written Resistor on Coaxial Cable

Discussion in 'General Electronics Chat' started by baby_1, Mar 28, 2012.

  1. baby_1

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    Jun 3, 2011
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    Hello
    what the value of resistor on coxial cable identify?
     
  2. KJ6EAD

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    Apr 30, 2011
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    Do you mean the printed characteristic impedance of the cable?
     
  3. baby_1

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    Jun 3, 2011
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    Yes of course
     
  4. MrChips

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    What is the number written on the cable?
    RG-174 is 50-ohm
    RG-58 is 50-ohm
    RG-59 is 75-ohm
    RG-62 is 93-ohm

    For cleanest signal transmission, the cable must be terminated with a resistor or load of the same impedance, i.e. put a 75-ohm resistor from signal to ground at the receiving end of a 75-ohm cable.
     
    Last edited: Mar 28, 2012
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  5. Papabravo

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    The MAGNITUDE of the characteristic impedance of a cable, measured in ohms, tells you the value of a terminating resistor that will absorb all of the signal energy without reflections. It is primarily a function of the geometry of the conductors and the properties of the materials. It is mostly independent of frequency until the dimensions of the cable are comparable to the wavelength of the signal propagating along the cable.
     
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  6. bertus

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  7. baby_1

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    Jun 3, 2011
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    Thanks
    excuse me i until can't understand.according the explanation the resistor of the cable should be equal the load impedance that absorb all enrgy to it,we have inductance and capacitors in cable virtual.so if we cut a cable the ohm resistance of these impedance is different without calculate the ohm of conductor.what does printed characteristic impedance show us and say?
     
  8. MrChips

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    You are mixing up DC resistance and AC impedance.

    The AC impedance, 50, 75 or 93-ohm takes into account the inductance and capacitance and is independent of the length of the cable. The terminating resistor should match this impedance.

    The DC resistance is a function of cable length and will result in signal attenuation over very long cables. RG59/U DC resistance is about 16 ohms per 300 metres. So a cable would have to be about 600 metres to see 3dB loss when properly terminated with 75-ohm resistor.
     
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  9. baby_1

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    Jun 3, 2011
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    could you tell me more about this sentence

    why the inductance and capacitance doesn't change the impedance while the cable length has been changed?
     
  10. Papabravo

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    Feb 24, 2006
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    This is because the inductance and capacitance are distributed along the length of the cable. When you apply the rules for series inductance and parallel capacitance, what you get is a "characteristic impedance", a complex number, whose magnitude is a constant.

    http://en.wikipedia.org/wiki/Characteristic_impedance
     
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  11. baby_1

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    Jun 3, 2011
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    Thanks Papabravo
    but i can't understand agian better.it seems i need a good refrence and explanation about it with picture and other physical image
     
  12. Papabravo

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    You can try Gonzalez, Microwave Transistor Amplifiers, Chapter 1, for probably the best explanation I've ever seen. If you couldn't grok the Wikipedia article it is doubtful that his treatment would work any better for you.

    http://www.amazon.com/Microwave-Tra...=sr_1_1?s=books&ie=UTF8&qid=1333040453&sr=1-1

    In any case, all you need to know is that the geometry and materials used in the construction of a coaxial cable produce a wide range of effects, some of which are captured in a number we call "the magnitude of the characteristic impedance". It is also important to realize that in the complex impedance plane, the locus of all the points with a magnitude of 50 ohms is actually a circle. We don't usually deal with impedances with a negative real part, so I guess you could limit it to a semi-circle in the right half-plane.
     
    Last edited: Mar 29, 2012
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  13. Papabravo

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  14. bertus

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  15. KJ6EAD

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    PB, I deleted my superfluous post. I hadn't noticed that you'd already posted the Wiki link.
     
  16. Papabravo

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    No problem -- I appreciate the vote of confidence
     
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