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[TowerTalk] Changing Beam Polarity

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Subject: [TowerTalk] Changing Beam Polarity
From: Eric Moore" < (Eric Moore)
Date: Tue, 24 Apr 2001 11:47:29 -0400
Hello TT'ers,

Has anyone ever attempted to use an old Alliance TV antenna rotator to
change the polarity of a 2 meter beam?

I have a Cushcraft 13B2 (13 elements, 15' boom) on top of my tower,
currently mounted horizontally.  I use FM 60%, and SSB 40% of time and
would like to use this rotator to change the polarity by rotating the beam
from horizontal to vertical and back again depending on what mode I want to
use.  Currently I just suffer poor FM performance so the Contesting on SSB
does not suffer.

Originally I thought I would just run the boom thru the  rotor, which would
be mounted on it's side, but the boom is just a tad to large to fit, and I
don't see an easy way to change the rotor to accommodate the larger

I am now planning to mount the rotor on it's side, on top of the mast, with
a flat plate welded to the mast pipe.  Then cut a 24" piece of 1" Rigid
Conduit (because it fits in the rotor) with a miter saw at 45 degrees,
welding the  pipe together, (avoiding breathing the fumes) to form a right
angle.  Then mount the beam perpendicular to the pipe and use the rotor to
turn the assembly. The beam would be about 16" away from the rotor, so the
leverage would be fairly small.  I was also planning to drill a hole in the
side of the rotor to allow moisture to escape and maybe wrap a piece of
rubber over the top to try and keep some water out.  I am 100% sure this
rotor was not intended to operate on it's side and it won't do the bearings
any good, but I have a few of these unit's and if I could get a few years
of service, I would be satisfied.

I figure the rotator display will let me know when the beam is horizontal,
indication north or vertical, indication east or west.

I have no specs of the Alliance rotator, but have heard of them being used
for elevation control before.  The Cushcraft 13B2 weighs about 10lbs and is
around 3 sq. ft of windload.

A few concerns I have are:

1.  Can the rotor turn the beam.

2.  Can the rotor hold the beam in the vertical position (which will mean
the "arm" is parallel to the ground).

3.  How long will the rotor last.

4.  Will the rotor, being so close to the beam wreck the antenna
performance for all use.

5.  Is there a better way to do this?

Thanks for any ideas you might have and as always, I enjoy reading items
posted here!


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