far field
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.