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I understand far field propagation conceptually: you have a sphere propagating outwards, and you capture only part of the surface with your antenna aperture, which you convert into an electrical signal.

What about near field coupling? I've seen efficiencies in the 99% range. How is this possible? Clearly electromagnetic waves do not propagate in the same way. Also, what assumptions are made in the far field? Linear wave front? What are the necessary conditions to couple to antennas?

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In the far field, the electric and magnetic fields do not exist independently from each other, they cause each other, because they are part of "free" electromagnetic waves. Also, there is no effect back on the emitting antenna if you capture these electromagnetic waves. In contrast, the near field produced by the source can be predominantly electric or magnetic and you can have a strong reaction back on the emitter. An extreme case of near field coupling (by induction) is a transformer, where you can have efficiencies much larger that 99%.

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  • $\begingroup$ Any resources on some quantitative analysis? Maybe two parallel conductors etc. $\endgroup$
    – user110971
    Commented Oct 1, 2016 at 18:31
  • $\begingroup$ I would first consult a standard textbook like: John D. Jackson, Classical Electrodynamics. $\endgroup$
    – freecharly
    Commented Oct 1, 2016 at 18:47
  • $\begingroup$ I have Griffiths book, but I don't seem to remember a discussion on this topic. I'll have another look. $\endgroup$
    – user110971
    Commented Oct 1, 2016 at 18:51

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