21 MICROWAVE TRANSMISSION APPARATUS-NAMELY, MAGNETRONS; TRAVELING WAVE TUBES; BACKWARD WAVE OSCILLATORS; MICROWAVE DUPLEXERS AND DIPLEXERS; POWER DIVIDERS; MICROWAVE PRESSURE WINDOWS; MICROWAVE FERRITE DEVICES-NAMELY, CIRCULATORS, ISOLATORS, FILTERS AND PHASE SHIFTERS; MICROWAVE SOLID STATE TRANSMITTERS AND RECEIVERS; [ MICROWAVE REFRACTOMETERS; ] SOLID STATE POWER SOURCES; OSCILLATORS; MULTIPLEXERS; SWITCHES; FREQUENCY MULTIPLIERS; TRANSITIONS BETWEEN WAVEGUIDE AND WAVEGUIDE, BETWEEN WAVEGUIDE AND COAXIAL LINE, AND BETWEEN COAXIAL LINE AND STRIP-TRANSMISSION LINE; ROTARY AND MOTIONAL JOINTS IN WAVEGUIDE; HYBRID JUNCTIONS IN TRANSMISSION LINES; MODULATORS AND MIXERS; AND LIMITERS