what is radiative field in antenna

the problem makes me puzzled, i hope some one can help me .can the far- fields of any antenna be regarded as spherical wave?

well as far as i know , when you are using far field assumption it means you have a plane wave and not spherical wave.

regards

thank you for your answer. can you explain it for me? why it is a plane wave ?

An electromagnetic wave propagating through space is always a plane wave. It means that its E and H component alternate in a sinusoidal manner, perpendicular to the axis of propagation. It is a "transversal" wave.

Only close to radiators that generate the wave, the shape of the wavefront runs off a "point", so it is spherical. As it travels farther from the radiator, it flattens in a planar wave.

Some radiators utilize an aperture (waveguide horn, parabolic dish); the wave energy is concentrated in a desired direction (therefore those are directional antennas). The larger the aperture measured in wavelengths, the farther from the antenna the energy beam forms. Until the beam is fully formed, the wavefront is not exactly planar; it has a "transition" form between the spherical and planar, distorted by the beam-forming aperture.

Like in optics, we divide the space in two zones: the "near" zone closer to the radiator, and the "far" zone farther away. In the "far" zone, the wavefront is cleanly planar; such wave we want to be.