[Top] [All Lists]

[TowerTalk] Changing Beam Polarity

To: <>
Subject: [TowerTalk] Changing Beam Polarity
From: (
Date: Sat, 28 Apr 2001 08:39:06 EDT
ERIC:  The main problem here is when you rotate the boom for vertical 
polarization the mast will be exicted with RFand upset the vertical pattern 
and F/B.  You have to mount the entire beam on one side of the mast and with 
good F/B if you rotate the boom.  Make it a long boom and add another antenna 
pointing the other way on the other side of the mast for balance.  k7gco

In a message dated 4/24/01 8:47:47 AM Pacific Daylight Time, writes:
<< 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!

List Sponsor: - Trylon self-supporting towers,
safety equipment, rigging gear, LOOS tension guages & more!

FAQ on WWW:     
Administrative requests:

<Prev in Thread] Current Thread [Next in Thread>