[TowerTalk] Aluminum tubing insert overlap...

[Robert Chudek - K0RC]

Thanks for all the feedback so far...

After I posted my question I dug around in my book library and found 
"Physical Design of Yagi Antennas" by David B. Leeson W6QHS. That was 
ARRL publication #3819 in 1992 but is now out of print.

There are 11 Chapters which cover many aspects of mechanical design, 
including improvements for survivability of 10, 15, and 20 meter yagis 
and the popular Cushcraft 2 element 40m yagi. I have not found an answer 
to my specific "overlap" question, although I haven't read the book 
cover-to-cover yet. It might be in there somewhere, yet to be found.

The book contains all the math needed to make wind survivability 
calculations for homebrew elements and masts. A quick Google search 
turned up a Lotus WK1 spreadsheet created by the author at:  
http://www.realhamradio.com/Download.htm

Excel 2003 would not import the WK1 format "for security reasons" 
implemented by Microsoft. I solved that problem and now have a working 
tool that can be used interactively to examine different tubing sizes, 
wall thickness, and lengths. It models tubing of any metal type and up 
to 10 taper segments per element half. It even allows you to add a 
radial ice component. Unfortunately, this spreadsheet does not address 
tubing overlap either.

Experimenting with this tool last night was a real eye opener... It 
reveals what you thought would be stronger element (extending the larger 
tubes) is in reality a potential failure point! Longer "whippy ends" 
help create survivability. I also learned that you model an element 
starting at the tip and working toward the boom with the larger 
sections. This mechanical modeling is separate from designing for a RF 
characteristic and resonance. The spreadsheet has a section on 
electrical characteristics although I have not looked closely at that 
section yet.

As suggested, I am also reviewing manufactured antennas. The shortest 
overlap I have found (so far) is 9 inches used on the boom of the 
Cushcraft 40-2CD. But I have just started this investigation. The 
original Telrex element used a 3 inch aluminum step-sleeve that took the 
2" OD tubing down to the 1.5" OD tubing. This was NOT the failure point. 
It was the 2" OD tube that crushed where it entered the 2.5" OD tube. 
Yes, the 2.5" tube is close to a 0.250" wall thickness and is both 
slotted and bolted at that joint. The overlap was 9 inches.

My original plan was to investigate shortening this full sized dipole 
using top hats or linear loading. My goal was to reduce the windload and 
torque exerted on the tower. The failed antenna was over 70 feet long 
and presented a big lever at the top of the tower. My target length was 
50 feet +/- 10% without using loading coils.

73 de Bob - K0RC in MN
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On 9/6/2011 6:30 AM, towertalk-request at contesting.com wrote:
> Message: 8
> Date: Mon, 05 Sep 2011 21:25:30 -0700
> From: Kevin Normoyle<knormoyle at surfnetusa.com>
> Subject: Re: [TowerTalk] Aluminum tubing insert overlap...
> To: Tower Talk List<towertalk at contesting.com>
> Message-ID:<4E65A0BA.9070808 at surfnetusa.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> interesting question. No one really addressed how to come up with the length of
> splice.
>
> If this was a normal bolted joint, you might say something like 8-10x bolt
> diameter for spacing and end spacing.
> But this is not a normal joint. The through bolts really don't do anything
> except keep things from sliding apart.
>
> If it's a riveted joint, then it's different..it would be more like a compressed
> joint.
>
> I think what happens, no matter how closely spaced, for short joint overlap, is
> that the tubing deforms a little under load, so that you get a lever effect,
> where the fulcrum is the bottom area near the lip on the big tube, and one end
> of the lever is the top end of the smaller tube (pressing against the inside of
> the big tube)
>
> I've noticed that the aluminum corrosion seems to match these areas, when taking
> tubes apart. (in through-bolted joints)
>
> So, if you have a 6" overlap, and an overall half element that goes out 20-33'
> feet, you can imagine there's quite a leverage multiplier between that fulcrum
> and the other lever point.
>
> So you can imagine how a small overlap, like 3" could end up with distorted tubing.
>
> I think with enough overlap, this lever effect gets spread over more area, which
> is better.
>
> Now bigger tubing that takes bigger loads, is also wider in diameter and
> thicker. So what's interesting, is that for all our joints, the length need may
> be similar.
>
> I think emprical data is probably your best bet..looking at what similar
> antennas do for overlap, at similar cases.
>
> The looser the joint, the worse it is. But you will have some looseness. the
> outer and inner diameters don't match.
>
> Actually you can probably tell what's good by putting the tubes together at a
> splice length, and holding it out horizontally and rocking it up and down. The
> overlap is "about right" when it doesn't feel like it rocks up and down so much.
> Shouldn't need more than 12", shouldn't be less than 4" (you start getting not
> enough material around the bolt holes then.)
>
> Even with loose fitting joints, the rocking decreases with increased splice
> length. So that's the model I would use: minimize rocking/oscillation to some
> amount.
>
> -kevin
> ad6z