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Coaxial Cable ( Transmission line )

  1. Dec 29, 2011 #1
    How does current flow in a coaxial cable in each of the following cases :
    1. When the cable is connected to a DC voltage source.
    2. When the cable is connected to an AC voltage source
    3. When the outer metallic shield is connected to ground and DC and Ac sources are connected separately
    I don't understand how current flows in one direction through the inner conductor and in the opposite direction through the outer conductor-Please explain this to me with proper diagram for each of the cases (case 1, 2,3) as mentioned above.
     
  2. jcsd
  3. Dec 30, 2011 #2

    vk6kro

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    A coaxial cable is just two conductors, so if you connect a DC voltage source, no current will flow unless you add a load of some sort.
    Then current will flow one way from the positive terminal of the power source, along the centre or outside of the coax cable, through the load and back down the other side of the cable, to the negative terminal.

    If you connect an AC source whose wavelength is comparable with the length of the cable, then you get transmission line effects where it depends on the load what happens next.

    Briefly, coaxial cables have a "characteristic impedance" which for many cables is 50 ohms.
    If the load is 50 ohms, all the power travelling in the line will be absorbed by the load.

    In this case, current actually flows on the INSIDE of the shield of the coax cable.
    It is quite possible to have a totally different AC signal on the outside of the coaxial cable shield.

    If the load is more or less than 50 ohms, there will be reflections on the line which will interfere with the incoming power. This produces interference patterns along the cable known as standing waves.

    There are books written about these effects and you need to attend a class or two to get the idea, but the following site might be a good starting point:
    http://en.wikipedia.org/wiki/Transmission_lines
     
  4. Dec 30, 2011 #3

    sophiecentaur

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    Any two conductors will have a characteristic impedance. A parallel pair or a twisted pair of wires will also have a characteristic impedance, the value of which depends upon their diameters and spacing. Coax cables can have different impedances also but 50Ω is way the most common choice.

    This business of 'matching' loads to cable impedance can be a bit confusing but any sort of wave will be reflected to some extent at any discontinuity (hence the small amount of reflection by even a totally clean glass window).

    I have a feeling that the OP looks a bit like an 'assignment' by asking "Please explain this to me with proper diagram for each of the cases". I would recommend looking in Wikipedia for Transmission Lines and Coaxial Cables, in particular..
     
  5. Dec 30, 2011 #4

    vk6kro

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    Yes, I had the same feeling and omitted the obvious diagrams as a precaution.

    Q1 was at a different level to the other questions, though, so I'm not sure.

    Mismatches in load and characteristic Z can be quite severe before it makes much difference. Mainly it is an indication that something bad has happened to an antenna and that can have a huge effect.

    Happy New Year!
     
  6. Dec 30, 2011 #5

    sophiecentaur

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    We're so cynical!
    :wink:
     
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