far field

the far field means the power distributed pattern in the space is not change with the distance from the transmitter,the only change is the amplitude,it not the same as the near filed that its power distributed pattern is changed with the distance from the transmittter!
it is important of the far field to the transmitter,because it determine how the power of the transmitter is distributed in the space,it determine the efficieny of the transmitter's power for the desired traget you want to communicate with

Generally all "transducers" antennas, electrical, but sonic also, generate a pattern that change its parameters along the transmission axis.
This change is smooth but for classification, has been creeted almost 2 zone: the near fiels (also called Fresnel Region) and the Far field (also called Fraunhofer region). Generally the far field boundry is 2D^2/lamda (D=dish diameter).
In far field, the wave is assumed as "plane" (i.e. not spheric) and the beam is pencil shaped.
On the near field the wave plane is curved, quasi spheric, and the beam is quasi cilindrical.
Of couse to do businnes with radio signals, antennas are on the far field.

At far field, you can assume that your waveform is a plane wave.

T Zul