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Direction of a coil antenna

  1. Feb 18, 2014 #1

    I am afraid this might be a simple question but I am really confused.

    I am currently working with ferrite core coil antennas, and I am having trouble at understanding their directivity. The cylindrical ferrite core is placed in the middle of the loops.

    What I have read is that the reception is maximal when the direction of the transmitter is perpendicular to that of the ferrite core. It is minimal when the direction of the transmitter and the core are in parallel.

    I am confused because, from Faraday's law, the induced voltage in the wire should be maximum when the magnetic field and the surface of the coil (whose vector is its normal vector) are in parallel. Since the normal of the surface is parallel to the direction of the core, this seems to contradict what I have read.

    Could someone please explain why the directivity of a loop antenna is the way it is? Does my confusion come from the fact that the direction of the transmitter is not the same as the direction of the magnetic field? (or is it?)

    Thank you!
  2. jcsd
  3. Feb 18, 2014 #2


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    Science Advisor

    Consider a vertical transmitter tower.
    RF current flows up and down the tower.
    A resulting RF magnetic field forms circles around the tower.
    You want the most magnetic field possible to pass through your antenna coil.
    So you keep the axis of your core and coil, horizontal, and perpendicular to the transmitter direction.
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