[TowerTalk] Antenna & Tower Wind Load Ratings

[jimlux]

On 6/15/17 9:56 AM, JVarney wrote:
> The core problem here is the ham consumer is not being given
> compatible and concise data from which to make an informed choice.
> There's only one way for antenna ratings and tower ratings to fully
> compatible and that's through the "Aa" projected area term in
> TIA-222-G.  Antennas and towers should be rated like this:
>
> ===
>
> TRIBANDER MODEL XYZ
>
> Aa = 11.0 square feet*
>
> * TIA-222-G 100mph wind speed, height 100 ft, Exposure Cat B, Risk
> Category I, Topography Cat 1.
>
(Aa is "Area of appurtenances", As "Area of structure", etc.)

Aa, though is before you add the force coefficient (Ca) and you need to 
know the orientation(s).  That's independent of the exposure and wind. 
The ones that vary are (EPA)N and (EPA)T  (normal and tangent to the 
wind direction)

The challenge is that the Ca coefficients depend on the diameter and the 
wind speed.

A 2" diameter tube at 100 mi/hr has a Ca anywhere between .7 and 1.2 
depending on aspect ratio

One nice thing is that for Yagis, I think all the members will be in the 
TIA222G "subcritical" category where "C<32" - C=(I*Kzt*Kz)*0.5 * V 
(mi/hr)* D(ft)

ALl those leading values (I,Kzt,Kz) are "about 1" and none are 2.  so 
we're really looking at when V*D>64.

Well, for 100 mi/hr and a 6" diameter element (which is pretty big), 
you're only up to V*D=50.  If you're building an antenna with a 6" 
diameter boom, you're already in a fairly exotic area.  For the usual 
1"-4" sort of thing, and reasonable wind speeds..(<150 mi/hr) you're 
still in the subcritical zone.