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[TowerTalk] W6NL 40m Moxon (again)

To: <towertalk@contesting.com>
Subject: [TowerTalk] W6NL 40m Moxon (again)
From: "Jim Thomson" <jim.thom@telus.net>
Date: Wed, 20 Apr 2016 09:41:58 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Date: Tue, 19 Apr 2016 23:58:07 -0700
From: Ken K6MR <k6mr@outlook.com>
To: "towertalk@contesting.com" <towertalk@contesting.com>
Subject: Re: [TowerTalk] W6NL 40m Moxon (again)

A week ago or so AC0H posted a simple question: how much wind load for a W6NL 
40M Moxon yagi. As I had just built one and installed it (but not bothered to 
do the mechanical model) I thought I?d dive in.

I posted some values from Yagi Stress, and a good conversation got going on how 
to calculate the maximum wind load. Some numbers were posited but I still 
wasn?t sure. Even though I?ve used Yagi Stress a fair amount, I was not quite 
sure how it did what it did.

VE7RF and VE1DT pointed us to an old posting by K7NV regarding a modern wind 
load calculation model published in Communications Quarterly. Thanks to AC6LA 
who found the article and posted a link to the article and K7NVs posting. In 
essence, I found out ?you?re doing it wrong?.

After reading through the article and (I think) understanding the concepts I 
ran through the calcs again. Needless to say, the results are different.

In summary (assuming I did it all correctly), the antenna has a maximum wind 
load of 6.22 sq. ft.. This occurs with the boom broadside to the wind. The 
model includes RG8 coax to the DE and the balun box is included. For 
comparison, the wind load with the antenna pointed into the wind (so loads from 
the elements and standard Cushcraft mounting channels) is 5.98 sq. ft..  
According to the K5IU model, the maximum wind load of the antenna is the larger 
of these two values.

I also used Yagi Stress to calculate a torque compensator plate. Because of the 
coax and balun there is a small value of rotating torque when broadside. 
Allowing for the torque development by the different sizes of the element tees, 
the plate is pretty small. But it does raise the total broadside area to 6.42 
sq. ft.  So I believe we can safely say ?under 7 sq. ft.? is a pretty safe 
value.

The element and boom sizes I used were assuming converting an XM240 and using 
the latest (2012) design that is labeled ?W2SC 100 mi/h?. This is the one that 
puts new 1.5? center sections in the elements.

I?ve posted the summary I wrote up if you are interested in the numbers. And 
please let me know if there is some bone-head math error or if I really don?t 
understand the concepts. The original XM240 spec is 5.5 sq. ft., so the new 
values seem reasonable.
http://bit.ly/1qYohrZ

Thanks to everyone who posted info on this subject. The best part about this 
hobby (for me) is learning new stuff. This was definitely new. 

Ken K6MR

## The cross flow principle has been around for a looong time in the real 
world. The French used the cross flow principle when they designed the Eiffel 
tower.  The aircraft industry also uses it, along with  everything else I 
looked at.
Dunno where the yagi  screwup came from. It was always thought that max 
windload occurred around 45 degs.... when in fact, MIN windload occurs at 45 
degs.   So you either use the boom area + boom to mast plate.... or the total 
of all the els,
whichever is greater.  But that Dick Weber article was written a long time ago, 
so I assumed everybody was used to the correct way of doing the calcs.   The 
Torque compensator plate is very effective..esp on yagis that have all the els  
bunched up at one end.

Jim  VE7RF

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