[Towertalk] Antenna Advice

[Pete Smith]

At 09:06 AM 12/23/02 -0500, Wes Cosand wrote:



>I, also, am interested in the Force 12 230/240 and 5BA solution.  Here are 
>some back of the envelop calculations about  what the yield strength of 
>the mast would have to be for this application.  Let's assume 20 feet of 
>mast above the tower with the 30/40 at 16 feet and the 5BA at 1 
>foot.  Lets assume 0.25" tubing wall.  I am not an engineer and these 
>calculations are only for discussion purposes.  They are based on the 
>method of W7NI's article.  (Do you really want a chemist doing your 
>engineering?)
>
>
>mast diameter    2"           2.5
>
>90 mph             121K psi       72K psi
>80 mph              98K           58K
>75 mph              68K           40K
>

I'm not familiar in detail with the W7NI method (I read the article once 
ages ago) but I think there's one important caveat.  Force 12 states the 
wind area of its antennas in their worst-case alignment to the wind, but in 
terms of effective area, which as they state in their brochure is 2/3 of 
the projected area for cylindrical surfaces.  Most antenna manufacturers 
use this number, but Rohn, for example, uses projected area for its tower 
ratings.  I'm not sure which W7NI used.

It also seems intuitively important to consider not only the risk of 
bending the mast, but also the forces at the top of the tower in this 
configuration.  Seems to me you would definitely want to guy at the very 
top of a tower that had this much load this far above the top plate.


73, Pete N4ZR
Happy Holidays