Another Mechanical Engineering Question re: free-stand antenna mast

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Charles L. Cotton
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Another Mechanical Engineering Question re: free-stand antenna mast

Postby Charles L. Cotton » Thu Mar 23, 2017 3:51 pm

Quite some time back I asked a question about a trolley system for an antenna mast. I have another question about a free-standing antenna mast, but rather than mix apples and oranges, I'm starting this thread.

I'm putting up a repeater at a location other than my house and I want to use a free-standing mast going to about 70'. I like the telescoping mast KD6X designed and built here, but his goal was to have a fully retracted unit that he could easily move in and out of his yard. (He as concerned about his HOA.) I don't need that feature, but I do want to be able to drop the antenna down in bad weather to lessen the likelihood of a lightening strike.

Here's my question. I want to use three 24' joints of square aluminum (possibly steel) tubing in a telescoping pulley system similar to KD6X's five-section mast. I would prefer to use 3", 2 1/2" and 2" each 1/8" wall to limit the complexity of the unit. However, I don't know if there is an engineering taper ratio that must be maintained for the mast to be self-supporting (i.e. without guy wires). The mast will also have a tilt base, but that's not relevant to this issue. High lighting masts are a continuous taper from top to bottom and KD6X used five sections ranging from 3" to 1" for a mast that extends to 28'. Again, he did his that way to make it easier to move in and out of his garage, nevertheless he has five different sizes of aluminum for a 28' span where I'm proposed to have only three different sizes for a 70' span.

Okay mechanical engineers, please tell me I can do it will three joints with a one to two foot overlap!

Thanks,
Chas.
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Lynyrd
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Re: Another Mechanical Engineering Question re: free-stand antenna mast

Postby Lynyrd » Thu Mar 23, 2017 4:20 pm

I've never worked with aluminum on that scale, but I've worked a lot with steel tubing. You'd be surprised how limber a 24" joint is when the walls are that thin. If you went with steel you would definitely have to go to at leas 1/4" wall and then you would have about 700 lbs of weight to deal with. Friction is also going to be tough to overcome on a 22 foot slide. I noticed on your link they had some kind of polymer in between the joints. It is probably silicone based to reduce friction.
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