[TowerTalk] 2 inch Chrome Moly mast needed

[Jim Thomson]

Date: Thu, 7 Apr 2016 22:49:27 -0500
From: Fred Sanborn <cc-6569 at comcast.net>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] 2 inch Chrome Moly mast needed

First of all many thanks to all who responded here on the reflector and 
also in private.  Several options were offered.

Jim VE7RF has it right on for what happened here at this QTH. Besides 
being hit by lightning, I also had a wind gust that bent the mast at 
about 45 degrees.  I hate to admit what I was using for mast at that 
time - 2 inch sch 80 pipe.  I knew at the time it was probably not a 
good idea and learned the hard way.  I have now paid many, many more 
times what it would have cost for a proper mast in order to clean up the 
mess by hiring a crane with a crew of 3 and pick the antennas off the 
tower.  So maybe I want a slight over kill this time.


    73,  Fred  KG9X

##  somewhere, buried in my files in basement, I have at least  35-40
old photos of bent mast pix.   The last one included was just a few years
ago with dale, VE7SV bent mast horror show.  His  2 el shorty 40m yagi
at top of the 10 ft mast, folded the mast 90 degs, 1.5 ft above the top of the
110 ft tower.  

##  You guys have no  clue how damned dangerous it is to try and remove a 
bent mast.    In some cases, due to weight of yagis..think Telrex, the ice laden
yagis  +  ice laden folded  mast,  eventually broke off, with  mast + yagis  crashing down,
and  slicing some guy wires  on the way, bringing the entire tower  down..onto
a fellows  roof !    That or the snapped off mast + yagis is hopelessly mangled
and mixed up in the top or next set of guys..+  parts of yagi eles and  or booms
sticking through the tower.  Its  dangerous, even if a crane is used. 

##  hams are cheap, they are also stupid.   15-30 mins  spent with the arrl  or dxe
mast calculator  will tell you everything you ever wanted to know.   Even then,the
mast will deflect quite a bit, b4 it folds over.  So if the mast calc sez it breaks at 100 mph,
rest assured, the same mast  will be deflecting  like crazy, at just 90 mph.   The problem
with a mast deflection is... the center of gravity is now away from the tower.   That puts
a downward moment on the mast, further complicating things..esp with a heavy yagi at
the very top of the mast.   This is real bad news for stuff like  1.9 OD x sched 80, 
35 ksi masts.   The boom is no longer parallel to the ground !  That puts even more  offset 
weight onto the deflected mast. One side of the mast compresses, the other side stretches. 
( this is right where the mast folds).  The engs call that... taking a set.  

##  1.9 x  sched 40  was popular here in town, back in the 70s.  Since it could be bought
already galvanized, was lighter than sched 80, and was relatively cheap.   A buddy
across town had a 4 el cubex quad on a 30 ft boom, and decides to mount the  quad
9 ft above the top of the rohn 25 guyed tower.   60 mph gusts..and it bent.   pita.
But they wouldn’t listen.     Trust me on this,  go through this gong show once, and
you will not even blink an eye, when it comes to writing the cheque for the 
DOM / CM mast.   If its  doable, use a 3.0  OD  x .25 mast.    7 lbs per ft....
who cares.   Use a 2nd thrust bearing, into the tower, but with no transverse
bolts tightened.  Its there just to minimize mast deflection inside the tower...
and also to support the mast, when rotor has to be removed... after lifting the
mast + yagis up a little bit..to get rotor out.   

Jim   VE7RF