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Subject: MAST V.S. TOWER
From: (Bill W. Cox [AA4NU])
Date: Sun, 14 Jul 1996 16:39:43 -0500 (CDT)
 > Question for the panel:
 > Heres what I would like to do with my tower project:
 >      - Put up 80' 25g
 >      - Guy it as a 60' tower (guy at 30' and 60')
 >      - Leave the top 20' unguyed and use it as a mast
 > I want to phase two lightning bolt quads, one at 40' and the other
 > at 80'.  The quad is about 6 sq ft in wind surface area.
 > Can anyone think of reason why I would not want to use the top 20' 
 > of the tower as a mast.  Seems to me it would be much stronger than a 
 > conventional 2" chrome moly mast, plus I can climb it.
 > These will be the only two antenna's on the tower (a total of 12 square
 > feet).  I'd like to put another 40 meter yagi on the tower (another 6 to
 > 7 square feet) but I think that would be pushing it. If the 40 meter
 > yagi goes up it would be installed at the 60' level a few inches above
 > the guy point.
   The 'thing' (or one of them) to watch is the total bending movment
   on the 25-g ... Rohn states something like 5400 ft-lbs MAX for 25g ?
   So a 10 sq ft antenna in a 30psi (70.7mph?-this may be wrong mph)
   mounted 10' up above the guys would be ...
    10 (sq-ft)   x  30  (psi)   x  10'  =    3000 ft-pounds ... or within
                                             the Rohn limit ...
   Here's where (other than using a too light mast) many "amateurs" like
   ME get into trouble ....
   Figure a TH7 (say 10 sq-ft) at 80' on a Rohn 25g guyed at the 35' (max)
   spacing for 25g. So it's guyed at 35' and 70' ... FB ... so far ...
   So the TH7's "bending movement" is 3000 ft-pounds ... OK so far ...
   Then Joe (the) Amateur decides he wants to be stronger on 40m ... Up
   goes a 2L 40m CC on top of the TH7 ... and everyone said space it at 
   least 10' above the TH7 ... 
   Now what do we have sitting out there in the yard ? Is it SAFE ?
   Let's see .... TH7 @ 80' =  
   10 sq ft x  30 psi x 10 ft above guy point = 3000 ft-pounds ...
   AND we must now ADD the CC 40m to this ... CC 40-2-CD @ 90' =
   6.5 sq ft x 30 psi x 20 ft above the guy point = 3900 ft-pounds ...
   So the TH7's bending movement PLUS the CC 40m bending movement 
   now equals a total of 6900 ft-pounds ... well beyond the Rohn
   rating of 5400 ... Yes I know that Rohn has a 'safety factor'...
   but why risk it ?  I really like to sleep well at night and not
   have to put things up twice, or be sued IF/WHEN it fell, someone
   or something was hurt or damaged ... Do you want to risk it ?
   Add to the above the fact that most 'hams' don't use the correct
   mast for supporting the TH7/CC40-2 and the risks get higher ...
   As to using "tower for the mast", I seen several folks do that, seems 
   CQ mag had an article by a "2-er" showing how to do it a few years 
   back.... BUT again watch the bending movement !  Also the "weak spot" 
   is whatever is used to HOLD the UPPER "tower mast" in place in the rotor. 
   I've asked the locals here who are using "tower masts" the following: 
                   "Who welded it, and HOW STRONG IS IT ?"
   Answers were such as "He##, I welded it (which is enough to make you
   want to run AWAY from the tower, much less climb it...) and "Who the
   He$$ knows how strong it is, it "feels/looks" PLENTY strong for the job"
   I have yet to climb one or work on any of those towers using them for
   the fear of "the MTBF" occuring the DAY I am on it or near it !
   (MTBF=Mean Time Before Failure)
   The Rohn tower sections I trust, the "thing used to hold it in the rotor,
   I do NOT trust it ... Why should I ? Is it SAFE ? How do you KNOW ???
   BTW *** Always plan on the tower holding your "dream setup", as in WOW
   those 2L quads sure do play well ... Man-o-man (woman-o-woman - PCorrect)
   just think what a PAIR OF FOUR ELEMENTS 30' BOOM QUADS would do up
   on THAT TOWER ... see what I mean ? ... dream AND PLAN ahead NOW ...
   Perhaps you should INVEST in your future and go to 45g now ???
   IF you don't have a Rohn book, get one ... the Commercial one is about
   2" thick and full of useful info, the other one is about 1" and still
   very useful. Mine are several years old now, but were used to make sure
   all the towers here were to Rohn's spec. That why I don't have to worry
   about them coming down, and IF they might. I can also show how they were
   installed to "Rohn specs", and *IMPORTANT*, I used ROHN turnbuckles and
   harware whereever I could, even if it meant abit more $, just to make 
   sure there were no 'weak links' that could cause problems. Why build it
   to 'specs' and then try a save a few bucks by using CHEAP (read UNSAFE)
   Well there's my 2.01 cents worth ... Get some paper out and doing some
   figuring on "what is going on out on the dream tower" before you start
   digging the 1st hole. I would on a large piece of graph paper, plot the
   guy points and also study the 'turning radius - out and UP and DOWN' on 
   the quads to make sure the guys will clear it FB, again you might want  
   to turn the lower quad 'someday' so the guys need to be placed out far  
   enough to allow that option. 
   Your project sounds worth doing. Using quads does add to the planning
   due to the spreaders going UP and DOWN and around the tower. But it
   can be done safely if you plan well ... I used quads for many years hr.
   When I went to add a 40m yagi, the quads came down because I could NOT
   find a SAFE method to have the 40m yagi stacked ABOVE the quad ... Do
   the 'bending movement math' and you will see why now ...
   73 Bill AA4NU

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