A dielectric resonator (DR) is mounted on the base of a housing (1) and a coupling pin (K) is positioned between it and the side wall. The coupling pin has a part section (H) covered by a sleeve of coaxial material. The capacitive action of the pin in the vicinity of the resonator introduces an additional resonant effect. The geometry is such that the resonant effect lies below the fundamental frequency. A typical length ratio between the sleeve and the free pin region lH/ls is 1/3 to 2/3.
