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Author Topic: What works best for the Mast?  (Read 732 times)
NZ4Z
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« on: October 15, 2007, 08:39:45 PM »

I have a Rohn 25G tower going up and was needed advice on what type of mast pipe to use, and what length to consider? Was also wondering what to use for a stand off to have an inverted V?

Thanks in advance

73


Steve Parker
NZ4Z

www.nz4z.myqsx.net
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KA5N
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« Reply #1 on: October 16, 2007, 03:02:54 AM »

You can buy steel or aluminum masts.  There are pros and cons for both types.  What you select depends on what antenna(s) you are going to use (weight, wind loading, etc.).  Water pipe and fence poles are not suitable masts for large arrays.  Texas Topwers' catalog shows a variety of lengths and wall thicknesses for their 2 inch dia. masts.  If you tell us what you are going to put on the tower, you will get better answers to your question.
Allen
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WA3SKN
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« Reply #2 on: October 16, 2007, 04:09:54 AM »

And you can just use a piece of 2x4 board for a standoff for your inverted Vee!

-Mike.
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KL0S
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« Reply #3 on: October 16, 2007, 05:09:35 AM »

Or for VERY light weight applications a SHORT section of PVC electrical conduit works well.  I recently used a 4 foot section on top of a aluminum tower as a support for a Weather Satellite antenna.  And it's cheap too....about $3 for a ten foot section.  Cuts easily with a hack saw.

Dino KL0S/4
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NZ4Z
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« Reply #4 on: October 16, 2007, 05:14:38 AM »

Sorry, I should have listed the antennas......... At this time I have a 3 element SteppIR, and a 10 element Cushcraft for 2M going on the mast.

Thanks again

73


Steve Parker
NZ4Z

www.nz4z.myqsx.net
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KG4RUL
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« Reply #5 on: October 16, 2007, 06:56:56 AM »

Here is what we did at the Berkeley County South Carolina  EOC to mount our antennas.  The G5RV mid point is suspended from the halyard on the pipe sticking out to the right of the tower.  The other ends of the G5RV are supported on one of the halyards of a flagpole and a pulley and halyard installed on a light pole, respectively.

http://home.comcast.net/~dzabawa/P1000324.JPG

Pictures of the whole tower erection process are here

http://home.comcast.net/~dzabawa/AntennaParty.htm

Details of our Radio Room are here:

http://home.comcast.net/~dzabawa/RadioRoom.htm
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N1LO
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« Reply #6 on: October 16, 2007, 07:34:40 AM »

Hello Steve,
 That's a (wind) loaded question, and there are many solutions.

 I have a similar setup: a Force 12 C3E @ 2' above the top plate and a 13 el Cushcraft 2m beam @ 6' above the top plate. They have similar wind loading to your antennas.

 At those same heights, 6061-T6 structural aluminum tubing, 2" OD, 1/4" wall thickness, 12' overall length, 4' in the tower, 8' above, will handle the job through 100 mph.

Anything over 6' is not UPS shippable. Look in the yellow pages for a local metal supplier or machine shop that can order it for you so you don't have to pay freight.

--...MARK_N1LO...--
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N1LO
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« Reply #7 on: October 16, 2007, 07:37:45 AM »

Oh, and for the side arm, a 4-5' length of chain link fence top rail works nicely. It's plenty stiff and hot dip galvanized.

--...MARK_N1LO...--
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K3GM
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« Reply #8 on: October 16, 2007, 11:44:56 AM »

I use an aluminum mast on my tower that's made from a 22' length of 6062-T6 alloy, Schedule 80 tubing. That's an extremely thick wall. I have the 3L SteppIR about 3 feet above the bearing with a large 2 meter yagi mounted 5' above that. No problems so far, but you won't find 6062-T6, Sked 80 tubing just anywhere. I got mine at a large materials supplier.
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W3JJH
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« Reply #9 on: October 16, 2007, 05:19:59 PM »

If you figure 6.1 ft^2 for the SteppIR; 1.2 ft^2 for the 2 m beam; and around 2.5 ft^2 for the rotor, mast, and effective load of the inverted-vee and its bracket; then you'll have just under 10 ft^2 of wind load at the top of the tower.  I'd keep the mast short so that the wind load in not amplifed by the lever arm up to the 2 m beam doesn't get out of hand.

The Rohn design data allows for loads in that range with a *properly guyed* 60 ft tower with wind speeds up to 70 mi/h.  I suggest keeping the actual tower to 50 ft and using a high-strength mast material.

Let's say that you'll have the hf beam about 2 ft above the top of the tower, the 2 m beam a half-wavelength (or about 3.5 ft) above it.  In a 70 mi/h wind the bending moment at the top of the tower will be around 490 ft-lb.  The yield strength of the mast material should be around twice that to allow safety margin.  2 in OD (0.12 wall) aluminum should be just adequate for such an application, but I would probably use 1026 steel.  Common water pipe would not be safe.
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W8JI
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« Reply #10 on: October 16, 2007, 07:14:03 PM »

I hope the tower is new or really good shape and actually 25G.

There is a good bit of misrepresentation about Rohn 25G.

I see people selling 20G all the time as 25G because they look very similar!!!

73 Tom

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