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[TowerTalk] centermast

To: <>
Subject: [TowerTalk] centermast
From: (
Date: Sat, 14 Aug 1999 12:11:15 EDT
In a message dated 99-08-11 14:52:13 EDT, writes:

> I have a 100 Ft. SSV tower sitting in 16 Yards of concrete.  I have a (I
>  believe been awhile) 2 in. ISD gas pipe for a center mast with I think 3/8
>  wall thickness.  Anyway I have 5-6 ft. in the tower on a tailtwister
>  rotator.  The mast is in a thrust bearing so there isn't any weight on the
>  rotator.  at about a foot above the tower is a TH-6DXX Tribander,  about 3
>  Ft. above that is a pair of Rutland arrays one being horzonital, other
>  being vertical, above that is another pair of Rutland 2 meter same setup,
>  both of the arrays are on about a 12 ft. boom horzonital from each other.
>  On top are two verticals for 432 & 2.  
>  I am going to remove the top verticals and replace it with a diamond 6,2,70
>  cm antenna.
>  I also just bought a 6 meter beam M2 5 element which I want to mount at the
>  top of the mast.  Top of the mast is about 15 ft. above the tower.
>  Should I replace the mast with a larger one or can I just insert another
>  mast inside the present one and bolt through ?
>  I am concerned with bending the mast over in high winds and particulary now
>  that I would be mounting the 6 M beam at the top of the mast so that I can
>  get the proper spacing.

     First of all, Chippewa County is an 80 MPH windspeed zone.

     Second, unless you know the strength of the existing gas pipe, it 
probably isn't up to the job. Pipe is not rated for strength - just liquids - 
so by itself it's probably doomed.

      My MARC (Mast, Antenna and Rotator Calculator) Program shows a bending 
moment of over 28K in-lbs and recommends a mast with 55kpsi yield strength 
and a 1/4 inch wall thickness. (MARC is available from

      You need either a carbon alloy steel tube or seriously reinforce your 
existing one. I don't know what would be suitable - maybe we'll get some real 
engineering input.

Cheers,   Steve    K7LXC
Champion Radio Products

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