[TowerTalk] Sidemount yagis

[Pete Smith]

At 10:47 AM 4/4/00 EDT, K7LXC@aol.com wrote:
 
>      Well, everytime I've talked to an engineer at a tower manufacturer 
>they've said that the capacity figure is over the whole tower. Obviously a 
>big headload creates the biggest overturning moment and other forces. The 
>poster's tower was 25G so there's not a lot of engineering overhead left to 
>take care of additional antennas and also the forces from the sidemount.
With 
>a 3-foot sidemount the side torque is increased by a factor of 3 so that can 
>create some big stresses down the tower.

A couple of comments -- I suspect tower engineers will indeed say that,
because placing the wind loads below the top of the tower, with total area
not exceeding the rated area at the top, *probably* won't result in an
overload.  As Barry pointed out, K7NV has a pretty sophisticated
calculation on his web site to illustrate how this works, which is
somewhere between Rich's assumption and what the tower engineers are saying.

As for the sidemount, a 3-footer would surely be overkill with Rohn 25,
wouldn't it?  My horseback analysis sayss that a side-mount with NO
swinging gate, mounted close to and directly outboard from one leg of the
tower, would give a tad over 240 degrees of rotation.  On Rohn 25, if you
added a gate that's 12" plus the distance the centerline of the rotator is
from the outside of the leg, the resulting arrangement should give you
close to 300 degrees.

>
>      There are several nuances involved here, i.e. windspeeds are higher up 
>the tower than near the ground, etc. And you still need to observe the LXC 
>Prime Directive to "DO what the manufacturer says" so that you end up with a 
>reliable installation.

But this is precisely the problem.  Rohn *doesn't say* how to figure the
safe wind loading in a stacking situation, and antenna manufacturers' area
numbers for the most part aren't calculated in the same way Rohn's are.
PEs aside, we really don't know what's safe, and have no resource for
cookbook solutions.

73, Pete Smith N4ZR
n4zr@contesting.com 

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