[TowerTalk] Aluminum tubing insert overlap...

Tue Sep 6 08:58:00 PDT 2011

With the goal of shortening the rebuilt antenna, you might consider a 
Moxon derivative from your Al farm.  Dave Leeson has published a mod to 
the Cushcraft 2L 40m (Visalia & Dayton 2004).

I'm halfway through a scratch build of the his "W6NL 40m Moxon Yagi 
120mi/h" (with K6KR, we are building two).  This is V3 of his 40m Moxon 
designs.  It is just under 49' tip to tip.

While his book recommends a minimum 2" overlap for sections  (page 4-25) 
this design has 3" overlaps for the elements.  Also, as I recommended 
earlier for this repair,  the bolting is cross bolts at 90 degrees  (I 
think there has been evolution in his designs since the book).  Per the 
W6NL design, we are using  8-32 and 10-32 spaced 1"  up to 1.25" od then 
1/4-20's spaced 2". The boom sections use 5/16" bolts and 6" overlaps 
and 2" bolt spacing.  I think cross bolting has the advantage of 
constraining the tube in two axis, eliminating any wiggle from the slip 
fit clearance.  For these thin tubes, a modest amount of bolt tension 
brings the tubes in contact in two planes.   The orthogonal forces 
reduce the distortion by having the bolts fairly close together.

Does your spreadsheet analysis show the maximum stress dimension and 
value for each section?  Yagi-Mech only gives the max value.  It seems 
to me the the "optimal" mechanical design would have the max stress (and 
at some value < yield stress) the same at the root of each section.  The 
other thing I've observed in playing with mechanical stress analysis 
with yagi-mech is that internal sleeves are "free" from a wind load/ice 
perspective.  That makes for a lot more design alternatives.

Grant KZ1W

On 9/6/2011 7:06 AM, Robert Chudek - K0RC wrote:
> 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
> ------------------------------------------------------------------------
>
> 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
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