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Signals on a single wire

Discussion in 'Electronic Basics' started by Andrew Howard, Apr 3, 2005.

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  1. Could someone please explain how a signal can be transmitted on a single
    wire? I have always been taught that a signal, or current, needs a circuit
    to work. But then I see cases that when only one wire is used to carry a
    signal.

    eg. You touch the input on an audio amplifier, and you get a buzzing
    noise, even when you only touch the positive wire.(the power supply used
    doesn't have a grounding pin)

    Is the second "wire" a path between you and the negative via your legs,
    the ground, the table, then the casing or something? Or is it that there is
    a path between negative and positive within the amp with some resistance,
    which has a voltage drop, creatinge a potential difference between the
    positive an negative? Am I even on the right track? :D

    Another example, which probably has a different explanation, is antennas
    on radio transmitters. How do they transmit anything, when there is only a
    single wire?


    Thanks,
    Andrew Howard
     
  2. Yes. And there is a capacity in big bodies, a buffer of charges, both
    positive and negative which usually are in balance, and it works as a
    kind of fixed point voltage-wise.

    If we think of a short piece of water pipe, not connected to anything
    at the ends, just air outside. A small amonut of water inside the pipe,
    and a pump in the middle of the pipe.

    Then if we start the pump it will pump the water in one direction,
    but after a very short time there is no more water to pump. If the
    amount of water from beginning in the pipe is only a nanoliter and the
    pump pumps 1 liter per second the water will be gone in one nanosecond.

    If the water from the beginning is a million liters, if the pipe is
    connected to a swimmingpool, the pump can pump water for a million
    seconds in one direction before the water stops coming.

    The capacity is very different in these cases, but there is a certain
    capacity
    for delivering current in both cases.
    The capacity determines how strong AC currents we can create if there
    is no DC connection which completes the circuit.

    Even if you body does not touch the ground, and you touch the probe to
    an oscilloscope you can see a strong signal on the scope. That is
    because your body has a certain capacity, measured in nanoFarad, and
    that is enough to deliver AC current, as the AC current goes forth and
    back. And because magnetic noise induces a voltage in your body.

    If you touch the probe with a much smaller object, like a 1mm piece of
    copper wire, you will not see much difference on the scope screen.
    Because that little object cannot deliver much as much AC current as
    your body, or the earth.
    The higher the frequency the easier it is to use these small
    capacitances of the neutral parts of the antenna as virtual ground
    planes.

    At lower radio frequencies the earth itself forms the neutral part of
    the antenna.

    The antenna converts electric signals into electromagnetic waves which
    can travel through space.

    Well, the signal is actually an electromagnetic wave already when it
    travels through the antenna wire. The antenna is just a bridge between
    electromagnetic waves in conductors and electromagnetic waves in space.
     
  3. Don Bruder

    Don Bruder Guest

    Sounds like a variation on the explanation a Ham operator once gave me -
    Boiled down from a conversation that lasted about half an hour, I came
    away with the understanding that a transmitter and receiver operate in a
    similar-in-result, although very different in actual mechanism, way to a
    physically huge, ultra-tiny-rating capacitor, with each set's antenna
    acting as one plate of the cap, and the "universe between them" as the
    dielectric. It makes sense on one level, but on others, it kinda falls
    apart. I've still never managed to wrap my head around the details of
    "why" radioo works, even though I like to think I have a reasonable
    grasp on the "how" part.
     
  4. Jim Gregory

    Jim Gregory Guest

    Andrew
    You are invoking at least two phenomena, open-circuit AC conduction and
    electromagnetic wave radiation or radio (meaning wireless) transmission and
    reception.
    The earth plane has a lot to do with it in some cases. Don't forget the
    neutral wire in a power line is earthed and that there are stray
    inductances, capacitances and resistances, deemed as some of the losses, in
    transmission. You are a huge capacitor and humidity plays some part. You
    cannot touch something and send DC, well, only a static discharge.

    In portable radio (covers radio, bluetooth, mobile phone, Tvs, etc) Txs and
    Rxs, the chassis plane is unwittingly the 2nd wire.
    Antennae and tuned capacitive circuits are responsible for conveying to/from
    the frequency band/s concerned through the ether, in air or not in air.
     
  5. If we think about photons we usually think of visible light photons,
    like extremely small particles or small bundles of electromagnetic
    waves.

    It is more difficult to image the photons created by the 1440 kHz MW
    radio transmitter in Luxembourg, (it is still exists).

    It creates photons which are in the kilometer size range.

    It may not help but rather make it all even more strange :)

    But we have to piece together all these aspects of electromagnetic
    waves into one mental concept.

    Electromagnetic waves as mainly magnetism, as mainly electrical forces,
    as photons.

    It is like we are blind people trying to find out what an elephant is.
    We touch different parts of the elephant and see different pictures,
    but to understand the whole elephant we need to put all the pictures
    together. And try to make some sense of it.
     
  6. Andrew,

    Guess you do not mean an antenna. The so called single wire divices use one
    wire for power and signal *and* a common ground. Long signal lines,
    telegraph lines for instance, used old mother earth as a common. An example
    of the Dallas (Maxim) single wire thermometers can be found on:
    http://pdfserv.maxim-ic.com/en/an/app162.pdf

    petrus bitbyter
     
  7. Bob Eldred

    Bob Eldred Guest


    "a transmitter and receiver operate in a similar-in-result, although very
    different in actual mechanism, way to a physically huge, ultra-tiny-rating
    capacitor, with each set's antenna
    acting as one plate of the cap, and the "universe between them" as the
    dielectric."

    Wrong, Wrong, Wrong! Radio energy is electomagnetic energy like light, just
    lower in frequency. Physics tells us that any charged particle will radiate
    electomagnetic energy when it accelerates or decellerates. Electrons in a
    wire or antenna moving at high frequencies are under acceleration by their
    motion they therfore radiate EMR. Likewise a EMR field will cause a current
    in a wire of its same frequency. You can't get away from it. It's a matter
    of physical principles. Look up Maxwell, Maxwell's equations and Hertz.
    These guys figured this stuff out in the 19th century. It does not require a
    return circuit, capacitance between antennas, eather or any other such
    things, just empty space.
    Bob
     
  8. Thanks for all the responses! Now I have the general idea, so I can find
    more info on my own.

    Thanks,
    Andrew Howard
     
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