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
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