[TowerTalk] Aluminum tubing insert overlap...

Tue Sep 6 10:13:07 PDT 2011

Grant,

I was aware of the Moxon antenna design but it never entered my mind as 
an option until you mentioned it!

I see the main horizontal element(s) are about 50 feet long and the 
folded tips are about 8 feet. I don't know any reason why the ends 
couldn't hang down in a single element design (other than some pattern 
distortion, I suppose).

The existing antenna used some 8-32 and 10-32 stainless bolts, although 
there was only one bolt per joint. I did see the effect from years of 
vibration, where the inner tube had "slots" cut from the movement. 
Bolting in two directions should solve that issue, as you pointed out.

I attached a copy of the Excel workbook (it won't come through the 
reflector) so you can compare it to Yagi-Mech and let me know what you 
think. I did notice there was an issue (with the WK1 to XLS conversion) 
in cells B41 ~ B49. I do not know what the descriptions should be in 
that area. Everything else seems to be working, although I have not 
hand-calculated any of the formulas. The current sizes and lengths are 
not final... they're just experimental values while I gain some 
experience with this tool.

And Roger... my brother-in-law turned a replacement insert for me over 
the weekend so I'm back to "playing with aluminum" this week!

73 de Bob - K0RC in MN
------------------------------------------------------------------------

On 9/6/2011 10:58 AM, Grant Saviers wrote:
> 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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