Microwave generation by a relativistic electron beam injected through a screen or foil into a cylindrical waveguide is investigated. The electron beam is generated by a field‐emission diode with an anode‐cathode gap of ∼1 cm. The electron density exceeds the space‐charge limit for the beam, which forms a virtual cathode inside the waveguide. The instability of the virtual cathode causes oscillations which couple to bunched reflexing electrons trapped in the potential well. Frequency locking between the oscillating virtual cathode and the reflexing electrons is found to generate a much purer, narrowband microwave output. Using diode mismatch to increase the beam impedance is shown numerically to be a useful technique to improve the microwave generation efficiency and spectral purity. The peak conversion efficiency is calculated to be 2.2% at 7 GHz. The numerical results are compared to experimental results, and the agreement is within 8% in frequency and 25% in peak power. Experimentally, we describe the qualitative effects of different cathode materials and dimensions. The anode materials used were screens and foils of various transparencies and thicknesses. Microwave generation efficiency dropped as the mean scattering angle increased with the thicker foils. Also, the increased transparency of fine screens caused an upward frequency shift due to the increased electron density.