What is the difference between the near field and far field

What is the difference between the near field and far field for a rigid piston source? Why can the directivity only be determined from the far field?

Solution

solution

The near field is remarkable for reproducing classical electromagnetic induction and electric charge effects on the EM field, which effects \"die-out\" with increasing distance from the antenna (with magnetic field strength proportional to the inverse-cube of the distance and electric field strength proportional to inverse-square of distance), far more rapidly than do the classical radiated EM far-field (E and B fields proportional simply to inverse-distance). Typically near-field effects are not important farther away than a few wavelengths of the antenna.

Far near-field effects also involve energy transfer effects that couple directly to receivers near the antenna, affecting the power output of the transmitter if they do couple, but not otherwise. In a sense, the near field offers energy that is available to a receiver only if the energy is tapped, and this is sensed by the transmitter by means of answering electromagnetic near fields emanating from the receiver. Again, this is the same principle that applies in induction coupled devices, such as a transformer, which draws more power at the primary circuit, if power is drawn from the secondary circuit.

This is different with the far field, which constantly draws the same energy from the transmitter, whether it is immediately received, or not.