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## [TowerTalk] Calculating Antenna Wind Load

 To: [TowerTalk] Calculating Antenna Wind Load K6NR@ARRL.net (Dana Roode) Sun Feb 9 13:04:54 2003
 ```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 has been replaced by a newer method.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" > To: "TowerTalk" > 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. > >>_______________________________________________ >>TowerTalk mailing list >>TowerTalk@contesting.com >>http://lists.contesting.com/mailman/listinfo/towertalk >> > > > > ```
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