A dielectric cone radiator sub-reflector assembly for a reflector antenna with a waveguide supported sub-reflector is provided as a unitary dielectric block with a sub-reflector at a distal end. A waveguide transition portion of the dielectric block is dimensioned for insertion coupling into an end of the waveguide. A sub-reflector support portion of the dielectric block and the waveguide transition portion provided with a plurality of longitudinal ribs and grooves coaxial with a longitudinal axis of the assembly; the longitudinal grooves open to a proximal end of the dielectric block. The unitary dielectric block may be manufactured as a single contiguous monolithic portion of dielectric material via injection molding.