NiBr₂, pure or Fe-doped, undergoes a phase transition from commensurate to incommensurate structure at T_IC=22 K. The incommensurate phase (helical magnetic structure) is accompanied by the appearance of a hexagon of satellites around the (003) and (101) magnetic reflections, which moves out from the commensurate peak as the temperature is lowered until it is complete at 4.2 K. The propagation vector in pure NiBr₂ lies in the basal plane a*b* and along the (110) direction whereas in doped NiBr₂ it can lie in almost any direction within the a*b* plane. A model that fits intensities distributed around a ring to those of the hexagon of satellites around the (003) magnetic reflection is discussed and used as a basis for calculations.