[TowerTalk] Coupling 3" boom sections

[NPAlex at aol.com]

General comment-
In the past few years I have had the need to put together hundreds of 3"  
booms, and joining two or more sections.   W6WRT's problem is more  challenging 
if the boom wall thickness is 1/8" as suggested in one of the  responses he 
received. Another readily available wall is 0.083" tubing  often stocked in 24ft 
lengths by many aluminum distributors.  The  method of coupling was the use of 
about 36" of 2-7/8" dia. x 0.083  wall tubing, that was slotted along its 
axis.  The width of  this slot is not too critical but should be at least 3 to 4 
times the wall  thickness of the coupling piece.  
 
One of the other responders suggested this same technique, however I think  
it would be very difficult to implement using the material  suggested.  The 
first challenge would be using the boom material for  the insert, which requires 
a significant reduction in diameter, and more  distortion of the shape.  Next 
the insertion length (half of 6ft)  would be very difficult especially if the 
wall thickness was also  1/8".   Any boom length longer then about 15 ft 
should be  supported by a bridge truss, and booms of 30 or more feet using 3 inch 
material  would need to be supported.  Given this fact, the role of the coupler 
is  primarily alignment of two halves and preventing them rotating  
independently.  A coupler section 3 feet long (18" insertion) is adequate  to do that 
job.  The slot should be positioned "up" and a series (about  4) of stainless 
steel sheet metal screws positioned on either side of the  slot will be 
adequate to prevent collapse of the slotted section.  For  added "anti-torque" 
control another row of screws can be applied on a line  opposite of the slot.  Be 
sure to liberally lubricate the insert and the  inside of the boom before 
attempting to insert.  Also be sure all burrs and  sharp edges/corners on the insert 
have been removed. 
 
Remember the boom to mast plate and associated clamps also contribute to  
preventing the boom halves from twisting.
 
Although the describe method may not be elegant, it has proven to be a  
successful technique in a commercial boom application for many years.
 
The other solution is locate some 2-1/2" sched. 40 pipe that is nominally  
2.875 OD, have about 0.010 removed (lathe job) which should be a nice slip fit  
into 3" x 0.083 wall boom.  If your boom material is 1/8" wall you will  have 
to find sched. 80 and take about 0.065 cut to fit the nominal 2-3/4" ID of  
the heavier wall boom.    Unless you are boring holes in the boom  for element 
attachment (ala Telrex) I think the 0.083 is adequate wall.
 
Regards,
Norm W4QN
 
PS, 2-7/8" sched 5 pipe is available from Tennadyne/Cubex in 12 ft  
lengths.(for slotted coupler)
====================================================
>I'm considering  building a long yagi for 20 meters and I see the ARRL
>antenna book  recommends using 3" aluminum tubing for the boom. One
>thing I don't find  is how to connect the sections together. There
>doesn't appear to be a  standard tubing size which would slip either
>inside or outside the 3"  tubing to allow clamping or riveting. Or
>maybe I just missed  it.
>
>Any suggestions?
>
>Bill, W6WRT