antenna element with "doubled up" nested aluminum rods for added stiffness

Question

I'm building a full-size quarter-wave 20m vertical antenna (approx. 18 feet long, plus a trio of Hustler mobile resonators at the top to add 30, 40, and 80 meters). To make the top 12-14 feet stiff enough to survive random 40-50mph (non-sustained) wind gusts from daily summer South Florida thunderstorms, I'm planning to "double up" the aluminum tubing for the portion for the top 12 feet (basically, making the element from three 6-foot 3/4" diameter tubes, joined together and strengthened on the inside with snugly-nested 5/8" tubes offset by 50% from the outer tubes).

1. Should the "inside" tubes firmly butt up against each other, or should I try to separate them slightly to ensure that the gap between the inside tubes is always at least as great as any gap between the inside and outside tube?

2. Is it OK to drill a hole through (and perpendicular to the direction of) both tubes & secure them with a stainless steel nut and bolt that passes through both sides of both tubes perpendicular to their orieintation? Or would it be better to use a pair of shorter screws that are just long enough to penetrate the outer tube and dig into the inner one, without actually bridging all the way across?

I know that with DC, providing multiple parallel paths for current to flow is generally a good thing... but from what I've read, RF is "weird", and providing multiple parallel paths for RF to flow along the same antenna isn't necessarily desirable. But, I've also gotten the impression that with HF, there are literally so many things that can unpredictably go wrong to screw things up, it's almost futile to even bother worrying about the effect of things like one long perpendicular bolt penetrating from end to end vs two.

Answer 1

As you are discovering, antennas are primarily mechanical beasts.

I recommend the book "Physical Design of Yagi Antennas" by David B Leeson - it has lots of relevant detail about element strength, tapering, connection, mounting.

A few thoughts that might help with your design:

Adding metal to make a pole stiffer and more resistant to wind, is much better added as extra diameter, than extra wall thickness. And a thicker wall is much better than a second tube free sliding inside.

Finally, the part you need to strengthen most is the bottom, which has to withstand a larger total wind force, and a longer lever arm. Thinner at the top is also good to reduce the wind load there, but the hustler resonators provide a minimum wind load so very thin won't help.

So in order of preference in my opinion:

1. 5/8" is quite thin for a self-supporting 18' aluminium mast. Perhaps in steel... Go at least 1.5", 2" is better if you don't have other constraints like visual appearance. Wall thickness for 2" need only be 1/16", if you strengthen it carefully at the bottom, 1/8 is better...

2. Use a larger diameter tube rather than a thicker wall tube, especially for the lower half of the vertical where the stress is greater.

3. Use a thicker wall tube, rather than two tubes that fit into each other. The inner tube adds very little strength.

4. Make joints overlap by about 3 or 4 diameters, if they fit snugly, more is not much better.

5. Bolt holes through the tube make it weaker, and drilling through all four walls in the same place is the worst thing. If the tubes fit tightly, one bolt in the centre of the overlap portion is probably fine. Bolts through clearance holes in both tubes do not stop the concentric tubes sliding against each other by the tiny amount they move when bending.

At HF don't worry too much about RF effects. Make sure there is some tight metal-to-metal contact at the point where the outside diameter changes, a bolt or a hose clamp and some slots cut in the outer. Tubes inside are invisible to the RF so don't make any difference. The only thing which will make a small difference is if you form a deep coaxial well where the inner is well separated from the outer for some distance, then connected.

Answer 2

You may find it very difficult to "telescope" aluminum tubing for more than a few feet. In my experience, it's critical to lubricate the interface between the inner and outer tubing with a loaded grease commonly known as NoAlOx, as in "No Aluminum Oxidation." Not only will it ease the initial telescoping, but it will prevent seizing over time and allow you to pull the assembly apart in the future.

It seems probable that you will build an 18-ft radiator from 6-ft lengths of tubing. A key to strength is to be sure that the joints of the inner and outer sections are offset. This may be most easily done by positioning 3-ft lengths at the top and bottom of the inner section, with 6-ft lengths between. Making the outer section from 6-ft lengths ensures that the outer section joints are bridged by continuous inner sections.

Long overlaps create ample opportunity for low-stress connections. Without specialized equipment and/or ability, it could be challenging to through-drill inner and outer sections of tubing with sufficient accuracy to make parts interchangeable; i.e., the antenna could only go together one way. Drilling and screwing can create stresses, especially in the windy conditions you anticipate, that could fracture the tubing over time. Sawing slots into the ends of outer tubing sections and compressing the slotted sections over the inner sections with hose clamps is a long-lasting way to create strong, reliable joints that can assembled with great flexibility.