2016 9:43 AM, Ken K6MR wrote:
Thanks for the link. This is definitely not an intuitive result. The good thing
is that the loads are smaller using this procedure.
Yagi Stress calculates these loads, but I’ve never done pencil and paper to understand
how it does it. Kurt notes that YS is using this method so the numbers I have for normal
yagis are good. Unfortunately Kurt didn’t allow for unusual designs like the W6NL
Moxon.
Ken K6MR
From: gboutin@infinichron.com<mailto:gboutin@infinichron.com>
Sent: Wednesday, April 13, 2016 05:17
To: towertalk@contesting.com<mailto:towertalk@contesting.com>
Cc: k6mr@outlook.com<mailto:k6mr@outlook.com>
Subject: Re: [TowerTalk] W6NL 40m Moxon
Ken,
This isn't exactly new. I think you'll find what you're after by reviewing
K7NV's summary post from back in 1998.
http://lists.contesting.com/archives//html/Towertalk/1998-08/msg00499.html
<http://lists.contesting.com/archives/html/Towertalk/1998-08/msg00499.html>
Gerald, VE1DT
...snip...
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|