Using nearfield measurements of antenna field patterns, the magnitude and phase of the current can be determined for every element of a phased array antenna. Analysis will be performed for an antenna slot array fed by rectangular waveguides operating in the TE10 mode. Each element will be described by using a magnetic current, which will in turn be used to calculate the corresponding electric field. A transistion matrix (T-matrix) will be formulated to relate the nearfields directly to the element magnetic currents. The Conjugate Gradient Method will be used to solve the resulting matrix equation since the transition matrix is rectangular.