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Author Topic: Moaning Mosely Antenna  (Read 2380 times)
NJ3U
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Posts: 123




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« on: December 08, 2011, 06:00:55 AM »

Guys,  I've mounted my Mosley TA33JR onto the roof and during times of moderate winds it begins to Moan !  This would be bad enough, still the sounds it is generating seem to be amplified by the roofing materials and attic space.

I've already found that facing it 90 degrees into the prevailing wind stops this, but I'm not always available to do so.

Any damping ideas, what else have you found that can help me stop the Moaning !

Either way my otherwise understanding wife is kindly asking me to address the issue.

Ideas please as I'm gonna have to take action otherwise she won't be happy and you know where that leads !
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K5LXP
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« Reply #1 on: December 08, 2011, 06:20:52 AM »

The cure I'm aware of is to run some rope through the element tubing to dampen vibration.  Seems simple enough, other than having to take the antenna down to do it.


Mark K5LXP
Albuquerque, NM
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K2QB
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« Reply #2 on: December 08, 2011, 06:22:48 AM »

The moaning is probably due to the elements resonanting/oscillating in the wind. One trick is to run a piece of light weight rope inside of each element. I used to have a Moseley TA-33 roof mounted and that cured the problem.
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WX7G
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« Reply #3 on: December 08, 2011, 06:36:06 AM »

If you don't want to disassemble the antenna to install the rope it can be mounted on the outside of the elements. Take some 3/8" nylon rope and tie wrap it to the elements. I would place a tie wrap every 6 inches.
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W8JI
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« Reply #4 on: December 09, 2011, 04:48:16 PM »

NEVER install a Yagi without rope inside the elements.

I followed Force 12's advice and installed a 10-15 meter dual band Yagi with rope. It moaned, and then about a year or two later an element cracked and fell right off in a mild breeze!


I also knew a CBer who has a 100 foot aluminum tower, and the tower or his antenna vibrated in the wind. After about a year or two the top of the tower just fell right off. You could see where the legs got hard and cracked from the vibration.

 
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W7MJM
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« Reply #5 on: December 09, 2011, 05:55:26 PM »

Very interesting discussion, guys. In essence, an antenna system, including its supporting structure (tower, roof, whatever) has a resonant audio frequency in addition to its resonant radio frequency. Installing rope inside the aluminum elements of an antenna is a bit like adding the right amount of neutralizing capacitance to an amplifier circuit to prevent it from going into self-oscillation.  Wink
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WB6BYU
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« Reply #6 on: December 09, 2011, 06:00:34 PM »

Actually it provides damping (lowers the Q of the resonance) more than neutralization.

That's why most standard car broadcast antennas have a wire wrapped around them, as it
throws off the resonance.  Otherwise at the right speed it will whip back and forth and
eventually wear out.

I have the same problem with one of my wire antennas where one end is tied to the barn -
the whole building hums, depending on the wind speed.  My solution was to take the tension
off the wire by letting the counterweight sit on the ground (lowering the Q of the resonance).
When the tree sways, it can still lift the weight, but usually it doesn't have enough tension
to sing.
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VE7RF
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« Reply #7 on: December 10, 2011, 08:30:56 AM »

NEVER install a Yagi without rope inside the elements.

I followed Force 12's advice and installed a 10-15 meter dual band Yagi with rope. It moaned, and then about a year or two later an element cracked and fell right off in a mild breeze!


I also knew a CBer who has a 100 foot aluminum tower, and the tower or his antenna vibrated in the wind. After about a year or two the top of the tower just fell right off. You could see where the legs got hard and cracked from the vibration.

 


###  Several folks here in town have had rope inside yagi els for more than 20 years, with zero problems.   I never did myself, but noticed that IF my hygain and wilson yagi's  were just at the correct angle to the wind [ light breeze], they would vibrate badly..and always about 5' out from the boom.[ both sides of the boom]  My HB yagi's did the same thing.  Unless u dampen down the vibrations, they will fall apart from severe metal fatigue.  They would osc up/down a good 5" in total, 2.5" above and below.

BTW, F-12 says not to install rope inside their ele.  Why would you?  The typ F-12 ele goes like  9/8, 8/8, 7/8, 6/8, 5/8,4/8,3/8"  The whole idea is to get rid of  'vortex shredding' ....and the way that F-12 does it, is to keep dropping the OD of each section down by 1/8".  They also do not use any swages anywhere.  I have several F-12 yagi's  from 80-15m. None of em vibrate, and none have rope in the else.  By not useing swedges, you get a more uniform tapering of the element, and the differing diams will kill any osc's.  It's a slick design.

I don't know of anybody who has ever used rope inside a F-12 yagi.   Is this something that is recent ?  Mine are all 5-8 years old.

Later...Jim  VE7RF
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W9CW
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« Reply #8 on: January 01, 2012, 12:37:29 PM »

The last commercial Yagi's I bought and assembled were Hy-Gain (the original Hy-Gain in Lincoln, Nebraska, before MFJ's acquisition).  A 105BA 10M long-john and a 205BA 20M...  Hy-Gain included black polypropylene rope within each box for inclusion within each element.  I never had a "moaning" noise with either antenna.  And, I never had any elements raining from the sky.

73
Don W9CW
 
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W6AOA
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« Reply #9 on: January 03, 2012, 11:58:48 AM »

I remember back when I was in the Navy we had a very large antenna system that had nylon rod guy lines (over 120Feet long). they would sing in the wind also and the vibrations would weaking and fatigue them.  The solution to place a piece of rubber tube over the line at a certain point on the guy to dampen it. I guess it interfered with the resonance and stopped it.  Just a possible idea. I wouldn't use rope because it will hold moister and be very heavy. 

Ed Schaff  W6AOA
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