Towertalk
[Top] [All Lists]

## Re: [TowerTalk] wind load

 To: "Tower and HF antenna construction topics." Re: [TowerTalk] wind load Charles Coldwell "Tower and HF antenna construction topics." Thu, 18 Feb 2010 14:55:44 -0500 mailto:towertalk@contesting.com>
 ```On Thu, Feb 18, 2010 at 2:02 PM, Gene Smar wrote: > >     To get these figures into engineering terms, you'd have to convert the > wind speed into pounds-per-square-foot > http://www.arraysolutions.com/Products/windloads.htm  and multiply by the > EFFECTIVE area of the antenna. Then this force is applied to the top of the > tower; let's say the tower is 100 feet tall.  In the example, the force of a > 70 mph wind is 12.54 psf.  The total forced applied to the tower at 100 feet > AGL would be 12.54 X 15 = 188 pounds. Well, that is substantially less than the 450 pounds I came up with from the "30 pounds per square foot in a 70 MPH wind" rule mentioned upthread. >     The trick comes in determining what the effective area (sqft) of the > antenna really is.  Manufacturers are unclear in their literature in how > they calculate the advertised areas of their antennas. I suppose this is due to uncertainty in the drag coefficient since the geometric cross-section should be unambiguous. Rewriting the Array Solutions formula as F = A * 0.00256 * W * W * Cd the effective cross-section is the product A * Cd, where A is the geometric cross-section, and Cd is 1.2 for long cylinders and 2.0 for flat plates. > That calculation showed that my D40 rotatable dipole had a spec from > Cushcraft of 1.3 sqft, whereas the "shadow area" of the elements calculated > out to be 3.55 sqft.  Go figure. Yeah, I can't make up that difference no matter how I play around with drag coefficients. -- Charles M. Coldwell, W1CMC "Turn on, log in, tune out" Winchester, Massachusetts, New England (FN42kk) GPG ID: 852E052F GPG FPR: 77E5 2B51 4907 F08A 7E92 DE80 AFA9 9A8F 852E 052F _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```