The study of interference due to scattering by hydrometeors within the common volume of two antenna beams, has centred previously on the scattering characteristics of the hydrometeors. Little attention has been paid to the nature of the common volume and its modelling, which can have order of magnitude effects on the magnitude of the interfering signal in extreme cases. The paper compares two existing models of the common volume which are currently in use with more accurate models developed within the text. Results are given for a variety of geometries in terms of the effective common volume; the common volume weighted by the antenna gain patterns. These demonstrate the severe limitations of the existing models. All current models assume the antennas to be aligned exactly, such that the beam axes intersect. A model is developed within the paper that allows for cases where the axes do not intersect. Results are presented for this arrangement showing that significant common volumes exist, provided the distance of closest approach of the axes is small.