Quantitative analysis of the amplitude difference between near and far field directivity

In this short paper we briefly revise the criteria defining near and far field. After an extension of the radian sphere notion to finite size sources, we generalize the amplitude and phase criteria for arbitrary source structures. Next we identify with a representative example the wavelength distance where the near field has reached the shape of the far field directivity, then we quantify the difference between near and far field directivity in function of the position to the source.