To transport electromagnetic energy at high power levels, a coaxial-to-waveguide power combiner/divider comprises a length of single-conductor closed waveguide terminated at one end by a conductive end plate. A plurality N of holes is formed in the end plate. A conductive matching plate is positioned within the waveguide opposite and spaced apart from the conductive end plate and spaced apart from the inner walls of the waveguide. A plurality of coaxial input/output ports each comprise an outer conductor that is electrically and mechanically terminated at the end plate about one hole and an inner conductor that extends through the associated hole into the waveguide and is electrically and mechanically terminated at the underside of the matching plate. The location and geometry of the matching plate and physical arrangement of the N ports are chosen so that the sum of the direct reflection and the N−1 coupled reflection contributions are small.