Need assistance with transmission lines formulas

Discussion in 'Homework Help' started by dannyj999, Jun 7, 2015.

  1. dannyj999

    Thread Starter New Member

    Jun 7, 2015
    What I already have;
    Working frequency of the cable = 1.75Khz
    Attenuation = 0.40 Np/km
    Phase shift = 0.25 rad/km
    Sending end voltage = 10V rms
    Sending end current = 20mA rms

    What I need to determine is the voltage and current in polar form at a particular point down the line, for example 20km.

    I've been trying to figure this out for hours but keep coming to a dead end.

    The formula I have for the current is;
    Ir = Is e^-na

    What I can't figure out is, is "n" the legnth? so for this example would it be 20? and "a" is the attenuation which I'm given?
    so would this be;

    Ir = (20x10-3)e^-(20)(0.40)

    This equals a very small number, which is why I'm thinking its incorrect.

    Or am I looking at this the complete wrong way? I thought originally I'd need to work out the characteristic impedance Zo but I'm not given the values for R L G and C ?

    So yeah! Any help would be greatly appreciated.
  2. MrAl

    Distinguished Member

    Jun 17, 2014

    I've never done it this way, but that looks right.
    'a' would be the attenuation factor 'alpha' and that would make 'n' the length in km.
    The voltage formula should look similar.

    For example, for a cable with a=0.8 Np/km there would be an attenuation of about 48.6db at a distance of 7km. Keep in mind that we are usually working in the range of thousands of meters here so there could be a lot of attenuation.