The Cavity Magnetron

Radar’s usefulness is set by its wavelength: to see a small object you need a wavelength comparable to it, and to form a narrow beam from an antenna of practical size you need centimetres rather than metres. In 1939 nobody could generate serious power at centimetric wavelengths, because a valve stops working when the electron’s transit time across it becomes comparable with one cycle of the signal. Randall and Boot’s answer at Birmingham in 1940 abandons the whole idea of pushing electrons back and forth in step with a signal. A magnetic field bends the electron stream into a rotating spoke pattern, and that rotating space charge sweeps past a ring of resonant cavities machined into a solid copper block — inducing oscillation in them by proximity alone. Transit time stops being a defect and becomes the mechanism. The result made ten kilowatts at 10 cm from a device the size of a fist, and it is gated here because a magnetron is a genuine radiation and high-voltage hazard.