I. Introduction
In phased array radars with electronic beam-steering, excitation phase and amplitude of the array elements are adjusted in order to shape and point the array pattern along the desired direction. Periodic calibration of the gain and phase of an array is essential because it reduces the array errors. The smaller the array errors are, the closer the implemented radiation pattern is to the theoretical pattern of the phased array. Thus, in either Tx or Rx mode, errors in the relative phase or amplitude are detected and suitably adjusted for individual elements [1 –3].