[TowerTalk] Optibeam OBLY14-5

[Jim Thomson]

From: JVarney <jvarn359 at gmail.com>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Optibeam OBLY14-5

<It's straightforward to show that a square boom is
<inferior to a round boom.

<A fair comparison is 2.25-in round boom vs. 2.0-in
<square boom because they have the same cross-sectional
<area. Bending is proportional to the moment of inertia
<of the cross section:

##  Its  actually  a lot worse.   A  2.25 inch OD  round boom has a 
circumference of  7.069 inches.   A  2.0 inch  square  boom has 
a   larger circumference of  8.0 inches. 

##  assuming same wall thickness, the  2 inch sq boom is
8 / 7.069 =   13.2%   HEAVIER...and more  expensive, if  cost 
per pound is the same.   Circumference of  the 2 inch sq boom is 
13.2 %  bigger.   

##  A  real  world  fair comparison would be  equal circumferences. 
In that case the round boom would have to be  8 / pi =  2.546 inches
in diameter.   They would both weigh..and cost the same.

##  Ok, now  compare the 2 inch sq boom vs the 2.546 inch round boom. 
Using your calcs, the round boom  now  solidly trounces the  square boom..hands down.   
Closest tubing readily obtainable is of course a 2.500 OD  boom. 

##  The  square boom will require the  use of one off ... square shaped...U bolts.
Flat plates could instead be welded directly the the square boom.  

##  I will stick with my round booms.   A buddy tried building full sized 40m eles,
using  rectangular, telescopic tubing,  6061-T6.   This was in the early 80s.   With the
long side of the rectangular tubing in the vert plane.    3 els on a  48 ft boom.  Boom made
from  4 inch OD  tubing.   Worked great..till the wind eventually destroyed  the  eles. 

Jim  VE7RF