![]() ![]() But as the EM plane wave gets very close to the isotropic antenna, it begins to cause current to flow in the isotropic antenna. The isotropic antenna cannot "look ahead" and see the plane coming when the plane wave is in its far field because the EM plane wave is not yet having any effect on the isotropic antenna. Now consider an EM plane wave approaching the isotropic antenna. By the theory of reciprocity, the isotropic antenna must then receive equally in all directions. By definition, the isotropic antenna radiates equally in all directions. It is considered a point source - small enough in dimension compared to any other incorporated dimensions that it is dimensionless and infinitesimally small. Now we turn our attention to the elusive isotropic antenna. This coupling is why the energy is not radiated or dissipated by a transmitting antenna. In a more general sense, this is considered the maximum distance from which EM (electromagnetic) waves can couple to a nearby object. The non-radiating, non-dissipating (thus reactive) part of the Fresnel zone is generally considered to extend 1∕(2π) times the wavelength from the surface of the antenna. The Fresnel zone is the specific area of interest. The answer to the question lies buried in the mechanisms of Fresnel (near field) and Fraunhoffer (far field) zones of antennas. While you will find some reasonable references to thermodynamic equivalencies in some texts, it seems the genesis of the isotropic effective aperture equation has been rarely published. This is a topic that troubles most students and even finds it way into many technical papers and textbooks in the form of incorrect assertions and conclusions. ![]()
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