[TowerTalk] Mast for C-3SS

[K7LXC@aol.com]

In a message dated 05/13/2000 1:04:37 PM Pacific Daylight Time, 
wd3q@erols.com writes:

> I've read all that was posted here re:masts, but am wondering if
>  there is an alternative to the large, expensive 4130 masts that
>  have been praised.
>  
>  My situation appears to be quite different than those detailed here: 
>  
>  6 foot roof quadpad with a C-3SS (27 lbs, 4.4 ft2 windload) and
>  a small (Austin) 1/4 wave 2m/70cm vertical.  The existing
>  aluminum 1-3/4" mast is 9 feet tall, of which ~5 feet is above
>  the thrust bearing, putting the antenna ~10 feet over the roof.
>  We've had 40 mph winds here this past winter/spring with no
>  impact on the antenna/tower/roof. I'd like to raise the yagi up
>  another 5 feet, putting it ~15 feet above the roof (which is
>  wood and shingles over an unfinished attic). 
>  
>  The obvious questions that comes to mind:  is it worth the
>  effort to raise the antenna another 5 feet; 

       First of all, the District of Columbia is only a 75 MPH windspeed 
zone, barely above the lowest rating, so no big deal wind-wise.
 
     My first question is why do you want it 5 feet higher? If it's to 
'improve' the takeoff angle by being 5 feet higher, I don't think you'd ever 
notice the difference in performance between the two.

      If it's because you've got loading problems with the C3-SS because it's 
too close to the roof, then it's not a bad idea. 

     To address the 4130 question, investment (and I do mean investment) in a 
4130 mast is for those stations who have big, aggessive stacks of antennas on 
the mast and/or you're in a really high windspeed zone. You've got none of 
those so you don't need the 4130.

      The Mast, Antenna and Rotator Calculator Program (MARC) shows 6,275 
in-lbs of bending moment. The recommended mast should have 25kpsi yield 
strength and .120 inch wall. (The 4130's yield strength is arround 115kpsi.)  
      

> can I use an aluminum mast (And if so, what type?); 

       A 2" mast of 6061-T6 has a yield strength of 35kpsi so that should be 
plenty. Other grades have different specs.

> is it safe to have another 5 feet coming out of the quadpod, or
>  will I have to re-do the roof mount (by adding guys?)? 

     Don't worry about guys. You load is small and the tower has lots of 
strength so as long as the feet have adequate anchoring - feel free to go 
nuts. 

Cheers,   Steve    K7LXC
Tower Tech 

--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm