Transmission lines - reflections

Discussion in 'General Electronics Chat' started by pyro, Sep 5, 2006.

  1. pyro

    Thread Starter New Member

    Sep 5, 2006
    I have a little question about your wonderful page!
    It's all about AC Chapter 13: Transmission lines.

    First the most simple case, where there is only a battery, 1 Volt DC. So the frequency is 0, or if simulating via Spice very very low. So for a very short time, the battery will send a 1 Volt wave. Now the wave will travel to the end and - as this is open - travel back. But what happens next? What about this resistance of the source? When this 1volt wave travels back, why is this wave not dissipated into heat at this resistor? Another pretty general question: In DC, when the wave travels back, the source is still at 1 Volt, the reflected wave is also 1 volt. Is that why it "stops" sending a new wave? But again, what about RSource?
    Why will the battery first see the cable, and then whatever is at the end? There is this analogy with the train, but I don't get it, or at least not what happens when the reflected wave comes back!
    For example this picture:
    The source resistance equals the line resistance. Why is the wave which is coming back not converted into heat?

    Another question:
    In all examples, the source is always fitted to the line (mostly ZSource=75 Ohm). What will happen if the source isn't fitted at all? If this resistance is close to zero or let's say zero, what will hapen with 1)open and or b) shorted? The wave can't leave the cable, but will the transmitter still send more waves onto the cable (of course this is very abstract, but just to understand it)! What will happen if the sender is at +1Volt, but all of a sudden a -1Volt wave comes back, will the sender then send a 2V wave, or what?

    And, finally: If I have a normal battery and I connect a normal cable, not coaxial, do waves behave like in all these examples, or is this different? What if I send pulses on a normal cable? Are there also these reflections and fields between the cables (even if I put them in a distance of e.g. 10 metres)?

    I hope you understand what I mean! I read the chapter a couple of times, but this question bothers me!
    Thanks in advance