(a) Equation (9.6) is derived under the assumption that the motion is in the direction of the line of sight from sending antenna to receiving antenna. Find this field under the assumption that there is an arbitrary angle between the line of sight and the motion of the receiver. Assume that the time range of interest is small enough that changes in can be ignored.
(b) Explain why, and under what conditions, it is reasonable to ignore the change in over small intervals of time.