Why we need to connect shield of shielded cable to earth ground?

Discussion in 'Wireless & RF Design' started by anhnha, Jul 5, 2012.

  1. anhnha

    Thread Starter Well-Known Member

    Apr 19, 2012
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    Hi,
    Why we need to connect shield of shielded cable to earth ground?
    I knew that the shield is used as shielding from electromanage interference.
    But why we have to connect it to ground?
    Thanks.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    The shield acts as a Faraday cage and prevents EMI from reaching the internal signal wire.
    If you do not ground the shield, EMI will induce voltages in the shield which will also be seen on the signal owing to the capacitance effect of the coaxial cable.

    (Note that the shield should be grounded at one end only in order to prevent current loops in the shield... but that's another story).
     
    simozz, cork_ie and anhnha like this.
  3. anhnha

    Thread Starter Well-Known Member

    Apr 19, 2012
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    Thank you, MrChips
    Did you mean that when the shield is grounded it will have ground potential and therefore EMI will not will induce voltages in the shield?
    Suppose that the shield is not grounded both end, then the shield is not create a complete circuit.How EMI is induced voltage into the shield? And when we ground the shield, are there any current flow from shield into ground?
    As far as I know, current only exists when have a loop.Is this right?
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    An ungrounded conductor becomes an antenna and its potential will rise and fall with the induced EMI signal at low frequencies. It is not necessary to have a current loop for this to happen. Current can still exist in a conducting surface. This is why ground planes on sensitive circuits are especially important.

    When the shield is grounded the entire shield is at earth potential and does to rise or fall above earth potential.

    Grounding the shield at one end or at both makes no difference in this case.
    Ground loops when both ends are connected is a different issue which is not being discussed at this point.
     
    geb77093 likes this.
  5. gootee

    Senior Member

    Apr 24, 2007
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    It depends what the cable is being used for. There are many cases where an actual earth ground is not used, and/or not needed.

    Except for certain types of antennas, I can't think of a reason that EARTH ground would ever be especially necessary or desirable. Even with antennas, an earth ground is not necessary. Think of all of the aircraft and vehicles and spacecraft that use radio communication just fine.
     
  6. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    It is indeed another story,& an incorrect one!

    With very low levels,associated with high gain amplifiers,such as with microphone preamplifiers,the shield of twin shielded cable is normally only connected at one end for just this reason.

    In coaxial cable,the shield (or more correctly,the outer) is one conductor of the cable,& must be connected for the cable to work.
    In racks at TV stations,hundreds of coaxial cables are connected from one unit to another,all with a common earth to the equipment rack,connected to both ends of the cable.

    Very long lengths of coax can indeed,pick up hum,due to fact that the "earth" at one end is not at exactly the same potential as that at the other.
    In this case,"Hum bucking" coils are used for video cables.
    These are very similar to the "Choke Baluns" used when feeding a balanced antenna from an unbalanced feeder (coax),& consist of a number of turns of coaxial cable wound on a ferrite toroid.
    The similar RF devices often use use an "air core".

    To answer the OP's question:-
    It is definitely not essential to connect the cable outer to real earth at both ends.
    Ground plane antennas often have no physical connection to "real earth",
    & the coax outer is connected to the 4 or more horizontal conductors at the base of the antenna.
     
  7. PRFGADGET

    Active Member

    Aug 8, 2011
    52
    7
    In aircraft and other "MOBILE" applications, bonding the shield to "Chassis Ground" presents a neutral "common point" in respect to the vehicle electrical system / battery / generator / alternator / power source.
     
  8. cork_ie

    Active Member

    Oct 8, 2011
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    Beautifully simple answer
     
  9. geb77093

    New Member

    Nov 26, 2012
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    To Mr chips. I enjoyed your post about Ground on July 5 2012. Did you mean that the EMI in the shield will NOT rise or fall above ground. In a shielded coax cable will there still be eddy CURRENTS even though the outer shield is grounded?How is it a block to the ELECTRIC FIELD? Thanks a million!
     
  10. MrChips

    Moderator

    Oct 2, 2009
    14,496
    4,268
    You can have eddy currents in any conductive material, grounded or not grounded.
    The purpose of shielded cable is to create a Faraday cage so that no current is induced from external EMI on any conductor inside the cage.
     
  11. eetech00

    Active Member

    Jun 8, 2013
    818
    148
    Just to add to mrchips answer,

    If the shield is not grounded, then emi from unknown sources can induce voltages on the internal copper wires of the cable. When that happens, the signal in the cable can become distorted. If Ethernet is being used on this cable, CRC errors will occur, causing retransmission. Too many retransmissions can cause applications to fail,
     
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