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Experiments are described in which a 5-Â¿ 1-MeV pulser drives virtual-cathode systems optimized to emit microwave radiation in the X band. Radiation is extracted both axially in TM modes and radially in TE modes. The threshold for microwave emission is found to be several times the space-charge limited current. The radiating frequency is approximately proportional to the electron beam plasma frequency with a proportional constant of (27Â¿)1/2. Anode/cathode spacing is varied to demonstrate the frequency tunability of the virtual cathode as a microwave source. The radiation is broad band and frequency chirping is observed. Radiation patterns of the axially extracted radiation are described. The instantaneous efficiency of energy conversion from electrical to microwaves is ~2 percent. External excitation from an external source does not alter microwave generation.