Towertalk
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 ```TT: I won't feel offended if you hit "delete" at this point. Following up on my recent postings regarding wind loads presented by Yagi antennas, I now know that I made the same error that numerous hams and ham publications have been making for a couple of decades. That is, I assumed (logically) that the wind force on a Yagi tubular element was in line with the wind itself, and that one compensated for the angle of wind on the element (angle of attack) by using trigonometry to reduce the effective area presented to the oncoming wind stream. However, I have since read three of Dick K5IU's various papers (available through ARRL's QEX/Communications Quarterly) in which he cites several references on the subject of wind flow over tubular structures. One author, Hoerner, explained that the net force of wind blowing across a tube is normal to the tube, i.e., at right angles to the axis of the tube. This is NOT logical but I'll accept it as true. As I wrote to Dick, this is the "and then a miracle occurs" part of the explanation. I'd need to find and read Hoerner's book to see a presentation of the physics involved here. Once you accept the principle of wind force as described (normal to the tube's axis), then all the trig calculations are easy to understand. The net result is that, indeed, the maximum wind force on a Yagi is either normal to the boom or to the elements, depending on which represents a greater area, and NOT at some intermediate angle as I had calculated. Another point: The nonsense of multiplying the area of a Yagi element by 0.67 or 0.6 or anything less than unity (1) to account for its shape is WRONG! Dick's article saved me from making that mistake in my tower calculations. The correct number (which Dick calls drag coefficient) is 1.2 for long, thin tubes used in Yagi elements. The drag coefficient for a flat plate is 2. Divide these two coefficients and you get 0.6, which might explain why some folks used this number as a shape factor. Another point: Armed with this new knowledge, I set about to calculate the wind area of my still-in-the-box Bencher Skyhawk. I took out the assembly manual and calculated the exposed area (L X W, no shape factor) for the boom and each tube of each element. Guess what? I calculated nearly 50% greater area FOR THE ELEMENTS ALONE than Bencher advertises for the entire Antenna. My figures show 12.2 sqft of element area and 4 sqft of boom, whereas Bencher specs the Skyhawk at 8.5 sqft. I sent Bencher an e-mail last week asking them to verify my figures. No reply as of this evening. A final point: If you blindly point your mast to due north (please don't start!) and attach all your long-boom Yagis from the same side of the tower, you're unbalancing the torque on your tower system. Dick wrote a couple of papers on aerodynamic balancing of antennas to minimize tower torque. I'm now going to mount my four antennas (Skyhawk, 2M Yagi (23 foot boom), D40 rotatable dipole, and triband V/UHF vertical) on the mast on alternating sides, depending on how the math shakes out. No point in deliberately putting unnecessary stress in your tower system, if it can be avoided simply. 73 de Gene Smar AD3F P.S. I'd advise you actually to calculate the area of your antennas, and NOT rely on manufacturers' specs (Unless you know how they came up with the numbers. And I apologize for starting another re-thread.) If you're contemplating the purchase of an antenna, maybe the manufacturer will sell/give you a manual for this purpose. (But then again, maybe not.) List Sponsor: Are you thinking about installing a tower this summer? Call us for information on our fabulous Trylon Titan self-supporting towers - up to 96-feet for less than \$2000! at 888-833-3104 www.ChampionRadio.com ----- FAQ on WWW: http://www.contesting.com/FAQ/towertalk Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com ```