Coaxial cable - why the current would flow only on the surface

Discussion in 'General Electronics Chat' started by alexei_kom, Dec 22, 2012.

  1. alexei_kom

    Thread Starter New Member

    Nov 22, 2009
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    Hello,

    The question is why the current would flow only on the surface of the inner conductor and not through the entire conductor? Both the conductors are assumed to have an infinite conductance. It is somehow related to that, but can't figure how...

    Looked at two books and googled it but didn't find any answer.

    Would appreciate any help!

    Thanks,
    Alexei
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,062
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    Search "Skin Effect".
     
    Last edited: Dec 22, 2012
  3. PackratKing

    Well-Known Member

    Jul 13, 2008
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    215
    Higher frequencies, follow the " Skin effect " [ Google ] on single-strand co-ax cables. The best setup I was taught for CB work, was multi-strand signal carrier in the co-ax. More "skins" = better propagation.
    The other possibility is the signal carrier in a lot of co-ax, is a steel wire plated w/ copper... Conductivity may be governed by the thin copper 'skin" copper being the better conductor of the two.
     
    Last edited: Dec 23, 2012
  4. jmoffat

    Member

    Jul 18, 2012
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    Have you tried looking up "Skin Effect" on wikipedia? Their explanation is a little over my head and uses a lot of math. Maybe you can explain it to me.
     
  5. alexei_kom

    Thread Starter New Member

    Nov 22, 2009
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    Hi,
    Thanks for the reply, but I think that it's not about the skin effect. Skin effect is about nonuniform distribution of current, whereas what I am talking about is general and does not assume a specific frequency.
     
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    "return" current in the sheath conductor (shield) of the cable will equal the center conductors current in most cases. The exception would be if you have a second circuit route that is in parallel to either the center conductor or the shield conductor.
     
  7. dataman19

    Member

    Dec 26, 2009
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    DC current and low frequency AC Current flows within the conductor.
    High Frequency AC (ie: RF) do not follow the quantum mechanics of Direct Current.
    In the RF world higher frequency current flow is in effect similar to the skin effect - but not random. The electromagnetic field pushes the electrons across the surface area of a conductor (which is why most people in the RF world call it "skin effect"). The current patterns are influenced by the RF field.
    In the 800MHz to 1.2GHz Range (the RF bands that I am most familiar with - being I worked in the Tropo Communications field - with 10KW to 50KW Transmitters) the Power and Frequencies are most influenced by the E and H fields of ridgid transmission lines (ie: waveguide). Ridged Transmission lines are more efficient at higher RF Frequencies because of the way that RF current moves in the transmission lines. In a coaxial transmission line the energy is expended as heat, because current flow within conductors causes heat. In Ridgid Transmission lines the energy is transmitted as ionized energy, and is more influenced by the E and H planes of the waveguide, the power capability is more about the distance from the center conductor to the outer conductor. Since the Energy has less DC resistance the power is transmitted more efficiently. In fact, at higher power levels, the inner conductor is eliminated and the Radiated Energy travels in the E and H planes - which eliminates capacitive losses due to the elimination of the center conductor.
    ..
    Properly terminated and properly loaded a waveguide approaches the "perfect RF conductor" characteristics. In the old Bell System the Horn Antennas relied upon this characteristic to provide a near perfect match between transmission line and free air. The old horn antennas were infinitely better "antennas" than anything developed since.
    ...
    On a 10KW or 50KW transmission system, when you periodically break down the rigid transmission lines to repair abnormalities (which cause reflected power to increase - or you notice the soot being expelled out the vents at the diplexer, a really common occurence) you could see the RF patterns etched into the copper center conductors surfaces. This was a "very visual" reminder that the RF current was flowing over the surface of the conductor. Anything that caused that current to divert or even attenuated asymetrically (ie a drop of moisure) often caused eddy events that if left un checked led to heat. This often led to an RF Plasma event that was essentially a combination of incoming RF energy trying to overcome the resistance and capacitance of the moisture droplet(s) that were causing a disruption of the RF current flowing across the conductor surface. The plasma was essentially pumped up by incomming energy, fed by the capacitance of the energy bubble, which grew as energy was pumped into it and the abnormality increased. Eventually copper was ionized, which resulted in increased heat and RF energy was reflected back toward the high power single tube Klystron Amplifier. Eventually the Amplifier Reflected energy safety tripped and the system shut down, some times the klystron kept pumping energy into the fault and the rigid conductors melted and the system shut down. Sometimes the Teflon Insulator/Spacers melted and pumped soot down the pipe. If the rigid center conductors stayed within parameters and no arcing occurred in the power wave plane, the insulators simply melted and the system kept running until some one shut it down, or the system reached critical mass where the heat and energy simply melted the center conductor.
    ...
    The outer conductor surface provide a lot less resistance to current flow that the entire inner mass of the conductor. This is why high power RF systems use copper inner conductors that are essentially thick walled copper pipes. It is also why Electrical High Current Conductors are made up of multiple strands twisted together. No electrical current likes to flow through a solid conductor, and as the frequency of the electrical energy goes up, this resistance becomes more of a problem.
    ..
    Dave
    Phoenix, AZ
     
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  8. charanbandi

    New Member

    May 17, 2012
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    It is the basic property of any metallic conductor...It has some proofs using gauss theorem....
    It simply says that "Inside any conductor,electrostatic field is zero and whatever be the excessive charge flowing with some potential difference across conductor,the interior of a conductor can have no excess charge ...
    U can Google for "electrostatics of conductor" hope u can find some good results ..
    I have read this from a text which Is not available in online...
     
    Last edited: Dec 23, 2012
  9. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    The skin effect is very general--with higher frequencies, the current usually can't penetrate as far into the conductor. Any device that uses a coaxial cable operates at relatively high frequency, so the current tends to flow closer to the outside of the conductor. The Skin Effect does, in fact, apply to this situation.
     
  10. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The RF consists of electric and MAGNETIC fields. All AC and DC current flow does in fact create its own magnetic field, but it becomes very important at high RF freq.

    The easiest way to envision the physical process taking place is to remember the inherent delay between the creation of a magnetic field around a conductor and the time at which current flow begins in a particular direction in that conductor.

    The higher the frequency the shorter the time in which the field can 'build'.

    Skin effect is basically(without the math) a spatial limit imposed by the delay. The interactions between the magnetic and electric fields can only propagate a short distance before being reversed and destroyed and rebuilt in the other polarity etc. etc. etc.
     
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