Vircators (Virtual Cathode Oscillators) are sources of short-pulsed, high power, microwave (GHz) radiation. An essentially dimensional argument relates their radiated power, pulse energy and oscillation frequency to their driving voltage and fundamental physical constants. For a diode of width and gap 10 cm and for voltages of a few hundred keV the peak radiated power cannot exceed ${\cal O} (\text{30 GW})$ and the broad-band single cycle radiated energy cannot exceed ${\cal O}(\text{3 J})$. If electrons can be accelerated to relativistic energies higher powers and radiated energies may be possible.

6 pp