coaxial cable wire ?

Discussion in 'General Electronics Chat' started by Mathematics!, May 17, 2009.

  1. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
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    What is the meaning of the 75 ohm or 50 ohm ,...etc rating of a coxial cable.

    I have been told it is the impedence. But I don't understand it since impedance is the complex number resistance + reactance j in the phasor domain .

    So the impedence is purely resistive. But what does that even mean for a signal ? Even if it was for calculating voltage dropped per what ?

    I am lossed any help.
    How is this value important.

    Also how much bandwith does a 75 ohm coxial cable have (i.e what is the highest frequency it can handle and the lowest frequency it can handle ? )

    And would different ohm rated coaxial cables have an effect on the highest and lowest frequencies it could handle?
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    The impedance of a coaxial cable (75R or 50R as you mentioned) is the impedance seen by varying signals. It is mainly caused by the inductance and capacitance of the cable.

    For not varying signals the impedance seen by the signal is only the physical resistance of the cable and the leakage resistance of the insulator between the inner and outer conductors.
     
  3. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    1,022
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    But 75 ohms means what ?
    What can you calculate with this value.

    If it is just resistance of the wire then how many feet for a certain volatge drop.

    I still kind of don't get this value. Their is no inductance reactants or capacitance reactants involved since it is purely resistance 75 ohms?

    Or is this 75 ohms value the magnitude of the impedance?

    Still confused sorry?

    So is lower ohm values better or worse why?
     
  4. mik3

    Senior Member

    Feb 4, 2008
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  5. DonQ

    Active Member

    May 6, 2009
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    An analogy to wire impedance is light shining through a substance. If a beam of light shines through water, and then shines into some more water, nothing happens except the light beam keeps going. If instead it comes to an interface with air, some of the beam keeps going, but some of it is reflected back. The impedance (refractive index) of the air is different than the water, so energy is reflected at the interface.

    The "impedance" is not "per meter" or anything like that. It is an inherent quality of the air or water.

    (Dirty water is like a lossy cable, separate from impedance. Good cables have very low series resistance, and very high inter-conductor resistance)

    If you have a varying electric signal (AC just to be easy, but also pulses, etc) flowing through a 70Ω cable and it comes to more 70Ω cable, it just keeps going. But if it comes to a 50Ω cable, or a 50Ω resistive load, some of the energy is reflected instead of being allowed to continue on.

    (The purpose of sending the energy down the cable was to get it to the load, wasn't it?)

    This reflection happens at each interface where the impedance does not match. You can imagine several unmatched interfaces, each reflecting, then reflecting back from another interface, and again and again... It can make a big mess real fast.

    Every source has an impedance, often set to match one of the available cables. When the proper cable is used, and all connectors used are the same impedance, and the load is the same impedance, the maximum signal will get to the destination without reflecting.

    Wrong cables, connectors, or mismatch source/destination impedances can set up reflections that can distort the signal, rob power, and in extreme cases, damage the signal source.

    Cables do attenuate the signal somewhat, with the level of the attenuation increasing as the frequency increases. There are tables you can find to see the attenuation of a signal per length of different cables. Different cables, even those with the same diameter and impedance, can be very different regarding the attenuation of a high frequency signal.

    Hope this helps.
     
  6. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
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    So their is no better quality in the 50 ohm or 75 ohm coaxial cables.
    If you stick with using the same ohm cable to make connection between things?

    I was wondering if the lower ohm number is a better quality.
    Or attenuates less of a high frequency signal?

    Also is their a max frequency a coaxial cable can handle?
    Or could it handle TeraHz ... so on
     
  7. mik3

    Senior Member

    Feb 4, 2008
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  8. DonQ

    Active Member

    May 6, 2009
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    There are better and worse quality 50Ω cables, and there are better and worse quality 75Ω cables, but none of them are worse than using a 50Ω cable when you need a 75Ω, or using a 75Ω when you need a 50Ω.

    The only cable to use is one with the matching impedance.

    Then, you have to go to the individual cable data-sheet. There are numbers like RG-8/U, RG-58/U, RG-8X, RG-316/U, that all describe 50Ω cable, but with different characteristics like: diameter, propagation speed, frequency attenuation, etc.

    There is no way to get around actually looking at cable specs yourself to see the options besides impedance.

    I can tell you that most consumer coax attenuate gigahertz signals, like cell-phone signals, so much that you lose so much with a long cable that it almost isn't worth trying to put an antenna in the top of a tree. Unless you use a very high quality cable, you will attenuate more in the length of the cable than you will gain with the better position of the antenna.

    Terra-hertz? Not likely. Now you need to look-up "waveguide".
     
  9. bertus

    Administrator

    Apr 5, 2008
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