Stan,
Thanks, the emergence of a new method may explain why Force12 does things
the way they do. The square root of the sum of the squares method also
seems to be what exW6QHS uses in his "Physical Design of Yagi Antennas"
book as well, but I may have misunderstood.
For my C3, the 2 numbers are very close (5.7 vs 6.0), so this isn't really
an issue for me. The notion that I might need to multiply Force12 numbers
by 1.5 or use the total boom + element projected areas against my US Tower
12.3 sq ft wind load limit was what concerned me. I believe this is false.
Dana Roode
Stan & Patricia Griffiths wrote:
> Hi Dana,
>
> It has been pointed out to me several times that the method I used to
> calculate antenna wind area in my original article (published in about 1985)
> has been replaced by a newer method. The newer method used either the
> element OR the boom area and not the square root of the sum of the squares
> of those two figures. K5IU can explain this much better than I can.
>
> Stan
> w7ni@easystreet.com
>
>  Original Message 
> From: "Dana Roode" <K6NR@arrl.net>
> To: "TowerTalk" <towertalk@contesting.com>
> Sent: Saturday, February 08, 2003 6:41 PM
> Subject: [TowerTalk] Calculating Antenna Wind Load
>
>
>
>>TowerTalk Folk,
>>
>>As a tower novice, I have been looking into wind load calculations, to see
>>what I can really afford to put on my US Tower TX455 (rated at 12.3 sq ft
>>of wind load at 70mph). I had been using the antenna vendor's specified
>>wind load numbers, but wondered why a 6 element Force12 6 meter beam was
>>rated at 2.0 sq ft where as 5 element Cushcraft beam was rated at 2.9 sq
>
> ft.
>
>> Better windload design on the Force12 might explain some of it, but
>>something didn't seem to add up.
>>
>>I posted some questions about this on the Force12 reflector and was told
>>that Force12 uses an "effective" area calculation rather than a
>
> "projected"
>
>>area calculation. Sure enough, the Force12 brochure has the following
>>explanation of their wind load calculation:
>>
>>"WIND LOAD is the worst case wind resistance for the antenna. Using the
>>latest structural analysis, the wind load is either the total element wind
>>load OR the boom wind load, whichever is the larger resistance to the
>
> wind.
>
>>Most beams have more element than boom wind load. The figure specified is
>>the effective area, which is the projected area of the elements or boom,
>>multiplied by 2/3 for a cylindrical surface."
>>
>>It was suggested that I needed to multiply the Force12 number by 1.5 to
>
> get
>
>>a number that would be appropriate to compare against the Rohn (or in my
>>case USTower) "projected" wind load maximum.
>>
>>I did my own calculations on my C3E yagi, adding up each separate element
>>section's wind load computed by multiplying the diameter times the length.
>>I also read an article by W7NI (January 1992 NCJ & ARRL Yagi Antenna
>>Classics) that said you compute the maximum wind load on an antenna as the
>>square root of the sum of the squares of the boom area and the element
>
> area.
>
>> Calcs are multiplied by 2/3 due to the round shape of the elements.
>>
>>My calculations were:
>>
>> Total Element Projected Area times 2/3 = 5.67 sq ft
>> Total Boom Projected Area times 2/3 = 2.0 sq ft
>> SqRoot of sum of squares times 2/3 = 6.0 sq ft
>>
>>The Force12 catalog has the C3E wind load rating at 5.8, but their manual
>>says "5.9 square feet max at 21 degrees from boom center". So, I have 6
>>potential C3E wind load numbers: 5.67 (total of elements), 7.67 (total of
>>elements plus boom), 6.0 (square root of sum of squares), 5.8 (catalog),
>
> 8.7
>
>>(1.5 times the catalog) or 5.9 (instructional manual).
>>
>>Question  which one to use against my USTower 12.3 number? The 6.0 sq
>
> ft
>
>>number makes the most sense to me  seems like the real MAXIMUM wind load
>>that can be on the antenna, slightly off center from directly into the
>>elements. Wind doesn't blow simultaneously directly into the boom and
>>directly into the elements (the 7.67 or 8.7 numbers).
>>
>>I also computed the wind load for the rest of the antennas I'm currently
>>looking at. The results, listing vendor rating, calculation using square
>>root of sum of squares, and calculation of total of boom plus element
>>projected area were:
>>
>> Rated Squares Total
>>Cushcraft D3W 0.9 1.75 1.75
>>Cushcraft 3 element 6m 1.8 1.1 1.5
>>Cushcraft 5 element 6m 2.9 1.9 2.6
>>
>>Now maybe I made some errors in my calculations, I'll have to double check
>>them. Perhaps the vendors use a more sophisticated technique to determine
>>wind load numbers.
>>
>> Dana
>>
>>_______________________________________________
>>
>>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>
> Weather Stations", and lot's more. Call Toll Free, 18003339041 with any
> questions and ask for Sherman, W2FLA.
>
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>>
>
>
>
>
