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Homework Help: Transmission lines & reflections

  1. Apr 3, 2017 #1
    1. The problem statement, all variables and given/known data

    Can someone help me solving this problem ?
    A lossless coaxial cable is used in a data network. it has an er =3 (relative permittivity) It is driven by a pulse
    source with internal impedance of 10 ohm, 1 V amplitude, square wave with
    duration of both positive and negative pulse 50 ns, with negligible rise and
    fall-time. Sketch the voltage at the end of the line as function of time for 300 ns

    2. Relevant equations

    3. The attempt at a solution
    The Zc = 50 ohms from caluclutation. The one way time delay due to the cable length is 58 ns. At start a voltage of 5/6V positive will propagte to the end it will be there after 58 ns and it will last till 108 ns. after that the negative pulse of -5/6 V will reach the end on 108 ns and will last till 158 ns. I have no idea how it will be with reflections?
  2. jcsd
  3. Apr 3, 2017 #2


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    Science Advisor

    I think you are assuming that the line is terminated with it's characteristic impedance of 50 ohm.
    The characteristic impedance of the line is the voltage to current ratio of the travelling wave.
    If the line is open-circuit or short-circuit there will be either zero current or zero voltage at the end.
  4. Apr 3, 2017 #3


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    Science Advisor
    Homework Helper
    Gold Member

    +1 You need to know how it's terminated (or make an assumption about how it's terminated from the question).
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