>I just looked in my Rohn Ham Tower Catalog, and for the BX/HBX/HDBX towers,
>"NOTE: ANTENNAS DEVELOPING A LARGE TWISTING MOMENT DUE TO WIND MUST NOT BE
USED ON THIS TOWER. ANENNAS SHOULD BE LIMITED TO THOSE HAVING A MAXIMUM BOOM
>LENGTH OF 10 FT."
>Then it repeats the 10 foot boom length limitation note on the next page.
There are a lot of Rohn BX type towers around these parts, and most of the
amateur towers have much longer booms on them than Rohn allows. One way to
[help] mitigate the twisting problem is to drop the rotor plate down a few
sections so the twist from the rotor is in the larger sections. In one
setup I saw, the antenna was mounted on 2" mast, then it adapted down to
around 1 - 1 1/4 pipe just past the original rotor plate, then the down to
the rotor at about 32'. This provided some torque relief as it allowed some
twisting in the mast pipe. I use the same technique on my 100' of Rohn 45.
The rotor is mounted at the guy set at 66'. I use about 25' of 2" mast pipe
and the array isn't a rotor croaker with this setup. I also use two mast
bearings which do not cause binding as the smaller long mast has more than
enough flex. There is a lot of stuff up there, including a 2 el 40, 15 ft
above the top of the tower. One guy set at 100' bearing and another on the
second bearing at 90 ft. This keeps it straight.
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