The coaxial vircator generates microwave radiation by injecting a radial electron beam into a cylinder such that the space-charge limit of the region is exceeded. A virtual cathode forms and oscillates in radial position and amplitude, generating microwaves which are extracted through an attached waveguide with a circular cross-section. The cathode is a cold, field-emitting material applied to the inside of a grounded cylinder. The anode is a semi-transparent cylinder located inside of, and concentric to, the cathode cylinder. The anode cylinder is pulsed positive. Microwaves were produced for most of the tested geometries and when peak microwave powers are divided by peak electron beam powers, efficiencies of around 1% were calculated for the best case. However, the diode is not impedance matched to the pulsed power driver and reflections cause a second, much lower pulse of voltage to be applied to the diode. During this second pulse, the microwave power is at nearly the same level as during the primary pulse. Efficiency calculations during this second pulse are on the order of 10-20%. Based on this result, it may be possible to develop a source which generates long-pulse, high efficiency microwaves based on the coaxial vircator geometry.