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?