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SSB Antenna Noise

Discussion in 'Technical Discussion' started by K1W1, Oct 20, 2011.

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  1. K1W1

    K1W1 Senior Member

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    Hi,

    I am wondering if any of the other readers/members here has had any issues with SSB Antennas making a drumming noise when in any sort of wind?

    I was on a new boat yesterday in 30 kts of wind and the noise was unbelievable, it was a really deep vibration through the whole top structure.

    It went away on one side when someone touched the Antenna about 2m from the base.

    I have never encountered this before and have sometimes wondered why there are things that look like small fenders about midway up these Antennas.

    Any solutions gladly explored.
  2. Marmot

    Marmot Senior Member

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    Those things are subject to vibrating like that when the excitation frequency is close to their natural frequency (I'm talking about vibration, not RF) and they set up a standing wave. Sometimes you see an old one that has several "nodes" along its length where the amplitude is high enough to damage the glass.

    About all you can do is select a different length antenna and make sure the tuner can still handle it, or modify the mount (shorter or longer or more flexible or stiffer) so that it changes the resonant frequency. Also, moving the mount to a different part of the monkey island (if that is where they are mounted) might help as well if the substructure is stiffer or more resilient.

    Good luck.
  3. Blair

    Blair New Member

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    I have to admit that one simple fix that worked OK years ago was trimming the length but a quite small amount (relative to overall length) thus somewhat changing the amplification or frequency(?) of the vibration that caused the specific noise e.g. a low frequency 'drumming' as you describe. Note: I now understand that even if you cannot hear noise when it too low frequency for humans it is still a cause of distress for people close - similar to actually hearing it. Often an unrecognised problem.

    I felt guilty about doing this not knowing if there would be any reception consequences I am only now admitting it! There was no apparent loss in reception but that's from a dumb amateur of course. I have also seen long whip aerials braced again at some point above their mounts - perhaps that's a common practice?
  4. Capt Ralph

    Capt Ralph Senior Member

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    Are we talking about a fixed fiberglass stick with a base mount and a clamp mounting somewhere a few feet up? OR a dipole in or on a back-stay?

    If it's a stick and you have no room to relocate the in-term clamp or ad another in-term clamp, try a length of fishing string from the base of the whip (small ant on the top of the big stick) to some out of the way rail or deck hardware. It won't take much tension to keep the hum of the stick down.

    If it's a dipole, slacking the back stay or piggy-back leads tension should help.

    Other than the hum (some folks may like it) if installed correctly, it should not hurt anything.
  5. PropBet

    PropBet Senior Member

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    Agreeing with Marmot, it sounds like you have a harmonic vibration in the stick. The length, as well the mount can play a role in it. I've seen the doughnuts you put on the stick that reduce or get rid of it all together, and or shortening or changing the mount. Rather than a mount where the stick butt ends into / at the mount, I've seen or recall to have seen a type mount where the mount was essentially a collar around the stick that allowed 6" or so to extend below the mount. Easiest way around it is a back stay, anchored elsewhere on the boat with a slight amount of tension. Not much, just taut.

    Is this a hollow or solid core antenna?
  6. Innomare

    Innomare Senior Member

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    It could also have to do with "eddy's": vortexes which release at a certain frequency. If this frequency coincides with the eigen-frequency of the antenna (which is function of the length), you can get resonance.

    They had this problem on the Erasmus bridge here in Holland, and it's also a frequent issue on chimneys. On the Erasmus bridge, they had calculated this effect, but initially they didn't take into account that the shape of the cables changes when it's raining (more wing-like).
    790px-Rotterdam_Erasmusbrug_bij_avond_vanaf_hoek_Veerhaven-Westerkade.jpg

    The solution could be to wind a rope or something in a spiral shape from base to top. This has an effect on where the eddy's release and could help your problem.

    If it doesn't help, at least it wasn't very costly to try it.
    Scalar Chimney.jpg
  7. Innomare

    Innomare Senior Member

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    Maybe the things you've seen halfway on the antenna's are meant to prevent the rain from flowing down all the way (creating a wing shape). You could also try simulating that by fixing a small funnel or something on it.