Experimental results are presented demonstrating a substantial improvement in output resonator bandwidth of an extended interaction klystron (EIK) by utilization of two overlapping modes. The EIK output resonator -3 db bandwidth was greater than 6% at the 170 Kw level at low S-band. The output circuit consists of a 4 gap π mode resonator with the adjacent

mode located within the response of the π mode. Efficient interaction was obtained not only in the π mode, but also in the transition region between the two modes. A qualitative discussion of the operation of an EIK in the mode transition region is given.