Question for mechanical engineers - Not gun-related

[Charles L. Cotton]

I and a friend are trying to start an Amateur Radio Club for youth. I have two repeaters (both 70cm band) and 100' of Andrews Heliax. We have struck out trying to get a good antenna location, so I'm going to talk to my fellow Board members at the club about putting up a mast/antenna. I want to copy something I've seen done that involves using steel box tubing. (It comes in 24' lengths rather than 20' for pipe.) Here's a video showing the design. https://www.youtube.com/watch?v=cY-7VgZNypk&t=5s

I would like to get at least 70' AGL and I'm thinking of three sections of steel, each just small enough to fit inside the larger one. The pieces would either be welded or bolted, preferably bolted.

This mast would be sandwiched between two pieces of either box tubing or angle iron each 24' tall. The center mast would pivot at a point close to the top of the supporting angle iron or box tubing. The lower section would be weighted to make it easy to raise and lower the mast. (Since this would be a mast rather than a tower, it couldn't be climbed for antenna work, so it must pivot.) The concrete base would be sized close to the base required for an equivalent length tower. (That information is available from tower manufacturers.)

Is there a formula or reference I could use to make sure I used sufficient overlap with the telescoping sections? Just guessing, I would assume a 2' overlap would be sufficient. That would result in a 68' mast, plus about 10' of antenna, if the lower section is bolted to a mounting plate rather than being put into the concrete base. (24+22+22) If I were to put a section in the base, it would give additional height (24' less the portion in the concrete.) Also, I need to determine what wall thickness would be required.

Any input would be greatly appreciated.
Chas.

[puma guy]

I and a friend are trying to start an Amateur Radio Club for youth. I have two repeaters (both 70cm band) and 100' of Andrews Heliax. We have struck out trying to get a good antenna location, so I'm going to talk to my fellow Board members at the club about putting up a mast/antenna. I want to copy something I've seen done that involves using steel box tubing. (It comes in 24' lengths rather than 20' for pipe.) Here's a video showing the design. https://www.youtube.com/watch?v=cY-7VgZNypk&t=5s

I would like to get at least 70' AGL and I'm thinking of three sections of steel, each just small enough to fit inside the larger one. The pieces would either be welded or bolted, preferably bolted.

This mast would be sandwiched between two pieces of either box tubing or angle iron each 24' tall. The center mast would pivot at a point close to the top of the supporting angle iron or box tubing. The lower section would be weighted to make it easy to raise and lower the mast. (Since this would be a mast rather than a tower, it couldn't be climbed for antenna work, so it must pivot.) The concrete base would be sized close to the base required for an equivalent length tower. (That information is available from tower manufacturers.)

Is there a formula or reference I could use to make sure I used sufficient overlap with the telescoping sections? Just guessing, I would assume a 2' overlap would be sufficient. That would result in a 68' mast, plus about 10' of antenna, if the lower section is bolted to a mounting plate rather than being put into the concrete base. (24+22+22) If I were to put a section in the base, it would give additional height (24' less the portion in the concrete.) Also, I need to determine what wall thickness would be required.

Any input would be greatly appreciated.
Chas.

Having built some tripods with pipe and tubing for feeders it can get frustrating trying to determine what size or gauge will telescope with another. One thing I learned after the fact that you can order tubing with the flashing removed. ie. the weld bead inside that prevents seemingly properly sized tubing from being inserted into the larger size. The tubing used to make receiver hitches has the flashing removed. I assume it's done with a broach of some sort. I don't know if this company would have what you need but we've purchased from them; maybe they can help you find what you need - Standard Steel Supply in Texas City.

[Ipconfig]

Charles as a long time ham and installer of wireless internet, I would suggest investing in a Rohn 20g or 25g tower.

[Texas_Blaze]

Charles,

The failure mode I see is that of wind gusts imposing a drag force on the mast causing a moment at the base on the structure. This would cause a deflection from the base (small) to the tip (large). The key properties to consider in the analysis is the moment of inertia (of the tubing) and modules of elasticity of the steel tubing. The problem can be considered as a cantilever beam. The moment of inertia changes at the overlap sections. The drag force can be determined from wind speed and drag coefficients for the box tubing. Once you know the imposed force you can calculate the top deflection and moment at the joint were the tubing is telescoping. Then you can calculate the stress developed and compare to yield strength of material and establish a safety factor with which you are comfortable.

You can find these equations in an undergraduate ME textbook on solid mechanics and drag coefficients in a fluid mechanics text book... or online

[warnmar10]

A 60' mast stands in my mom's front yard over 25 years after my dad passed and about 40 years after he had it built and installed. It utilizes round tubing, the sections telescope and it breaks about 8 feet above the ground on a single pivot in double shear. His design somewhat mimics a steel street lamp post only taller. I'll be there for Thanksgiving if you're interested in pictures.