----- Original Message -----
To: <email@example.com>; <K8RI-on-TowerTalk@tm.net>
Sent: Monday, September 18, 2006 10:44 PM
Subject: Re: [TowerTalk] Recommendation for mast on bottom mount rotator
> In a message dated 9/16/2006 9:02:29 A.M. Pacific Daylight Time,
> firstname.lastname@example.org writes:
>>> I'm going to move my rotator to the bottom of my 80 foot tower and
>> would like recommendations for the mast. I will support it at the top
>> with a thrust bearing, and at least one, possibly two, non-thrust
>> bearings part way down to prevent side to side movement.
>> My question is what would be a good mast material? It would only have
>> to support it's own weight and I'm wondering if 2 inch water pipe
>> Remember this mast has to do basically what the top mast does. It is
> long torsion bar.
> That and with a thrust bearing at the top the tower will end up
> all the extra weight of the masting.
> Umm, I'll go with the long torsion bar. BTW, mounting antennas
> alternately up a mast goes a long ways to minimizing torque since they'll
> each other out. (Article in QEX or QST by Dick Weber, K5IU, PE.)
That is one of those, "it all depends" on many factors including the wind at
various heights at any given moment.
There is no gurantee the torque from those antennas will cancel out.
Although multiple antennas would tend to have a dampening effect it is
possible for the forces to add. Also when the rotator starts and stops you
are looking at the total angular momentum or inertial of all added together.
Just watch out for resonances. There is a lot of flex in an 80 foot mast
even if there is no play in the couplings. The easier you make it on the
rotator the more likey the harder it becomes on the antenna until you hit
resonance. Then the old antenna elements really begin to whip back and
Everything is a set of compromises, or tradeoffs.
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com (Use return address from home page)
> But the mast sticking out of the top of the tower is naked and is
> subject to sometimes large forces. If an antenna one foot above the top
> of the
> tower has x force on the mast, then the antenna up ten feet will exert
> bending moment on the mast; forces are becoming significant. Your
> captured mast
> inside the tower doesn't have anything like that so you can use more
> ordinary materials for this masting.
> I'm not an engineer so can't give you any calcs or specifics
> Steve K7LXC
> TOWER TECH
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