OK,
Well, if I assume your fiberglass boom is about 1.5 inch diameter to max opening of the G5500, maybe you could find some alum tubing that would just slide over the fiberglass for a nice fit. One trick that is used is to slit the end of the tubing with a hack saw about 1/2 inch in and place a hose clamp over the end to clamp it tight to the smaller (fiverglass) tubing. I did this with a 70-MHz yagi to extend the elements to work on 6m.
Aluminum sched-80 tubing should be strong enough for your mast if you are going to shorten it a lot. As I mentioned my setup uses about 8-feet above the tower. BTW I was quoting from a standard chart for 1-1/2 inch shed-80 steel pipe when I suggested 1.9 inch O.D. But a shorter mast of aluminum will probably be OK, but still go with a thick wall (thus my recommendation of sched-80).
You are thinking 16-foot mast, 4-foot inside tower, 12-feet above tower. Assume 6m is attached at 1-foot above tower this leaves 11-feet spacing between antennas when all are at zero elevation. That is way more than needed. The satellite booms extend ~9 foot so when elevated at 90-deg. they will still be 2-feet above the 6m antenna. OK that is max height I would try.
But if you use 10-foot of mast, 4-foot inside tower, 6m antenna 1-foot above tower you will still have 5-feet separation that is 1/4 WL on 6m. You would need to extend the crossboom so that as the sat antennas elevate they will pass outside the 6m elements. At 90-deg. elevation your sat antenna booms would extend about 4-feet lower than the height of the 6m antenna. Interaction between them ought to be minimal. Also since the time that satellites spend above 80-deg. elevation is very small this is not that much of a compromise.
So this should give you some ideas, GL!
At 01:13 PM 11/22/2007, 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' soa 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 Iend 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 ======================================
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 ======================================