Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Transient (Time domain) in transmission lines

  1. Jan 16, 2010 #1
    Hello.
    I am watching this video:
    https://www.youtube.com/watch?v=xIQtnQ9XPbE

    and he says there:

    I see that the wave turns into a "normal" sinus in the end, but it's not constant.
    I think, that even on an unmatched transmission line, the waves should go back and forth, but eventually turn into a constant voltage. Isn't that correct?
    If not, and the correct answer is that the steady state IS the final wave there - how do I calculate the voltage? is it the RMS of the wave?

    Thank you.
     
  2. jcsd
  3. Jan 16, 2010 #2
    When a TEM wave in a cable goes from one medium (cable) to another, If there is an impedance mismatch (and the signal source is a voltage source), there is a reflection and a standing wave. Furthermore if the propagation velocity changes, the waves "pile up" in the downstream cable. There are two equations that are relevant:

    Cable impedance is proportional to

    Z = sqrt(1/ε)

    and the propagation velocity is proportional to

    and v = βc = 1/sqrt(ε)

    where ε is the dielectric constant of the dielectric in the cable. A high dielectric constant reduces both the cable impedance and the propagation velocity.

    If the signal source impedance does not equal the cable impedance, there is a reflection of the backward-propagating signal, producing a standing wave.

    Bob S
     
  4. Jan 16, 2010 #3
    Thanks.
     
  5. Jan 16, 2010 #4
    For my previous post, the characteristic impedance of a coaxial cable is about

    Z = (1/2 pi) sqrt(μ0/εε0) Ln(R/r)

    where R and r are the outer and inner radii of the coax, and sqrt(μ00) = 377 ohms.

    Bob S
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook