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Author Topic: Making sure the antenna stays where it's supposed to....  (Read 918 times)
VA2FSQ
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Posts: 510




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« on: May 07, 2013, 09:01:22 PM »

This winter, my av620 vertical started acting strange.  It turned out that the nuts attached to the u-bolt that attached the antenna to the mast came loose, allowing the antenna to slide down the mast.  There was a lock washer on each bolt.  I tightened them (not easy at -10C) and a few weeks later they came loose again.

So now i'm putting up a new antenna mounted high on a wall on the side of my house.  It is a beam and the support structure will attach to the gable and the reinforced wall of the attic.  There will be 4 threaded rods extending out of the wall to which I will attach some custom brackets for the antenna.  The thing is, I need to rent a lift to attach the antenna and I want to be sure that it won't come loose over time, or at least for several years.

So I'm thinking of using a lock washer, and two bolts, the kind that have the nylon insert so are self locking.  The two nuts will be tensioned against each other and the bracket.  I will also use locktite.

In your experience, is this a good way to go?  Or is there some better arrangement?

Thanks for any advice.
Tom
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VA2FSQ
WD8KNI
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Posts: 144




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« Reply #1 on: May 08, 2013, 03:05:17 AM »

Tom, I wonder about the wall thickness of the mast, and if it compresses as the wind blows allowing it to loosen.  I also wonder about the size of the clamps and the mast diameter.  If you have a 1" mast and a 1.5" clamp it will work loose they must be the same size.  I also wonder about they type of steel the clamp is made of, is it stretching causing it to become loose? I have never had nuts come loose that had a lock washer on them, except when both sides of the lock washer did not have a flat surface to work against. They should always be installed flat washer, lock washer, nut.   Nylon inserts nuts (nylock) nuts should never have to be doubled up in a locking arrangement (only use one), they are used on aircraft they are that good at staying tight, note only use them one time to insure integrity.  If you go the Locktite route use blue, not red.  Have not looked at the specification in some time, as I remember blue will break loose somewhere around 20 ft lb of reverse torque, red will never break loose it destroys the threads when taking apart, unless the bolts are very good.  Good luck with the problem.. Fred
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WB6BYU
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Posts: 13149




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« Reply #2 on: May 08, 2013, 06:31:04 AM »

It is difficult to keep bolts/nuts tight when the antenna is vibrating in the wind
and the mast / brackets / bolts only withstand a limited amount of torque.  There
really are two problems:  keeping the nuts tight on the bolt, and making sure that
the brackets stay tight against the mast even when the bolts are tight.  It isn't
uncommon for mounting brackets to bend a bit over time so they don't grip the
mast as tightly, in which case you may need to tighten the bolts further even if
they haven't loosened.

(Temperature cycling can also cause problems:  torquing a bolt at -10C might not
provide sufficient force when it warms up.)

My suggestions:

1) use U-bolt plates, etc. with teeth that grip into the mast so you aren't just
relying on friction between two flat surfaces to hold up the antenna.

2) use brackets and hardware that are sturdy enough that you can torque them
down securely without damaging them or the mast.

3) use enough bolts / clamps so that a single loose bolt won't allow failure

4) plan that you will have to inspect / tighten bolts periodically.  Annual inspection
and maintenance is often recommended:  it might not be practical in your case, but
do give consideration to how you would access the antenna if needed.
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VA2FSQ
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Posts: 510




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« Reply #3 on: May 08, 2013, 08:18:02 AM »

Thanks for the info.
The previous antenna had 1.5 inch pipe connected to 1.5 inch mast. Mjf quality.
My new setup has 2" aluminum tubing with 1/4 wall and I'm using dx engineering clamps which fit perfectly.
I guess I'll have to buy a ladder that goes 30' high. If I do it right yearly checks may be enough.
Thanks for the suggestions
Tom
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VA2FSQ
W4HIJ
Member

Posts: 367




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« Reply #4 on: May 09, 2013, 11:02:48 AM »

My Cushcraft MA5B gets turned in the wrong direction by the wind on occasion. I though maybe the antenna was loose on the mast above the rotator but every time I've checked it, it's as tight as I can get. I just have to rotate the Rohn telescoping mast at a joint and aim it again and that usually last me for awhile. Fortunately I have easy access to the antenna and supporting structure.
73,
Michael, W4HIJ
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