Do we always need a return wire?

Discussion in 'General Electronics Chat' started by antennaboy, May 8, 2011.

  1. antennaboy

    Thread Starter Active Member

    Jan 31, 2008
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    Hello Forum,

    I always thought, naively, that a circuit powered by a DC or RF generator with a transmission line connecting it to a load, needed two conductors to close the circuit. One conductor being the signal conductor while the other the return conductor.

    Is the return conductor always necessary in a circuit? If so, why?
    The voltage is a concept that needs to be applied at two points (conductors).

    Does current in a circuit always need to form a closed circuit? Why?
    With two conductors we can then have two currents, equal and opposite. But what do we do with a transmission line made of only one conductor? Both equal and opposite currents exist on conductor: zero net current...maybe?

    If we connect a AC battery terminal to a single wire, that wire becomes an antenna.

    Has anyone heard about single-wre transmission lines (Goubau line)?
    How does a single-conductor transmission line like the Goubau line work?

    Is the longitudinal current inside it equal to zero? If it is zero, why so?
    How about the voltage on this line?
    All I know the field propagates as a surface wave wrapped outside the single metal wire. I tried to find simple conceptual explanation but not luck so far.

    (Microwave waveguide are composed of only one conductor too. But the field (modes) are trapped inside the guide and propagate by reflecting off the guide walls. There is then propagation without any conductors, free space propagation (free space can be viewed as an infinite spherical waveguide).

    Thanks
    antennaboy
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    2,534
    You are confusing electricity with RF. While related, they aren't the same things.

    As usual, Wikipedia is your friend.

    http://en.wikipedia.org/wiki/Single-wire_transmission_line

    Electricity in a circuit always requires a return line. A circuit is a loop, it can apply to auto racing as well as electricity.

    RF, on the other hand, is a mix of fundamental forces. It is electric and magnetic fields 90° coupled together, and it radiates. Light is an example of this too. With RF, when an antenna converts it back to electricity a dual conductor is again needed, look at cables and 300Ω twin lead. An antenna is a transducer (look it up on Wikipedia), it converts one thing into another, and in many cases back again. A speaker is another example of a transducer, it converts electricity into sound, and sound into electricity.
     
  3. antennaboy

    Thread Starter Active Member

    Jan 31, 2008
    45
    0
    Thanks Mr. Bill.

    I kind of think that an RF circuit is simply a AC circuit working at RF frequencies.
    The theory of transmission lines shows up and things get more complicated.

    The simple hydraulic analogy that current flows in one conductor and out of another works well only for DC.

    But AC, DC, RF are all, fundamentally, working based on Maxwell equations....
    Clearly it is a stretch to use those equations for a DC circuit.....
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    Depends on how you define DC. If you turn a circuit on and off in anything resembling a cyclic manner it has a frequency, but no antenna.

    Also, when you turn something on or off it has a squarish edge, which are chock full of frequencies. A square wave has many harmonics, it isn't just one frequency. This may be a bit advanced for you yet, but eventually you'll cover it if you are going to school.

    Before digital really took over everything RF was king. Think about it, it is almost like magic. You can talk to someone or control something over a long distance. It was what I cut my teeth on way back when.
     
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