I saw this setup once around 1970. K6RR had a LM-470 crankup with a 15
meter yagi as the lower one and a 10 meter delta quad as the upper one. As
I remember it the two booms were only a few feet apart, the thought being
that the delta quad increased the 'effective' distance.
The lower rotator was held in a cage hanging off the side, the mast rotated
by a chain running between them. I don't remember what rotators were used
(probably prop motors, but that should not matter). Somebody with some
steel and a good welder should be able to make you a mount pretty easily.
A thrust bearing could hold the larger mast vertical load. As long as the
two antennas don't interact too much it should work fine.
Ken K6MR
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of john@kk9a.com
Sent: Thursday, December 21, 2006 6:22 PM
To: TOWERTALK@contesting.com
Cc: xdavid@cis-broadband.com
Subject: [TowerTalk] Concentric Masts
I have never seen a set up like this, however I see no reason that it would
not work well. There may be some interaction with your tribander depending
on the antenna orientation, however the 10' spacing should help minimize
this. I'm assuming from your description that you have a bearing on the top
of the outer mast for the inner one. You don't say why a TIC ring would not
work. They fit my Rohn 65G and I think that TIC makes an even large model
so if it doesn't fit, you must have a huge commercial tower or someother
factor.
John KK9A
To: TowerTalk <towertalk@contesting.com>,CADXA <CADXA@LISTSERV.TEMPE.GOV>
Subject: [TowerTalk] Concentric Masts
From: David Gilbert <xdavid@cis-broadband.com>
Date: Wed, 20 Dec 2006 21:15:24 -0700
List-post: <mailto:towertalk@contesting.com>
I'm going to be limited to one freestanding tower at this QTH, but I'd like
to be able to operate two stations simultaneously (with appropriate
filters) for when friends visit. I expect to put a 40m yagi about 10 feet
above a decent tribander and would like to be able to point them
independently. TICs won't fit the tower, and side mounts won't give me
full azimuth. Has anyone ever tried to use concentric masts, such as a
longer 2 inch mast inside a shorter 3 inch mast, driven by two different
rotators? A sketch of the concept can be found at
www.stone-ridge-road.com/ab7e. It seems simple enough that I'm sure others
have tried it and I'm curious what issues they might have had with it (coax
loops getting tangled, etc). The rotator for the 3 inch mast might even fit
inside the tower. The idea doesn't look like it would cost much extra to
implement, aside from the second rotator and mast of course.
Thanks in advance for any comments.
73,
Dave AB7E
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