We report on theoretical studies to demonstrate the conceptual feasibility of a high‐power sheet‐beam klystron operating at 11.4 GHz. Numerical simulations are used to investigate beam dynamics and rf issues. Specific problems with the gun design, magnetic focusing, and rf coupling at the output gaps unique to the strip beam geometry of the SBK are considered. Very high efficiencies have been calculated for several values of perveance per square beam element of the SBK. We have found that a lowg‐perveance, x‐band, sheet‐beam klystron is capable of producing over 200 MW of power at 65% efficiency, using a flat electron beam with a reasonable size of 0.4 cm×16 cm, a moderate voltage of 400 Kev, a strip gun compression ratio of 10:1, a cathode loading of 124/cm2, a uniform focusing field of 3 KG, and a double‐gap standing‐wave output cavity. With only a constant longitudinal focusing field, the calculated efficiency of the SBK is among the highest of current or proposed HPM sources. (AIP)