[Towertalk] yagi spacings

[Pete Smith]

At 09:59 AM 10/15/02 -0700, Bob Nielsen wrote:
>Bob wrote:
>
>These total 21.5 square feet. According to the FCC database, K7AWB is
>in Spokane county, which has a 70 mph wind speed rating. Rohn 25G is
>specified for 18.4 sq. ft. with a single set of guys at 40 feet, 70 mph
>wind speed, so the installation would be pushing things a bit.  YMMV,
>but I think a more conservative approach (heavier tower or smaller
>antennas) should be considered.
>

Amen to that.  This is particularly true because Rohn's ratings are for 
projected area, while most antenna manufacturers persist in quoting a more 
optimistic "effective area" number which must be multiplied by 1.5 to be 
equivalent to what Rohn says.  Assuming the areas quoted are "effective 
area,", then the tower would be overloaded >1.75:1 even if all the antenna 
area was right at the top of the tower -- distribute it along a 15' mast 
(lever arm) above the top and the problem becomes far worse.

For those interested in more on wind loading calculations, I highly 
recommend K7NV's Yagistress web site -- 
http://yagistress.freeyellow.com/K7NV/  He is a professional engineer who 
understands all this stuff, and has a good way of presenting it to the layman.


73, Pete N4ZR
Sometimes a tower is just a tower