A magnetron output arrangement includes a coaxial output adapted to be coupled in an endfire configuration to a rectangular waveguide. The waveguide has a flange for mating with the magnetron output arrangement. The coaxial output includes a central conductor. The output arrangement further includes a plate located in a predetermined position with respect to the central conductor. The plane of the plate is arranged to be substantially perpendicular to the axis of the coaxial output. The plate is located a predetermined distance from the end of the central conductor, such that the coaxial output in use couples with the plate so that the output coupling is substantially the same regardless of the type of waveguide flange employed.