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Why 75 Ohms not 50 Ohms?

Discussion in 'Electronic Design' started by Harry, Sep 12, 2005.

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  1. Harry

    Harry Guest

    Does anyone know why video connectors have impedance 75 Ohms not
    50 or any other number Ohms?

    Why is 75 Ohms better than 50 Ohms for video signals?


    -- Harry
     
  2. Genome

    Genome Guest

    You didn't read the FAQ......... did you?

    Sheesh

    DNA
     
  3. Reg Edwards

    Reg Edwards Guest

    Why is 75 Ohms better than 50 Ohms for video signals?
    It isn't.
     
  4. Joerg

    Joerg Guest

    Hello Reg,
    Reight, technically it isn't. But then again 75Ohms -> less copper for
    center conductor -> more plastics -> lower cost -> more profit...

    That could all change with the current oil price trend.

    Regards, Joerg
     
  5. Guest

    I once saw this explained ( maybe in the ARRL handbook): the higher
    impedance gives better voltage transfer,lower(35 ohms?) give better
    current match . 50 is a compromise. Actually, some missle and radar
    systems use 95 ohms.
     
  6. J. Davidson

    J. Davidson Guest

    Isn't it related to the FM/TV dipole antenna which has 300 ohm impedance????
    I'm not quite sure but the relation may have someting to do with 300 / 4
    giving 75.

    Jerry
     
  7. J. Davidson

    J. Davidson Guest

    Isn't is related to the dipole TV/FM antennas with 300 ohm impedance?
    300 ohms / 4 giving effectively 75 ohms.

    Jerry
     
  8. Joerg

    Joerg Guest

    Hello Jerry,
    This ratio had to be 4:1 because of baluns. However, it depends on which
    part of the world you are looking at. In much of Europe the flat cable
    and thus antennas were set at 240 Ohms and consequently the coax used to
    be 60 Ohms. I believe they also went to 75 Ohms now, probably assuming
    the old 240 Ohms antennas have all corroded away. Most of the late
    models had preamps anyway.

    Regards, Joerg
     
  9. Guest

    Guest Guest

    73.2 ohms is the radiation resistance of a dipole. 2 dipoles in a balanced
    configuration are thus 293 ohms. These numbers are rounded off to 75 and
    300 ohms respectively.

    Norm Strong
     
  10. Phil Allison

    Phil Allison Guest

    <...

    ** But what has this got to do with ** video ** ????





    .......... Phil
     
  11. Whn the video guys needed some coax, they just grabbed what was handy.
     
  12. Tam/WB2TT

    Tam/WB2TT Guest

    You get more voltage for the same power.

    Tam
     
  13. Phil Allison

    Phil Allison Guest

    "Paul Hovnanian P.E."

    ** Sounds quite plausible.

    The 75 ohm standard must date back to WW2.



    ........... Phil
     
  14. Phil Allison

    Phil Allison Guest

    "Tam/WB2TT"
    ** So what ?

    My point is that video is being confused here with RF.




    ........... Phil
     
  15. What frequency would you pick as the low end of RF?
     
  16. Phil Allison

    Phil Allison Guest

    "Richard Henry"
    "Phil Allison"

    ** What colour are the sox you have on ?




    ............ Phil
     
  17. Better colour rendering 75 ohms is violet-green-brown, while 50 ohms is
    only green-black-brown. Violet and green are complementary, so you can
    get any colour along that axis by mixing. Brown is low-brightness yellow
    (yellow would have given 750 kohms, which is obviously too high), thus
    introducing the missing red element.

    Or perhaps it's just a historical accident, based on early designs of
    coaxial cables in different countries. In Germany, 60 ohms was used.
     
  18. Reg Edwards

    Reg Edwards Guest

    Coaxial cable impedance has nothing to do with radio antennas or
    video. Its a matter of engineering economics.

    50-ohm cable maximises power handling capability for a given amount of
    expensive copper.

    75-ohm cable minimises attenuation for a given amount of expensive
    copper.

    If the price of copper had been different in the 1920's when coax was
    developed we might have had two different standard impedances.
     
  19. Phil Allison

    Phil Allison Guest

    "John Woodgate"


    ** Oh dear ..........




    ........... Phil
     
  20. Reg Edwards

    Reg Edwards Guest

    Because 50-ohm coax maximises power handling capacity it is most
    appropriate for feeding radio transmitting antennas and hence other
    radio applications.

    Because 75-ohm coax minimises attenuation versus distance it is most
    appropriate for long and short distance communications purposes such
    as the telephone system.

    Far more millions of miles of 75-ohm coax have been manufactured than
    50-ohm coax since the 1930's.
     
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