An RF applicator has an open-ended waveguide having an aperture and a dielectric cone extending through the aperture and is electrically connected to an RF source that is configured to generate RF energy pulses. A top fin is mounted to an inner top surface of the waveguide and comprises a conductive material, is electrically connected to the RF source, and has dimensions configured to optimize a bandwidth that the RF applicator applies to tissue. A bottom fin is mounted to an inner bottom surface of the waveguide and comprises a conductive material electrically isolated from the RF source, with dimensions configured to optimize a bandwidth that the RF applicator applies to tissue. A dielectric cone is inserted into the waveguide. A filler material between inner surfaces of the waveguide and the solid dielectric cone can fill gaps and has a dielectric constant similar to the dielectric cone.