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The theoretical and experimental results of a study of the factors which determine the efficiency of backward-wave oscillators are described. The dependence of power output upon space charge, circuit loss, beam thickness, velocity spread, and circuit mismatches is determined by a combination of theoretical and empirical means. In addition, the effect of circuit mismatches upon the starting current and frequency is discussed. The study shows that efficiency can be increased by increasing either the gain parameter C or the space-charge parameter Â¿q/Â¿. Circuit loss, beam thickness, and velocity spread are found to decrease the efficiency. The use of the results reported here permits the designer to control the factors affecting the power output and predict the efficiency of a tube with reasonable accuracy before it is built.