Radio interference caused by metal to metal contact

Discussion in 'General Electronics Chat' started by Von, Dec 1, 2008.

  1. Von

    Thread Starter Active Member

    Oct 29, 2008
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    Can anyone BRIEFLY shed some light on how vibrating "metal to metal" contact can cause interference in a radio controlled model.

    What is the basic physical phenomenon that causes this?

    Is it more or less likely in AM than FM and or 2.4 ghz DSM control systems.

    Sorry if the question is too broad, just looking for some insight to this.

    Thanks in advance...

    Von.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Never heard of it, though I would suspect either a intermittent ground plane, which would affect reception, or if there were a voltage potential between the metal parts a minor arc gap. Before tubes (aka valves) became common it was a common to use an arc a transmitter. It is incredibly inefficient and sloppy, since the arc radiates all over the spectrum (in spite of the tuned circuit they used with the transmitter).

    Of the 2 I favor the former most.

    AM is extremely susceptible to RF interference, much more so than FM or other modulation schemes.
     
  3. Von

    Thread Starter Active Member

    Oct 29, 2008
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    Bill,

    I said the same thing.

    But someone on a RC helicopter forum stated that they had a bad bearing and this was causing "radio noise" as manifested by servo glitching.

    A little google search seemed to give this some credibility but I am having trouble with it too.

    They actually "jumped" on me after saying it seemed like it was crazy.

    I do see how mechanical vibration could interfere with the resistive element used for servo position feedback and cause glitching but radio noise (EMI) seemed a bit of a strech.

    I'll wait to see if anyone else has any insight.

    Thanks.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    I have checked lubrication in a big bearing with an ohmmeter. The better the grease film on the balls, the higher the reading. If bad lube caused RF generation, I am reasonably sure the meter would have jumped a bit. Steel on steel is not much of a source for RFI.

    I do agree that vibration can cause problems with servo potentiometers. Antenna contact could also be affected with bad results. A really hard mount will allow engine vibration to be preferentially coupled into the servos and receiver.

    For RFI, you have to have a difference in potential enough to make a spark. Perhaps you could get radiation from a spark ignited engine, but hardly from airframe panels. Where is the voltage source?
     
  5. mik3

    Senior Member

    Feb 4, 2008
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    What if one of the metal is a bit magnetized and it causes high frequency currents induced in the other?

    Maybe there is an explanation, nature is so strange!
     
  6. Von

    Thread Starter Active Member

    Oct 29, 2008
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  7. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    As previously stated, I believe the problem is most likely one of vibration causing poor connections.

    I'm not sure if the and oil/grease and and rotating bearings could create ESD, but possibly And perhaps they are not ball-bearings, but bearing surfaces with differing metallic composition. If so, then that could be the problem. Note that ESD charges as low as 20V can damage an IC or electronic component when discharged quickly into the device, but that you don't really feel anything below 1KV or so.

    However, a spark isn't necessary to create EMI, just a difference in potential and a loop area that acts as an antenna.
     
  8. mik3

    Senior Member

    Feb 4, 2008
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    If someone, someday explains why a positive charge attracts a negative charge maybe we will be in a position to start understand how universe works!
     
  9. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    ANY metal junctions that aren't soldered can cause harmonics by partial rectification action. Every radio amateur knows the problem that using a metal roof or a chain link fence can cause when it comes to harmonic generation. Poor metallic contacts are inherently non-linear.

    eric
     
  10. mik3

    Senior Member

    Feb 4, 2008
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    Do you know what is this phenomenon called?
     
  11. KL7AJ

    AAC Fanatic!

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    Well, back in the dark ages it was called the Cat's Whisker effect....after the first radio detectors. Any dissimilar metals...or metals with intervening oxide....can form a low-quality rectifier.
     
  12. KL7AJ

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    Nov 4, 2008
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  13. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    The problem is pretty well documented in model aircraft that use ignition engines. Metal push rods and fittings, particularly on the engine, are thought to contribute to glitching with 72 MHz equipment. Also, in an airplane, some of the interference may be related to static charge that can build up. John
     
  14. draeath

    New Member

    Dec 3, 2008
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    A positive ion has too few electrons to be stable, while a negative ion has a surplus. They are attracted to each other by their pull to stabilize (one for the hole, the other for the electron).

    As to what allows them to pull toward each other, I think that's something way over my head ;)
     
  15. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    I think it's just a built-in, low-level, atomic-scale mating ritual.

    Sex is everywhere, you just can't escape it (and, why would you?).
     
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