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Radio waves emitted by an inductively coupled power transfer system

  1. Mar 10, 2014 #1
    I would like to know if a a inductively coupled power transfer system also emits electromagnetiv waves.
    The system contains of two inductance coils one generating the magnetic field with AC, the other picking up the magnetic energy and transforms it into an AC, also.
    The current frequency is above 100 kHz.

    In my oppinion, the self inductance of the coils do emit electromagnetic waves, but i have read that only an open oscillating circuit by meaning of a straight antenna emits electromagnetic waves.

    Does anyone knows can help me out here?
  2. jcsd
  3. Mar 10, 2014 #2


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    Some radio receivers will work down to 100kHz. Try putting one near your coils.
  4. Mar 10, 2014 #3
    Unfortunately, i use Ansys Maxwell to simulate the coils and wont be able to detect any fields in a real case.

    Since the magnetic energy from the field, that does not influences the receiver coil is not transfered or used, it must go somewhere, when my electric Energy continues feeding by an AC.
    By putting the circular shape together with infinitesimal straight lines, i do have infinitesimal antennas.

    As the shape of the coil is circular, the magnetic fields should even add up, so where to go with my magnetic Energie that is induced by the flowing AC?

    Since i havent found much information about radio waves and coils, i am still uncertain, if my though is right here
  5. Mar 10, 2014 #4


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    Hi BaPF
    welcome to the Physics Forums

    Yes it does :smile:

    its generating an electromagnetic field not just a magnetic field
    Any AC current generates an electromagnetic field

  6. Mar 11, 2014 #5


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    Unless you have perfect flux linkage between the two sides of your transfer system, there will be some power leakage. Transformers tend to be designed with a closed loop of magnetic material coupling the primary and secondary. This makes them pretty efficient. Any significant spatial separation between primary and secondary will result in radiated (wasted) power. The secret is to minimise the loss by using the right impedances on both sides.
  7. Mar 11, 2014 #6
    i guess that you mean coupling coefficient k= M/sqr(L1*L2)=1 by having a perfect flux linkage, right?
    That means, that the energy of the mutual coupled electromagnetic fields stay in the system with the two coils, the primary and secondary one.

    With respect to the perfect flux linkage, i now take a look at the self inductivity of the coils.
    Also i picture my coils as two antenna, that are able to emit radio waves, in principle.
    I picture two cases:
    The first is a perfect balanced resonant circuit of coil and condensator, so the energy of my magnetic field feeds my electric feld, so no electromagnetic radiation is emitted.

    The second one is not balanced so my electronic power source needs to compensate the idle power in addition.
    Since my electric power supply has added extra energy now in form of my magnetic idle power of the coil, the
    energy must go somewhere, if my electric power supply keeps continuesliy feeding and not gaining back the
    magnetic energy.
    With no condensator to feed, the energy is emitted as radio waves with the coil acting as an antenna.

    Is that right?
  8. Mar 12, 2014 #7


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    There is no 'magic' that can eliminate radiated power completely. The best you can do with a structure if finite size is to reduce the Radiation Resistance at the feed point. This is what your resonant circuit is doing. You seem to be considering mutual impedance alone.
  9. Mar 13, 2014 #8
    i fully agree that there is a rest of radiated power left in any real case.
    The main question, that probably narrows down my thoughts is, where the origin of the radiated waves that enter the vicinity arount my coils is.
    As there are two cases closer considered regarding my magnetic (and so my electromagnetic) fields of the coils, i split the radiation source in two parts.
    Even it is one source after all.

    So as i see the Radiation Resistance, i can use it as a faktor to determine if i have radio waves or not, right?

    So in a global view of my system when i meassure the radiated Power, i can calculate the Radiation Resistance.
    But still i dont really see, what in the system is responsible for it.

    By looking at the feeding point, there are both the electromagnetic field around only the coil and the one that also covers the second coil and one that is beyond the second coil and does not cover it.
    All electromagnetic fields are part of the source and so one field all in all.

    Now i picture this:
    The part of the field energy, that is not around the coil and is not " used" by the second coil will be radiated in the vicinity.
    The part of the field energy that is not around the coil but "used" by the second coil will not radiate.

    The part of the field, that is only around the coil still troubles me:
    Some part of the electic power of the feeding source is used to power the magnatic field only around the coil.

    In case of a perfect resonant case, i only need to feed Energy to the resonant circuit until it is settled. Afterwards, the coil and condensator constelation compensate each other.

    In this case, the only Energy of my feeding source after a short while becomes the Energy that is needed for the field, that covers the secondary coil and the part, that is emitted in the vicinity.
    So the only radition comes from the part that emitts in the vicinity

    If my system is not perfectly balanced, my source do feed continously Energy to create the magnetic field of my coil. As my source keeps feeding, this addition Energy will be emitted by the coil than to preserve the
    equilibrium of energy.
  10. Mar 13, 2014 #9
    is that right?

    I really appreciate your help. I hope i am not making things more complicated as neccessary here.
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