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Masts and stacking considerations

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Subject: Masts and stacking considerations
From: (
Date: Fri, 14 Jun 1996 11:58:36 -0400
In a message dated 96-06-14 10:19:15 EDT, you write:
>I've seen a lot of talk concerning the 3 inch Rohn thrust bearing .. does
>this mean many of you are actually using larger than 2 inch masts out
>there?  Do most beam's boom-to-mast brackets and commonly used
>rotators handle up to 3 inch diameter masts? 

Hi, Bry --

     The use of a real 3 inch mast is not very common.  The confusion may be
from the Rohn part number TB-3 is for their 2 inch thrust bearing.  The 3
inch TB is part TB-4.

     Boom-to-mast bracketery needs to be increased to 3 inch but that's
pretty simple.  
I guess the question you have to ask yourself is "if i need a 3 inch mast, is
the antenna windload excessive?".  Another complication is that many rotators
(i.e. Hy-Gain) are designed for 2 inch masts only and will turn eccentrically
if any other size is used.  
>I'm about ready to stack a TH6DXX  8-10 feet above a 3 ele 40m beam,
>and the larger diameter mast would eliminate the expen$ive 115Kpsi alloy
>needed for a 2-inch dia mast in 100MPH winds 

    If you go to a rotator with a clamshell type of mast clamp, a 3 inch is
no problem because it'll self-center.  Correct me if I'm wrong, but going
from 2 inch to 3 inch isn't necessarily going to save you any money because
it doesn't significantly increase the strength of the mast.  Besides, the
forces remain the same.
>ELNEC indicated that the tribander's pattern will be adversely affected by
>the 40M beam nearby, so I twisted the beams 90 deg. relative  .. muuuch
>better, almost as if the 40M beam disappeared.  I haven't seen any
>installations done this way.  Am I missing something; other than having to
>contend with the 90 deg. offset when pointing the beams?

     For some reason, the Cushcraft 402CD doesn't cause the pattern problem
that you describe where bigger antennas do.  Many hams with this
configuration do turn them 90 degrees (K7RI and WY7I locally for example).

73,  Steve  K7LXC

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