Radar works by sending high energy pulses of radio waves. I found this in the museum at Croome. It was used in wartime Allied radar devices. A resonant cavity in this case refers to the idea of reflecting electromagnetic waves back of one side of a cavity. They interfere with the incident waves and can set up a stationary wave in the cavity if the width of the cavity is an integer multiple of half wavelengths. I have read that electrons are whirled around inside the device, which is a vacuum tube, and go past the openings of a series of cavities, in which they excite waves. This is all very new to me so more research needed.
