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Far Field Radiation

  1. May 24, 2010 #1
    Hello,

    I am reading in my book about Hertzian dipoles. I am trying to understand the far field radiation of a Hertzian Dipole.

    Lets say I place a loop at the origin, with the plane of the loop in the x-y plane. Along the z axis at a distance of 1 km I place a Hertzian Dipole.

    Is the orientation of the dipole going to affect whether the loop picks up the emitted signal from the dipole? From reading my book, I think that placing the dipole axis along the z-axis wouldn't allow the loop at the origin to pick up the electromagnetic signal. Would it be better if the dipole was aligned parallel to the x-y plane?

    Perhaps someone could help me further. Thank you
     
  2. jcsd
  3. May 24, 2010 #2

    Born2bwire

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    The radiation pattern of a dipole antenna is something like sin^2 \theta. This pattern is a standard omnidirectional pattern. This means that at \theta = \pi/2, the directivity is at a maximum and at \theta = 0 the directivity is null. Here, we aline the z axis along the axis of the dipole. Thus, if you placed any receiving or transmitting elements along the dipole's axis in the far field, these elements would not interact with the dipole since they lie in a null of the radiation pattern. Instead, you would want these elements to lie in the plane of greatest directivity of the antenna, which is the x-y plane as you have suggested. In addition, the dipole and other elements need to be oriented so that they have compatible polarization. If the dipole and receiver are cross-polarized, then they will not interact regardless of their radiation patterns.

    So a dipole and loop antennas are both linearly polarized and you would want to make sure that you orient them accordingly. This is done by placing the loop in the x-y plane with the dipole at the origin oriented along the z-axis.
     
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