Abstract: Bistatic synthetic aperture radar (BiSAR) offers flexible observation geometries and distinctive scattering information by spatially separating the transmitter and receiver. However, its complex transmitter–receiver geometry makes the wavenumber-domain spectral support difficult to characterise intuitively, thereby complicating the analysis of two-dimensional resolution mechanisms. This study investigates the formation of spatial and planar spectral support regions for general far-field BiSAR configurations. The direction and magnitude of the spatial wavenumber-sampling vector are analysed to construct an equi-frequency sphere model (EFSM), which characterises the annular-sector-like geometry of the two-dimensional spectral support region. Based on this spectral-support geometry, relationships are established between the relevant geometric parameters and the range resolution, azimuth resolution, and angle between the two resolution directions. This formulation provides a geometric interpretation of two-dimensional BiSAR resolution in the wavenumber domain. Simulation results show that the predicted resolution trends agree well with those obtained using the conventional gradient method and point-target image measurements. These results support the validity of the spectral-support interpretation and provide an intuitive basis for BiSAR resolution analysis and geometric configuration design.