[TowerTalk] XM-240 Failure

Wed Dec 19 23:53:50 EST 2012

I have had considerable experience with both antennas (EF240X and XM240X) and the C3 for that matter. I originally had the C3 and EF240X up at about 35 M and just couldn't keep the 40 meter antenna up and working. First the studs that the linear loading wires attached to broke.  They told me it was a bad batch and gave me new ones to replace the bad ones. (That was helpful...they given me the 4 studs and I spent $400 taking the antenna down and fixing it.) Six months later one of the linear loading wires broke on the driven elements. Had to take it down again to replace that. When the linear loading wires broke again I took the antenna down and junked it. Bought an XM-240 and put it up. No problems with it till I moved to the new QTH.  Rebuilt it and put it up with the C3. Made it through two hurricanes with these antennas at 40 M. In the 11 years I have lived here I have had two lightning strikes that caused me to have to take down the XM240 and replace the loading coils and balun.

I love the C3. Even lightning that took out the 40 M beam and my FT1000/Alpha 87 didn't faze it.  It has been up for 22 years and two hurricanes and no problems with the antenna or the rivets.  I even riveted the elements on the XM240. I would never buy another EF240, it is flimsy and hard to tune. The XM240 does not like lightning but I don't hold that against it. :

-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Larry
Sent: Wednesday, December 19, 2012 6:03 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] XM-240 Failure

The EF240X was up about 8 years (a couple of hurricanes, a few Nor'easters, didn't survive the tornado). I never had trouble with the C3 I had. The EF240X elements are a bit bigger than the C3 and have a linear loading scheme (2 aluminum wires that go from about 3/4 of the element to tie points about 12 inches above the boom and the tie point is about 8 inches wide IIRC). Only one of the two elements was affected. Just a guess, but perhaps a wind resonance problem (ever see the Tacoma Narrows bridge crash? c 1940). 
Over time the repeated stress causes a failure. Of course, in your case of only 2 months that is quite short for stress failure unless there were defective parts to begin with. The EF240X had 4 good sized rivets on each side of the U shaped boom to element piece. The holes on both sides of the boom  were worn to a vertical length of about 3 times the rivet diameter but horizontally the holes were only a bit larger than the rivet diameter.

I replaced the rivets with 1/4-20 (or was it 5/16-18) SS bolts and had to put in a new boom piece of course.

73, Larry  W6NWS
-----Original Message-----
From: D. Drake
Sent: Wednesday, December 19, 2012 5:34 PM
To: 'Larry'
Subject: RE: [TowerTalk] XM-240 Failure

I had  Force 12 C-3 up for over 10 years and had no trouble. I'd bet your EF240X was up there a long time before it failed.  My XM-240 was up only 2 months!

73,

Dale AA1QD

-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Larry
Sent: Wednesday, December 19, 2012 3:12 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] XM-240 Failure

I had a similar failure on a Force12 EF240X. Force uses rivets. Eventually it elongated the holes on the boom and broke the rivets and the element fell
117 feet.  I fixed the element and then the following year a tornado finished off the antenna by bending one half of both elements to the other side of the boom.

73, Larry  W6NWS

-----Original Message-----
From: D. Drake
Sent: Wednesday, December 19, 2012 2:28 PM
To: towertalk at contesting.com
Subject: [TowerTalk] XM-240 Failure

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   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 werent 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.  Ive 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  thick.  I also suspect that the sheet aluminum the present U channel is formed from may not have been heat treated so its 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

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