Abstract: An extended interaction output circuit is provided for interacting with a modulated electron beam and for outputting RF electromagnetic energy. The circuit comprises a plurality of linearly disposed cavities having an axially extending beam tunnel to permit the traveling therethrough of the modulated electron beam as well as to couple electromagnetic energy between the successive cavities. Each of the cavities is separated by an annular disk having a hole providing the axial beam tunnel. The hole diameters increase in steps so that the impedance of the successive cavities decreases along the axial extent of the circuit. The diameter of the successive cavities is also increased as the associated width is decreased to maintain the same mid-band resonant frequency. The linearly disposed cavities act as an RF filter having successively tapered impedances to reduce reflections of the electromagnetic energy propagating through the circuit.