[amsat-bb] Re: Mast strength??

Edward Cole kl7uw at acsalaska.net
Thu Nov 22 13:15:30 PST 2007


Your dimensions of 2-inch diameter (I assume outside diameter) with 
0.25 inch wall thickness is closest matched to standard sched-80 pipe 
with 1.9 inch O.D. and 0.2 inch wall.  In steel pipe this weighs 3.36 
lbs per foot length.  Your 20-foot piece will weigh 67 lbs.  That's a 
lot of weight and wind load to be supported by only 2-feet inside the 
tower!  Add the weight of the rotator, crossboom and antennas and I 
think you will NOT keep it up in 70 mph winds.

Guessing that the element length of the 6m yagi is about 1/2 
wavelength (about ten feet), why don't you consider using a wider 
crossboom so that the satellite antennas clear the ends of the six 
meter yagi when you elevate them?  If the 2m antenna is mounted in 
the "X" configuration you need about 1/2 of 1.4x40 inches beyond the 
six meter antenna width to mount your 2m antenna.  Maybe make this 
about 1/2 of 1.5x40 = 15 inches.  A cross boom of 12.5 feet should 
do.  Since the satellite antenna only comes close the six meter 
antenna at extreme elevation angles spacing needs not be any more 
than that.  You can then lower the vertical mast requirement by half 
and end up with a lighter and more engineering stronger setup.

With this setup your roughly 18-foot antenna boom ends will drop to a 
foot or so below the level of the 6m yagi at elevation=90.  If you 
were to lower the most more then the satellite antennas would be 
correspondingly lower at 90-degrees (min mast spacing between 6m and 
satellite antennas is about 5-feet).  I think this would be a better 
design than having so much exposed unsupported mast with heavy top 
load.  I am putting up my satellite antennas on a single Rohn-25 
tower top section with about 8-feet of 2-inch aluminum mast and 
5-foot crossboom.  I will have the same rotor and UHF yagi, and the 
KLM-22C (very similar in size to M2CP22), plus a 33-inch dish.  All 
of this system survived several years use at my old QTH where is was 
supported by a 3-foot RS TV tripod tower.  You can see old photos of 
this on my webpage:

You should measure your antennas to be sure of the crossboom length 
and add enough so that antenna clamps have sufficient boom to clamp 
and antenna elements clear.  A metal crossboom will work (regardless 
of what you may hear).  If you worry about sagging of the crossboom 
this is easily strengthened by making a truss from rayon cord and a 
couple 1-inch pieces of tubing mounted either side of the elevation 
rotor.  Probably 2-foot long would suffice.  Use small turnbuckles on 
the cord to tighten .  This method has been used in the past to keep 
very long yagi booms from sagging.

Good Luck!
Ed - KL7UW

At 10:13 AM 11/22/2007, you wrote:
>Hi all,
>         I am installing 20' of Rohn 45 with a flat top from Normsfab
>and a Yaesu GS-065 thrust bearing.
>The lower end of the tower is already figured out but I'm not sure of what
>strength 2" mast I need.
>         All this short tower will be supporting is a 3-4 element 6M 
> homebrew beam
>just above the bearing on the tower,
>as well as, the elevation portion of the Yaesu G-5500 rotor with 8'
>fiberglass cross mast, and M2 2MCP22, and
>436CP42UG beams on each end of the cross boom.
>The 2MCP22 is 2.5 sq ft wind load.
>The 436CP42UG is 2.0 sq ft wind load.
>I'm not sure of the WL for the rotor and 6M beam.
>         The mast will have to be at least 18'-20' long to give 2-3 
> foot inside the
>tower to the AZ rotor and
>enough sticking out the top to give 1/4 wavelength (at 2M) spacing between
>the satellite antenna's
>and the 6M beam.
>The satellite antenna's are close to 20' long and mount from close to the
>center so I need at least
>10' (+1/4 wavelength) above the top of the 6M beam.
>         I'm not an engineering type so I thought I would ask on here. What
>and material would the mast need to be to support this.
>I am thinking T6061 aluminum, .25" thick wall, 2" outside diameter mast.
>I live in a 75mph wind zone.
>Any help would be much appreciated.
>A good friend will come and bail you out of jail,
>A TRUE friend will be sitting next to you saying.....
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Ed - KL7UW
  BP40IQ   50-MHz - 10-GHz   www.kl7uw.com
144-EME: FT-847, mgf-1801, 4x-xpol-20, 185w
DUBUS Magazine USA Rep dubususa at hotmail.com

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