Exaggerated illustration of the visibility phase perturbations due to the weak cosmic signal δV_j (green arrow). The horizontal and vertical axes represent the real and complex axes of the complex plane, respectively. The red arrow represents the foreground visibility, V_j^F, and the blue arrow represents the sum of the foreground and cosmic H I 21 cm visibilities, V_j = V_j^F + δV_j. The resulting phase perturbation is δϕ.