An apparatus includes a horn having a horn body including at least one horn sidewall defining a first opening that tapers down to a second opening in a direction of elongation and a port that is tubular and dimensionally uniform transverse to the direction of elongation and extends in the direction of elongation from a first port end that is in communication with the second opening to a second port end that defines an external opening. A dielectric rod includes a rod length extending between a first rod end and a second rod end with the first rod end extending through the external opening of the second port end and into the port cavity such that the first rod end is in a spaced apart relationship from the port sidewall along the light path.