Page "Electromagnetic radiation" Paragraph 11

Looking at the original charges and currents that are the cause of the wave brings into play terms involving charges and currents (" sources ") in Maxwell ’ s equations that produce a local type of electromagnetic field near sources that does not have the behavior of EMR.

In particular, according to Maxwell, currents directly produce a magnetic field, but it is of a magnetic dipole type which dies out rapidly with distance from the current.

In a similar manner, moving charges being separated from each other in a conductor by a changing electical potential ( such as in an antenna ) produce an electric dipole type electrical field, but this also dies away very quickly with distance.

Both of these fields make up the near-field near the EMR source.

Neither of these behaviors are responsible for EM radiation.

Instead, they cause electromagnetic field behavior that only efficiently transfers power to a receiver very close to the source, such as the magnetic induction inside an electrical transformer, or the feedback behavior that happens close to the coil of a metal detector.

Typically, near-fields have a powerful effect on their own sources, causing an increased “ load ” ( decreased electrical reactance ) in the source or transmitter, whenever energy is withdrawn from the EM field by a receiver.

Otherwise, these fields do not “ propagate ,” freely out into space, carrying their energy away without distance-limit, but rather oscillate back and forth, returning their energy to the transmitter if it is not received by a receiver.