[TowerTalk] Boom Construction

[KI7WX@aol.com]

About a dozen people asked for a summary of boom construction techniques so 
here it is. I'm no expert on this, merely passing on what was learned.  
Perhaps I'll screw it up enough that some of our more famous homebrewing hams 
will pipe up on how they do it and where they get the parts.  I was somewhat 
surprised by the interest in this and also by what I perceive as difficulty 
in obtaining the necessary aluminium in small quantities.  I hope the latter 
is simply my not knowing where to look.

I found the following points to be helpful and hope it is useful to others as 
a starting point.  Corrections and enhancements are of course more than 
welcome. 

1) Do the proper mechanical engineering calculations to make sure you are 
building a strong enough boom for the wind/weight load.  Also useful to 
calculate windload and torque on the entire antenna for tower and rotor 
evaluation.  K7NV's Yagistress software is useful and Dave Leeson's book 
"Physical Design of Yagi Antennas" (ISBN 0-87259-381-9) contains many helpful 
details.  N4CW lent me his copy - thanks Bert!!  N1LO has also captured a lot 
of useful info on his website including discussions on windload calculations.

2) This is the method that makes intuitive sense to me but requires access to 
a metal lathe which isn't standard issue home shop equipment.  You'll need 
access to a machine shop with the proper tools for this type of construction.

Use extruded aluminium tubing in 3, 2.5 or 2 inch OD (outer diameter) with 
0.125 wall thickness for the outer boom sections.  This tubing is available 
in different lengths. Texas Towers has a few sizes but it doesn't appear they 
have exactly what is needed to make longer booms. W3LPL has noted a tubing 
supplier previously on the reflector, I couldn't find it quickly though.  
Suggestions of vendors for tubing would be helpful. 12 foot sections might be 
a good choice for making a 36' boom with two joints or a 24 foot boom with 
one joint. Splices (joints) are formed by taking a 36" piece of tubing with 
OD that is slightly larger than the ID (inner diameter) of the boom material 
and turning it down on a lathe so that you can use it as an inner splice 
segment.  Fit should be to 0.005".   The inner splice piece overlaps the 
joint by 18" on both sides. Rivits or 1/4 20 bolts can be used at the splice.

Note:  What I really need to add here, but I don't have the info yet, is 
exactly what type of tubing (grade, OD, wall thickness) is used to make the 
inner splice piece given 2", 2.5" or 3" outer tubing.

(This would make a great NCJ artical: "Booms for the masses" or "Where's the 
ka-BOOM"??  Apologies, it's late and I digress.)

2.1)  Truss the boom for added strength.  Methods are shown in Leeson's book. 
 Truss can be single cable or "T" truss with two legs per side.  One could 
also triple guy as described by Leeson.  Phillystrand and clamps works well, 
some folks use marine rope.

3)  Method 2

Boom joints can be made using inner tubing having slightly smaller OD than 
the ID of the boom sections.  Aluminium "shim" stock is used to take up the 
"extra" space and provide a tight joint.  Machine shops have a device for 
rolling shim stock into cylinders for this sort of application.  Join with 
rivits or bolts as before.

4) Use nested 0.058" wall tubing to form longer length booms.

Leeson, on page 11-62 describes fabrication of a 36' boom using nested pieces 
of 0.058" wall tubing with thicker 1.625" x 0.125" wall material being used 
to further strengthen the center and outer joints (TT lists everything but 
the 1.625 x 0.125 tubing needed for this boom).  Figure 11-22 shows 
construction details, if anyone really can't find this text I'll send them 
the page assuming neither Dave or the copyright holder (ARRL) objects.

5) Buy a boom from a commercial manufacturer. Seems like cheating, but 
Antenna Mart lists 3" booms and HyGain may still sell 2" x 0.12" wall swaged 
boom sections for use as replacement parts (I haven't checked).  One could 
also ask M2 or F12 to sell you booms from current designs.  For example M2 
lists antennas with 36 and 24 foot booms which could work for the 15M and 10M 
OWA yagis.  I've no idea if M2 or F12 would sell their booms without the 
elements but asking can't hurt too badly.  

5.1) Make friends with someone having a big pile of used antennas and antenna 
parts and put some of it back in the air where it belongs.

6) Other than the boom the rest is pretty easy.  Aluminium plate can be found 
at several sites and the other materials are available at McMaster Carr from 
their online catalog.  FWIW, making some back of the envelope calculations, 
it looks like the 36' 15M yagis would run about $600-650, and the 24' 10M 
would be slightly cheaper - excluding building time of course.

Again, I hope all that was helpful. 

Mark
KI7WX

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