An 11th-harmonic slotted peniotron has been designed for reducing the external magnetic field and maintaining relatively high efficiency. Small-signal theory is used to get the start-oscillation conditions for an axis-encircling electron beam. The device employs a 30-kV 1-A axis-encircling beam to interact with the 2π modes of a slotted 11-vane circuit. As an oscillator, the 11th-harmonic slotted peniotron is predicated to yield 8.6 kW of output power with an electron efficiency of 28.7% at 99 GHz.