The KALI-5000 pulse power system was utilized to generate high-power microwaves using a coaxial virtual cathode oscillator. The typical electron-beam parameters were 290 kV, 23 kA, and 100 ns, with a few hundreds of amperes per centimeter square current density. Experiments were conducted using a graphite cathode for two different radial anode-cathode gap distances to investigate the dependence of microwave output magnetic field and frequency on diode voltage and current waveform. It was found that the coaxial vircator always has more than one frequency component. It was shown that the microwave B-field and the frequency do not depend on the time-varying impedance and perveance of the diode. Electron-beam emission area, uniformity, and the anode-cathode gap play a major role in deciding the power density and frequency at the point of measurement. The highest microwave power is emitted when all the power is delivered into a single frequency at least for a certain time of the accelerating pulse. © 2012 IEEE.