of generally circular shape and fabricated to be inserted into and seal the open end 14 of the conductive coaxial resonator 12 thereby confining all of the microwave energy within the coaxial resonator the insulator element 18 is provided with an inwardly extending portion 20 having a circular aperture 22 centrally disposed therein which functions as a receptacle and support for the anti node end 26 of an elongated cylindrically shaped resonant element 24 which preferably is one quarter wavelength long the other end 28 of the resonant element 24 is provided with an aperture 30 which functions as the input voltage receiving terminal and is in electrically conductive contact with the input terminal means 32 at point 33 via an electrically conductive wire path which forms the inner conductor of a coaxial input line the coaxial input line includes an outer conductor 35 and a dielectric 34 the edge portion 36 of the end 28 of the resonator element 24 is provided with a first annular ring 38 transverse to the longitudinal axis 40 of the resonant element 24 a second annular ring 42 is displaced from and in juxtaposition with the first annular ring 38 and coaxially aligned therewith and connected to the closed end 16 of the coaxial resonator.