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Metal to metal contact generates RF?

Discussion in 'Electronic Basics' started by quietguy, Aug 16, 2005.

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  1. quietguy

    quietguy Guest

    I fly (or crash) radio controlled model planes as a hobby.

    Engine speed is controlled by a servo which operates a piano wire
    control rod which attaches to the throttle lever on the engine.

    I am advised by the cluey and experienced guys at my club that metal to
    metal contact in glow planes is a no no - ie the metal throttle control
    rod mustn't attach to the metal throttle lever on my engine, as it will
    cause glitches etc - ie interference with the radio control operation

    However, on thinking a bit about it I don;t see how this can happen as
    there is no potential difference between the motor throttle lever and
    the control rod - in my current case the other end of the rod as is
    usual just attaches to a plastic/nylon/whatever lever on the throttle
    servo.

    I am not sure that RF 'noise' can be generated just by rubbing two bits
    of metal together when there is no potential difference between them

    I note that in other situations - eg mobile phones in cars, car radios
    etc there is LOTS of metal to metal contact surounding these devices,
    and is not a source of interference (ignoring the common problems of
    earth loops etc which are a different matter)

    If it is relevent the RC operates on 36.xxx megs

    So, I am thinking that maybe the problem doesn't really exist - in the
    situation as I have outlined above - but I have been wrong before,
    so....

    If you believe it could be a problem I would appreciate a short
    explanation of how the interference is generated, and whether your view
    is based on personal experience or information gleaned elsewhere

    Cheers and thanks in advance

    David
     
  2. Tom Biasi

    Tom Biasi Guest

    message

    Hi David,
    I have heard of pieces of metal, long metal cables, etc. when in the
    presence of an RF field can arc at the junction in the case of a high field.
    Even in a weak field the junction can act as a diode and may produce some
    broadband noise but I never heard of the warning WRT RC planes.
    Regards,
    Tom
     
  3. I'm pretty sure the level of RF in a model plane is unlikely to
    approach that needed to produce arcs as it's receiving, not
    transmitting. OTOH long conductors can also look like
    multiwavelength antenna elements, absorbing RF and storing it
    temporarily assuming just the right lengths, and if their length
    changes at just the wrong time that energy will dump out as sparks,
    producing interference with desired operation. But this smells
    improbable; one would have to work at it.

    The situation with phones etc is the opposite; engineers stay up
    late making sure this kind of thing can't happen.
    This sounds like a more reasonable concern, especially as the
    metals' work functions may be different allowing rectification. This
    applies even if the metals are the same as one bit may be more or
    less oxidized than the other.

    A somewhat less catastrophic version of the first idea seems more
    reasonable; that the moving connected metal parts will look like a
    variable antenna element parasitic to the actual antenna on the
    receiver, reducing/reinforcing the field strength seen by the
    receiving antenna making reception vary wildly as if the plane were
    jumping large fractions of a mile from the transmitter as the parts
    move.


    Mark L. Fergerson
     
  4. quietguy

    quietguy Guest

    Thanks for the replies - much appreciated

    David
     
  5. kell

    kell Guest

    Those guys could be wrong about the WHY, but right about the
    interference. You could get interference coming from the ignition. Do
    you have a high voltage coil in the plane to give you spark? If there
    is a conductive path from the engine to the radio-controlled
    servomotor, you might get interference from the high voltage spikes
    generated by the ignition coil.
     
  6. quietguy

    quietguy Guest

    Good thought Kell, but most RC planes - all of mine - use a glow plug so
    there is no coil or spark etc\\\

    David
     
  7. Chris

    Chris Guest

    Hi, David. Unless you have a brushless DC servomotor controlling the
    throttle, you might want to replace your brushes (and make sure you
    have the right ones -- the wrong brushes, even when new, may not make
    good contact). If they're worn, that can generate plenty of RF noise.
    I believe that's the most common cause of interference with the RC
    control.

    If you want to follow up on the question of static generation, look at
    any insulators rubbing against each other. That's a much more likely
    cause of charge generation (like shoes on a dry carpet, or a comb on
    wool). Sometimes, putting a tiny amount of dry graphite lubricant
    between the rubbing insulators can help.

    Good luck
    Chris
     
  8. quietguy

    quietguy Guest

    Thanks Chris - actually I am not having any problem with interference - the
    plane in question works fine and no glitches noticed. However, experienced
    club members (and lots of other guys on a RC plane newsgroup) suggested I
    not contine to use metal to metal contact in a new plane I built - so I was
    wondering about the how and what and such of this potential problem

    But I appreciate your reply

    David
     
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