An RF feedthrough has an electrically insulative cone that is hollow having first and second openings at first and second ends having first and second diameters. The first diameter is larger than the second diameter, defining a tapered sidewall of the cone to an inflection point. A stem is coupled to the second end of the cone, and passes through the first opening and second opening. A flange is coupled to the first end of the cone and has a flange opening having a third diameter. The third diameter is smaller than the first diameter. The stem passes through the flange opening without contacting the flange. The flange couples the cone to a chamber wall hole. Contact portions of the cone may be metallized. The cone and flange pass the stem through the hole while electrically insulating the stem from the wall of the chamber.