In order to transfer electromagnetic (EM) energy efficiently from a relativistic klystron amplifier (RKA) to a steerable super-array, the issue of mode conversion must be considered, i.e., the transfer of electromagnetic fields from a coaxial-line waveguide to a number of rectangular waveguides. A special converter has been designed to solve this matching problem. The converter consists of several sections including a multi-fin converter, a fan- shaped converter, and a twisted-rectangular converter. The mode conversion in each of these sections has been studied analytically and experimentally. The optimal parameters for minimum energy loss have been found for the different converter sections and the power breakdown in some of the sections has been studied. Formulas for engineering design are presented.