The output radiation of a dc to ac radiation converter is characterized. A relativistic ionization front passing through a capacitor array of periodd=1cm produces short pulses of tunable radiation between 39 and 84 GHz with a gas pressure between 0 and 30 mT. The frequency spectra of the produced pulses are discrete and exhibit full widths at half maximum between 12&percent; and 28&percent;, consistent with the expected width for six cycles’ pulses. An upper bound of 750 ps (detection bandwidth limited) is placed on the pulse widths. These are the shortest pulses produced by a source of coherent radiation in this frequency range. ©1998 American Institute of Physics.