Joe,
You are correct. You defeat the purpose behind using fiberglass by routing the cables in that manner. Actually you can use a metal cross boom if you follow some simple guidelines. Check out the following:
http://www.g6lvb.com/fibermetalboom.htm
73, Joe kk0sd
-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Joe Sent: Thursday, November 22, 2007 6:09 PM To: stargatesg1@verizon.net Cc: amsat-bb@amsat.org Subject: [amsat-bb] Re: Mast strength??
This gave me a remembrance to a thought of these fiberglass arms.
Is it really worthwhile in the first place, if you are running the feeds along it anyway?
if you keep the feed line away then i could understand.
But if you run the feed lines along the antenna boom,, then across the crossarm, and finally down the mast, with this metal running along the crossarm (feed line) are you not defeating the whole purpose of using the non conducting section?
Joe
Stargate wrote:
Ed, 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?
Thanks, RoD KD0XX
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@acsalaska.net] Sent: Thursday, November 22, 2007 4:16 PM To: stargatesg1@verizon.net Cc: amsat-bb@amsat.org Subject: Re: [amsat-bb] Mast strength??
Rod,
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: http://www.kl7uw.com/sat.htm
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 Normsfaband 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 6Mhomebrew 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 2-3foot 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 onhere. What
thickness 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.
Thanks, 73's RoD KD0XX
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"
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73, 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@hotmail.com ======================================
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_______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb