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Are the E and H field not orthogonal near the antenna?

  1. Oct 14, 2015 #1
    Hello, I've heard in many places that in the "near reactive field" of an antenna which is the region really really close to the antenna, the E and H fields are not perpendicular. But I just can't imagine how that is possible since In Maxwell's 3'rd and 4'th equations it is explicit that the curl of A is proportional d B / dt therefor they are in space orthogonal where A and B can be E and H.
     
  2. jcsd
  3. Oct 14, 2015 #2

    berkeman

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    Staff: Mentor

    The wikipedia article has some nice pictures... https://en.wikipedia.org/wiki/Near_and_far_field :smile:
     
  4. Oct 14, 2015 #3
    Yeah I've read this before posting, but it doesn't really give a good explanation on what I'm asking. It only says something vaguely about my question. And there are no images on how the EM field looks like in the reactive region.
     
  5. Oct 14, 2015 #4

    davenn

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    ohhh ?

    from that wiki article

    Felder_um_Dipol.jpg

    Near-field: This dipole pattern shows a magnetic field 55535f5b2af77c68e8201eb7dc99f36a.png in red. The potential energy momentarily stored in this magnetic field is indicative of the reactive near-field.


    if this isn't answering your Q directly, then try rephrasing your question :smile:

    Dave
     
  6. Oct 15, 2015 #5
    Hello Dave, as you can see the E and B fields in this image are orthogonal. Therefor it is a bad image since it clearly states that they are not orthogonal. Anyhow I came up with an explanation myself trying to figure it out. In the near field the electromagnetic field is not formed, therefor it is NOT an electromagnetic field yet, just electrostatic and magnetostatic and when they start to variate later on they form perpendicular E and H waves. I think that this is the only way to explain it. Unless it's some weird ass quantum physics effect of which I am not aware of.
     
  7. Oct 16, 2015 #6
    The fields near the antenna can be considered as being a radiation field, where Erad and Brad are orthogonal, plus a reactive field, where Ex or Bx can be anything, usually one of them being greatly predominant. Imagine that part of the antenna was a capacitor. The electric field between the plates is mainly the reactive component, and will be much greater and bear no relation to the radiation field which will also be present.
     
  8. Oct 16, 2015 #7
    The fields near the antenna is usually, but not always perpendicular (depending on the antenna type). Discrepancies are explained by charge distributions on the antenna. As Tech99 alluded, patch antennas and the like are exceptions.

    Also the actual far field radiation is a special relativistic effect caused by accelerating the charges setting up a dipoleish effect. I think the fields are still perpendicular, but in a slightly curved spacetime, so not at 90º as it were. (I could easily be wrong though. Relativity is not my field.) This curving effect is insignificant for most purposes and not usually taught to EE students.
     
  9. Oct 17, 2015 #8
     
  10. Oct 17, 2015 #9
    This may be of passing interest. My own measurements seem to indicate the following behaviour as we approach a dipole or slot antenna.
    At a great distance, the Power Flux Density falls off with the inverse square law (20dB/decade).
    At a distance called the Rayleigh Distance, the pattern starts to become cylindrical rather than spherical and PFD begins to fall of with 1/D (10 dB/decade).
    At distances closer than about lambda/5, the PFD remains constant, and either Bx or Hx also remains constant. The other reactive component continues to increase at 10dB/decade until the actual antenna is touched. As an example, for a dipole, equatorial plane, Bx increases right up until the probe touches the antenna, and the value measured then agrees with the B field corresponding to the antenna current and the wire radius.
    In the case of a dipole, the Ex field in the equatorial plane remains constant from about lambda /5 until very close to the antenna, but then rises very locally as the feed point is approached and the local field of the driving voltage is seen.
    If two dipoles are brought towards each other, the smallest path loss ever seen is 3 dB, even when they touch. This is because half the power is radiated and half is conveyed from one to the other. When two dipoles are brought together and touch, there is no jump in path loss.
    I would like to mention that the set of radiation contours close to a dipole, published by Hertz, and repeated by Kraus, do not appear to show the reactive fields.
     
  11. Oct 17, 2015 #10
    .......Correction, either Bx or Ex
     
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