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Re: [TowerTalk] Recommendation for mast on bottom mount rotator

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Subject: Re: [TowerTalk] Recommendation for mast on bottom mount rotator
From: "K8RI on TowerTalk" <>
Date: Tue, 19 Sep 2006 14:15:45 -0400
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----- Original Message ----- 
From: <>
To: <>; <>
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,
> 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 
>> one
> long torsion bar.
> That and with a thrust  bearing at the top the tower will end up 
> supporting
> 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 
> cancel
> 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 (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 
> 10x
> 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
> unfornately.
> Cheers,
> Steve    K7LXC
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