In the reactive near field (very close to the antenna), the relationship between the strengths of the and fields is often too complicated to easily predict, and difficult to measure. Either field component ( or ) may dominate at one point, and the opposite relationship dominate at a point only a short distance away. This makes finding the true power density in this region problematic. This is because to calculate power, not only and both have to be measured but the phase relationship between and as well as the angle between the two vectors must also be known in every point of space.