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
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