[TowerTalk] [Bulk] Re: W6NL 40m Moxon

[Ken K6MR]

I’ve been trying to find an archive of Communications Quarterly but no luck so far. This obviously takes some study to understand (at least for a non-mechanical type like me).

Grant: your idea of adjusting the boom to add the area from the tee sections seems like a good idea. I’ll have to play with this. 

Thanks to everyone for bringing this to my (our) attention. It is one of the disadvantages of using software for designs: you don’t have to understand the concepts to get answers. It’s like using Spice for circuit modeling: you don’t need to understand the circuit to make something work. Paper and pencil calculations really drive home why things work the way they do.

If anyone has a source for the original article please let us know.

Ken K6MR

From: Grant Saviers
Sent: Wednesday, April 13, 2016 12:24
To: Ken K6MR; gboutin at infinichron.com; towertalk at contesting.com
Subject: Re: [TowerTalk] [Bulk] Re: W6NL 40m Moxon

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.
>
> Y
agi 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 at infinichron.com<mailto:gboutin at infinichron.com>
> Sent: Wednesday, April 13, 2016 05:17
> To: towertalk at contesting.com<mailto:towertalk at contesting.com>
> Cc: k6mr at outlook.com<mailto:k6mr at 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...
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