[TowerTalk] Antenna Windload Manufacturer Ratings

[jimlux]

Gerald Boutin wrote:
> I am curious about how antenna manufacturers and antenna mechanical
> design programs determine the maximum windload ratings for an antenna,
> in particular yagis.
> 
> It occurs to me that for a typical yagi that is designed for say a
> maximum windload of 100 MPH, stress is calculated on the exposed surface
> area of the elements against a maximum allowed calculated stress,
> assuming that the wind in worst case conditions is at right angles to
> the largest exposed area - ie) at rights angles to the wind.
> 
> However, it is obvious from seeing pictures of yagis in very high winds
> that the antenna is not totally at right angles to the wind in high wind
> conditions. ie) The elements bend (substantially) and thus the exposed
> area is reduced.
> 
> Is the bending taken into account in designs and published ratings, or
> is this always just an inherent additional safety factor?
> 

I'm not an antenna manufacturer, but I'll bet they do NOT take into 
account any changes in shape from deformation under load. They're 
basically coming up with an approximate load, and using some convenient 
safety factor or margin in their design (e.g. if there is 10000 psi 
stress and the antenna is made of aluminum with 17000 psi yield)..

Or they design by empiricism and rules of thumb.. the converse of the 
ham dictum "if the antenna stayed up in last year's wind storm, it 
wasn't big enough"..

The YagiStress modeling program and related book, as I recall, assume 
fixed configuration and no deformation.