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Discussion in 'Electronic Basics' started by Peter, Dec 31, 2003.

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  1. Peter

    Peter Guest

    How do I determine the impedance of a length of coax that I have?
    I don't imagine an ohmmeter will do it?

  2. Is there any information written along the outside?
  3. Peter

    Peter Guest

    No it's just an old piece about 3 feet long.
  4. Well, impedance is primarily determined by the diameter of the center
    conductor and the inside diameter of the outer braid or foil, with a
    less sensitive dependence on the dielectric material. If you can
    measure center conductor diameter, and the overall outer diameter, and
    guess the kind of plastic used in the dielectric (teflon,
    polyethylene, foam, air with spiral spacer, etc.) you can probably
    identify it in a table of common types.
    It will if your hunk is infinitely long! ;-) Otherwise use a
    TDR (makes infinite - finite), or look up its specs.
  6. Hi,

    If you look at the characteristics of different coax
    cables with the same impedance and dielectric type but with
    various diameters, you will notice that the capacitance per
    foot is nearly independent of the diameter. IIRC it is
    about 22pF/ft for 50-ohm with a solid polythene inner and I
    expect then that it will be just as constant across the
    diameters for other types. This leads to the possibility of
    identifying the impedance of a length of feeder by using a
    simple DVM with a capacitance range. Of course the method
    would be more accurate, the longer the test piece.

    Cheers - Joe
  7. Jerry G.

    Jerry G. Guest

    To measure a cable's impedance yourself would be fairly complex. Look for
    an types of numbers on it to see if you can identify it from catalogue

    The impedance is determined by the type of dielectric material that the
    centre conductor is passing through and the shielding is around, and the
    distance between the centre conductor and shield. The gauge of the centre
    conductor, and outside diameter is also a determining factor.

    By doing precise physical measurement, knowing the types of materials
    involved, and the type of alloys used in the composition of the conductors,
    a mathematical model of the characteristics of the cable can be determined.
    This is very complex, and would not be accurate in all practicality for you
    to determine.

    The best way would be to use a cable network analyser, or to use a sweep
    signal generator and oscilloscope, with a RCL matching network to determine
    the impedance curve over a range of frequencies. From this the actual
    impedance can be determined.

    Accurately measuring the characteristics of any type of cable or wire can
    turn out to be a complex and expensive ordeal.

    If your piece of cable is from a cable TV distributor, or from a video
    distribution system, it is most likely a 75 ohm cable. If it is from an
    older Ethernet system, it is probably a 150 ohm cable. If it is from the
    antenna system used on communications radio, it is most likely a 50 ohm



    Jerry Greenberg GLG Technologies GLG

    How do I determine the impedance of a length of coax that I have?
    I don't imagine an ohmmeter will do it?

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