Abstract: A filter for a telecommunication system, designed to suppress a signal frequency F.sub.a and to transmit an adjoining frequency F.sub.b without significant attenuation, comprises a resonant cavity of prismatic shape longitudinally traversed by a conductive coil wound on a hollow dielectric core into which an adjusting screw is inserted. One end of the coil is grounded at the cavity wall while its other end is left unconnected and forms with the adjusting screw a variable capacitance C.sub.d with distributed constants; a lumped-constant tuning capacitor C.sub.1 is connected between ground and an input lead connected to an intermediate tap on the coil, the inductance L.sub.2 " of that lead and the inductance L.sub.2 ' of the coil portion between the tap and ground forming an inductive voltage divider bridged by the lumped-constant capacitor C.sub.1. The remainder of the coil, lying between the tap and the unconnected end, forms an inductance L.sub.1 which together with the distributed-constant capacitance C.