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We mainly focus on the role played by the antenna geometry in the direction estimation performance for a direction finding (DF) bistatic multiple-input multiple-output (MIMO) radar. First, we introduce the signal and the noise models which satisfy the space-time separability conditions. Then, we derive an expression of the Cramér-Rao lower bound (CRLB) on target parameters with the antenna locations for a single point target in the far-field scenario. We show that, under the space-time separability conditions, the temporal dimension parameters (range and range rate) and spatial dimension parameters [direction-of-departure (DOD) and the direction-of-arrival (DOA)] are uncoupled. We also show that DOD and DOA are uncoupled because of their spatial independence. In addition, we derive a set of the necessary and sufficient geometrical constraints for the uncoupled direction estimation which is based on the diagonal Fisher information matrix (FIM). We present that, when both the transmit and the receive antenna location parameters have equivalent auto-moment-of-inertia tensors of x and y axes, and zero cross-moment-of-inertia tensors of x, y and z axes, the bistatic MIMO radar system with uncoupled direction estimation can be obtained. The corresponding conditions are extended to monostatic MIMO radar case for their commonness of the colocated antenna geometry. In numerical example section, we show several representative antenna geometries to illustrate the antenna geometry conditions derived in this paper.