The Effect of Rain on Echoes in the Vicinity of mm-Pulse Radars

In recent years the development of mm-wave pulse radars was pushed forward very rapidly. It can be shown that precipitation echoes, the main interference source, cannot be computed from the well-known formula for weather radars in which a conical energy distribution is assumed. It is necessary to approximate the real nearfield energie distribution. In this letter this is done by a hyperbolic transition from the effective antenna area to the farfield cone. The new definition for a nearfield gain is used to derive the equation for rain echoes from 1m to 10,000 m. It is shown that in contrast to the behaviour in the farfield the echoes are proportional to the inverse of the gain. This is because antennas with high farfield-gain have a low nearfield-gain.