Stan,
Thanks, the emergence of a new method may explain why Force-12 does things
the way they do. The square root of the sum of the squares method also
seems to be what ex-W6QHS 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 Force-12 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 has been replaced by a newer method.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 TX-455 (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 Force-12 6 meter beam was
>>rated at 2.0 sq ft where as 5 element Cushcraft beam was rated at 2.9 sq
>
> ft.
>
>> Better wind-load design on the Force-12 might explain some of it, but
>>something didn't seem to add up.
>>
>>I posted some questions about this on the Force-12 reflector and was told
>>that Force-12 uses an "effective" area calculation rather than a
>
> "projected"
>
>>area calculation. Sure enough, the Force-12 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 Force-12 number by 1.5 to
>
> get
>
>>a number that would be appropriate to compare against the Rohn (or in my
>>case US-Tower) "projected" wind load maximum.
>>
>>I did my own calculations on my C-3E 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 Force-12 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 US-Tower 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, 1-800-333-9041 with any
> questions and ask for Sherman, W2FLA.
>
>>_______________________________________________
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>>TowerTalk@contesting.com
>>http://lists.contesting.com/mailman/listinfo/towertalk
>>
>
>
>
>
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