wave propagation in transmission line

Discussion in 'Homework Help' started by screen1988, May 18, 2013.

  1. screen1988

    Thread Starter Member

    Mar 7, 2013
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    I am learning about Transmission Line here:http://www.allaboutcircuits.com/vol_2/chpt_14/3.html
    And this is the part that I don't understand.
    Because the electrons in the two wires transfer motion to and from each other at nearly the speed of light, the “wave front” of voltage and current change will propagate down the length of the wires at that same velocity, resulting in the distributed capacitance and inductance progressively charging to full voltage and current, respectively, like this:
    [​IMG]

    In this picture, it seems to me that capacitors are charged graduallly, one after another. After the first capacitor is fully charged, the second capacitor will begin to be charged.

    I don't know why it happens like that. Why capacitors don't charge like this:
    At the beginning, the first capacitor is charged but when it has some voltage(not fully charged), it will act as a source and continue to charge to second capacitor. And the same happen for the rest capacitors.
     
  2. ninjaman

    Member

    May 18, 2013
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    i think its a representation. im starting amateur radio and find this a bit confusing myself. there arnt a lot of capacitors in the cable. its the inner core and dielectric surrounding it, then the braid. the inner core and braid are the plates and the dielectric in the middle. the braid acts as shielding stopping the signal inside transmitting. the inner core is an antenna that is not transmitting as a result of the braid. dont know if this helps
    all the best
    simon
     
  3. screen1988

    Thread Starter Member

    Mar 7, 2013
    310
    3
    Thanks simon,
    I'd like to know how these capacitors are charged.
    Are they charged in turn, one after another? I mean, for example, initially the first capacitor is charged fully to its maximum value then the second begin to charge from zero volt.
    I read the worksheet and it seems that the above is right but I wonder why the following is not:
    Initially, all capacitors are uncharged. Now the switch is closed and the first capacitor is charged but after a short time it has a small voltage across it (not fully charged to maximum), it will act as a voltage source and charge the second capacitor.
     
  4. ninjaman

    Member

    May 18, 2013
    306
    1
    hello
    that part is over my head. i think it just represents that the cable has capacitance across it and inductance along it. a cable like that will have a characteristic impedance. for radio use its usually 50 ohms, tv is usually 75 i believe. the impedance has to match the output of the radio to allow maximum power transfer. i dont think the length of the cable really affects it so much unless its really long.
    sorry i cant be of any more help.
     
  5. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Last edited: May 19, 2013
    screen1988 likes this.
  6. screen1988

    Thread Starter Member

    Mar 7, 2013
    310
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    Interesting video!
     
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