A method for manufacturing a radio frequency (RF) applicator which includes covering a ceramic insert with a coating, wherein the ceramic insert has dimensions that substantially match an internal volume of an open-ended, hollow waveguide, and wherein the ceramic insert has a recess therein configured to accept a radio frequency emitter, heating the waveguide to a temperature that is above a melting point of the coating, placing the coated ceramic insert into the internal volume of the heated waveguide, wherein the internal volume is completely filled except for the recess, and cooling the waveguide, ceramic insert, and coating to a temperature below the melting point of the coating so that the coating solidifies and fills gaps between facing surfaces of the insert and the waveguide.