[TowerTalk] Aluminum tubing insert overlap...

[Jim Thomson]

From: Robert Chudek - K0RC <k0rc at citlink.net>
Subject: Re: [TowerTalk] Aluminum tubing insert overlap...

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.

%%%  A bare min of triple the diameter  for any  overlap..and preferably 
a  lot more, like 4-6 times the diam...and a snug fit, not a sloppy fit. 





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

%%%%   The arrl has free mast software.  K7NV and also DX eng
has ele mech design software.  You will get a real eye opener
after playing around with them for hrs on end. 


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. 

%%%%  yes, start at the ele tips and work inwards.  Ditto with
booms on yagi`s. 




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.

%%%%  My  F-12 3-el 40m yagi  .... called a  340N  uses els that are aprx
51-53feet long...and use linear loading.  The linear loading consists of   three sixteenths
solid AL rod.   The LL extends OUT  towards the tips.  It`s a superb design.  The tips are
.375 OD....and the very middle is  1.875 x .125 wall.  Aprx 24 lbs  per ele..and good for
over 100 mph. Minimal wind load too.....like  3.3 sq feet per ele. 

%%%  capacity hats..or T bars as F-12 calls them  also work superb.  F-12  does not put the
capacity hats out at the extreme  ends either.  On a 50 foot ele... the  T bars  would not have to 
be very long either..like  aprx   16 foot...tip to tip.  And aprx 2  thirds the way out on each side.
That`s like adding a 10m ele part way out on each side.   A   pair of 16 ft  T bars  can easily be 
made to be very light.   The simplest conversion would probably be to take the stock 70 ft long
telrex el..and lop  10 ft off each end....then  add the  16 ft T bars   at aprx  15-20 ft on either
side of the center feedpoint. 

later... Jim   VE7RF 




73 de Bob - K0RC in MN
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