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Homework Help: Radiation in the far field for a current carrying loop

  1. Dec 7, 2014 #1
    Hi people, I have a problem with some integral here.

    I have a loop of radius a, with a current I = Ioe-iωt' and trying to calculate the radiating fields in the far zone, my procedement is:

    Current density: J(r',t') = Ioδ(r'-a)δ(θ'-π/2)e-iωt'/2πa2 φ (φ direction)

    Here t' = t - |r-r'|/c (retarded time)

    I evaluate the vector potential: A(r,t) = ∫vJ(r',t')dV/|r-r'|

    I approximate 1/|r-r'| ≈ 1/r because in the far zone this term does not affect too much compared with the exponential, and |r-r'| ≈ r - aSinθCos(φ-φ') for the exponential, that plays a major role in the far zone.

    The problem is that I reach integrals that I can't solve:

    ∫ sinφ' e-ikaSinθCos(φ-φ') dφ'
    for x direction

    and ∫ sinφ' e-ikaSinθCos(φ-φ') dφ' for y direction (both integrals from 0 to 2π).

    Maybe I'm doing something wrong, but I don't know, any help?

    (r' are the source coordinates and r the observer coordinates)
  2. jcsd
  3. Dec 7, 2014 #2


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    Usually it is assumed that you are working with low enough frequencies and a small enough loop that you can take ##a## to be small in relation to the wavelength of the radiation. Then you can approximate ##e^{-ika \sin \theta \cos (\phi - \phi ')}##.
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