The purpose of this paper is to derive a general analytical expression for the correlation coefficient of signals received by two different antennas as a function of the distance between them (baseline distance), the signal bandwidth, and the antenna beamwidth. The two antennas are supposed to be part of a radar system for detecting meteorological phenomena such as rain and clouds. One of the antennas transmits the radar signal and receives the echoes; the other antenna only receives the echoes. The good functioning of such a radar system depends on the correlation between the two antenna signals. The exact analytical framework derived here has been recently used in practice to assist in the design of a meteorological radar system for Orbisat Indústria e Aerolevantamento S.A.