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Confusion on Source of EM Radiation? (segment Serway)

  1. May 30, 2010 #1

    In my introductory course of Physics, we use Serway. I've typed over a little piece from the book that I find quite vague. It is giving an example of (after deducing the plane wave EM equations) EM radiation. It uses the following antenna:


    The positive and negative sides are continuously switching due to the AC voltage source in the middle and thus we approximate it with an oscillating electric dipole.

    1) So first they use the Poynting vector to argue energy is flowing out, but then they say it's actually not those fields that present the radiation, because they're out of phase.

    2) Then they go on to say that at a farther distance, the earlier E and B fields have gone and just the derivates are there, am I right? And they say the changing E-field produces a B-field, much like the law of Faraday and Ampere-Maxwell, but that would mean they are out of phase, wouldn't it? Yet a little further in the text they seem to take it as evident that the E and B are in phase.

    3) Also, in the description in bold "at a farther distance, something else...", they seem to imply the radiation itself only originates from a far distance? But the photons actually come from the antenna itself, don't they? It's not that if there is a certain point in space with varying E and B waves, that photons start to shoot out of that point, right?

    This section has confused me a bit on the production of EM-waves.

    The way they explain it, you would also expect EM-waves going directly up for example, because far (directly) above the antenna, there is also a changing E-field (no matter how weak), but the formula it gives a little bit later ("I ~ sin²(theta)/r²") gives a theoretical zero for theta = 90°...

    Is this a bad text or am I completely misinterpreting it? (or both...)

    EDIT: A NB that has nothing to do with Serway: and would you expect a uniformly accelerating charge to radiate? The derivate of the E-field is constant...

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    Last edited: May 30, 2010
  2. jcsd
  3. May 31, 2010 #2


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    An antenna is often described as having a set of near-field and far-field behavior. In the regions immediately surrounding the antenna, there is a large concentration of fields. If you were to construct the Poynting vector for these near fields, you woud find that the direction of the Poynting vector is largely inconclusive. That is, the dominant fields in the near-field are non-radiative. For a dipole, these fields have a 1/r^3 dependence (thus you can see that they are dominant for small r). If you move far away from the antenna, then these fields quickly die out (as we would expect since they are non-radiative). What you have left are the radiating portion of the fields from the antenna. For a dipole antenna, these fields drop off as 1/r. So, you can see from the relative r dependence that in the immediate volume surrounding the antenna, the non-radiative fields will be dominant and can hide the radiative nature of the antenna.
  4. May 31, 2010 #3
    It's a poorly written book. The diagram is ok.

    If you want this very clearly explained, pick up any engineering textbook on electromagnetics by Kraus, Harrington, Balanis, Stratton etc. The units will be MKS but the processes will be elucidated much more clearly than in physics texts albeit from a classical perspective (which is completely sufficient to describe antennas all the way up to millimeter wavelengths and even higher in frequency.)
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