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@contesting.com wrote:
Message: 8
Date: Mon, 05 Sep 2011 21:25:30 -0700
From: Kevin Normoyle<knormoyle@surfnetusa.com>
Subject: Re: [TowerTalk] Aluminum tubing insert overlap...
To: Tower Talk List<towertalk@contesting.com>
Message-ID:<4E65A0BA.9070808@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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