[TowerTalk] XM-240 Failure

[D. Drake]

Some of you may know I recently moved to a new QTH
in New Durham, NH where I have installed a 70 ft.
Rohn tower. On the tower is my old 4 el SteppIR
and a new XM-240.  The XM-240 is up at about 85
ft., above the SteppIR.  The stuff all went up the
week before Sandy blew through.  When Sandy
visited here she was pretty weak with the biggest
wind gusts recorded by my wx station in the 45 MPH
range.  A few weeks later we had strong winds one
night when a cold front moved through. The next
day I noticed the reflector of the XM-240 hanging
at an angle.  Since it seemed that we had not had
any extreme winds I assumed that I had not
adequately tightened the bolts that attach the U
channel (piece that holds the element) to the boom
clamp piece ( 4 – ¼ 20 ss hex head tap bolts).  A
few days later K1RX came up to fix it and we
discovered that the bolts were not loose but in
fact the bolt heads had torn (ductile fracture)
through the aluminum U channel on one side.  A
quick call to Cushcraft and they agreed to send me
a new U channel. They said they had never heard of
this failure before.  

 

The U channel holes for the mounting bolts are
actually elongated as are the mating holes on the
boom clamp.  This allows for some adjustment of
the elements to get them perpendicular to the boom
and parallel to each other.  Unfortunately this
leaves little metal for the bolt head to compress
against.  The instruction manual specifies a
hardware stack up of bolt head, u channel, boom
clamp, split lock washer and nylon insert locknut.
No flat washers are used.  When we installed the
new U channel we added some stainless steel fender
washers under the heads of the bolts and between
the boom clamp and the lock washers.  Fender
washers were also added to the driven element
hardware.  This allows the forces to be spread
better over the 1/8” aluminum pieces.  I know
putting a flat washer under the lock washer will
reduce the “locking” ability but with the
elongated hole the lock washers weren’t exactly
performing as intended and there is the nylon lock
nut.   If anyone is interested I can email you a
photo of the damaged U channel.  

 

The U channel is actually made from 1/8” aluminum
sheet formed on a brake.  I’ve been told by a
former Cushcraft employee (when Cushcraft was here
in NH with the previous ownership) that the
antenna was originally designed using an extruded
U channel that was ¼” thick.  I also suspect that
the sheet aluminum the present U channel is formed
from may not have been heat treated so it’s
toughness would be even lower than the original
design.

 

It was rather frustrating to experience this, not
just because of the effort in fixing it but I had
taken great pains when preparing the antenna to
try to compensate for all of the other failures
or problems that others have experienced with
these antennas. As an engineer I should have known
enough to question the assembly but hind sight is
20 20 as they say.  

 

 

Dale AA1QD