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Characteristic impedance - low vs high frequencies

Discussion in 'Electronic Basics' started by [email protected], Jun 7, 2007.

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  1. Guest


    Why characteristic impedance of transmission line isn't important at
    low frequencies (1000kHz) and it is one of the main parameters in RF?
    At high frequencies if a generator isn't matched with the impedance
    of the line the reflection of the power can damage the genarator, why
    isn't that the case at low frequencies? Or am I wrong about the
    destroying the generator?
    (example: If we connect high power audio amp to speaker we don't have
    to worry about the characteristic impedance of the cable) Can
    somebody explain this?

  2. Andrew Holme

    Andrew Holme Guest

    Transmission line effects are negligible if the line length is a tiny
    fraction of a wavelength.
  3. Totally wrong.
    Just ask a 'serious' electic company engineer on matching generators to
    loads with and without reflections.
    It is the physics that is involved, not beliefs or frequencies.


  4. Eeyore

    Eeyore Guest

    Who says it isn't ? That's the upper end of the DSL frequencies on a phone line
    and it's very important to use transmissin line thinking for that to work.

  5. Eeyore

    Eeyore Guest

    Audio only goes to 20kHz. The wavelength @ 20kHz is 1.5 km. A speaker cable is
    maybe 10m. There's no transmission line effect to consider here.

    In long-distance telephony, the long length of the cables make transmission line
    thinking important even at these audio frequncies.

  6. Guest

    Eeyore je napisao/la:
    Sorry, I meant to say 1000Hz not 1000kHz
  7. Phil Allison

    Phil Allison Guest

    "Andrew Holme"
    ** WRONG !!

    Same old ham radio bollocks trotted out as fact about a different area of

    ........ Phil
  8. Phil Allison

    Phil Allison Guest

    "Eeysore Fucking Pommy IDIOT "

    ** NOT AGAIN !!!

    Can't the Graham Stevenson MORON ever get a

    FUCKING decimal point right ?????

    Or any other point right ??

    ........ Phil
  9. Phil Allison

    Phil Allison Guest

    ** That is not a correct statement - just another myth from the mouths and
    keyboards of fools. All signal and power cables have a characteristic
    impedance and it can matter at low frequencies.

    The most important fact about characteristic impedance is that IF the load
    on the end matches the cable's characteristic impedance - the parallel
    capacitance & series inductance of the cable no longer has any effect.

    Capacitance in a mic cable 100 metres long can have a significant effect on
    the high frequency output of a mic.

    The inductance of a twin wire speaker cable of only 10 metres long will
    cause a drop in level at 20kHz of several dB if the load impedance is 1 ohm.

    ....... Phil
  10. Eeyore

    Eeyore Guest

    It's that damn Windows calculator !

  11. John Fields

    John Fields Guest

  12. John Larkin

    John Larkin Guest


  13. Phil Allison

    Phil Allison Guest

    "John Fields"

    ** However, in this case the man's " tool " is clearly the problem.

    ........ Phil
  14. Jasen

    Jasen Guest

    ??? how do you get that? I get 10km for free space so probably 5 or 6
    km for twisted pair - what propagation rate does a twisted-pair give?
  15. Guest

    Can someone recommend me a good book about that topic or a web site!
  16. John Larkin

    John Larkin Guest

    I'd vote for 15 km.

    Figure twisted pair at roughly 0.7, depending.

  17. Eeyore

    Eeyore Guest

    I get it using the MS calculator which I find very easy to accidentally move the
    decimal point one place.

  18. Rich Grise

    Rich Grise Guest

    Because in Phil's universe, everything is wrong? ;-)

  19. Rich Grise

    Rich Grise Guest

    It's lame to blame the calculator. Man up, say, "I made a mistake", and
    get on with your life.

  20. John Larkin

    John Larkin Guest

    umm, good point.

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