[amsat-bb] Re: Mast strength??
stargatesg1 at verizon.net
Thu Nov 22 14:13:58 PST 2007
That's an idea I had not thought of. Extend the satellite antenna's out to
the point where
they are past the ends of the 6M antenna. Hmmmm, I already have a fiberglass
crossboom that came
with the trade I made a while back and was planning on using it, but your
idea gives me some more
thinking to do. Thanks
The satellite antenna's turning radius is only about 10' so a mast 16'
would give me 4'
inside the tower and still allow for 1/4 wavelength of distance (at 145mhz)
between the 6M antenna
and satellite antenna's at 90 degrees vertical.
Your idea would certainly be stronger and may be the way I end up going,
but I still wonder
what grade of tubing I would need for that length and wind load.
Pipe is a no-no, from what I have read since it is not really made to
support lateral loads,
and tubing is rated in lateral load or bending moment. Also pipe is measured
in inside dimensions
rather than outside dimensions. A 2" pipe is not 2" outside diameter.
I'm still not sure what to use. I would think aluminum of the correct alloy
and thickness would suffice but
I would hate to see it laying on it's side one day:-(
Your idea may be cheaper in the long run due to the strength it would have.
Any other ideas from anyone?
A good friend will come and bail you out of jail,
A TRUE friend will be sitting next to you saying.....
"DAMN THAT WAS FUN"
> -----Original Message-----
> From: Edward Cole [mailto:kl7uw at acsalaska.net]
> Sent: Thursday, November 22, 2007 4:16 PM
> To: stargatesg1 at verizon.net
> Cc: amsat-bb at amsat.org
> Subject: Re: [amsat-bb] Mast strength??
> 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.....
> >"DAMN THAT WAS FUN"
> >Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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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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