Committee on Radio Astronomy Frequencies

Where is the transition from near field to far field for a radio telescope?

Answer:

The distance of the transition from near field to far field for a radio telescope is taken as the distance to the Fraunhofer area, i.e. > 2D²/ in meters, where D is the diameter of a radio telescope in meters and is the wavelength in meters.

Example 1 shows these distances for frequencies below 3 GHz for a radio telescope with diameter of 50 meters in the following figure.

Example 2 shows these distances for frequencies below 275 GHz for a radio telescope with diameter of 50 meters in the following figure.

For the evaluation of the compatibility between potential interferers and radio astronomy, using the assumptions considered in Recommendation ITU-R RA.769, it is irrelevant whether the interferer is in the near field or in the far field of a radio telescope.