Generally speaking, the near field is defined as the electromagnetic field within a range of less than 1 wavelength (or wavelength level); while the far field is the field (distribution) after the electromagnetic wave propagates to a distance, experience Generally speaking, many near-field modes of a general radiation field can be ignored after propagating 10 wavelengths. At this time, it is approximately considered that the field only has the 'radiation' characteristic of propagating far away, that is, it can be regarded as a far field.