<p>We present the results of theoretical and experimental studies of a relativistic backward-wave oscillator in the long-wavelength band of the centimeter-wave range (λ ≈ 10 cm). Using a nonlinear nonstationary model, weshow that the choice of the reduced high-frequency space charge parameter plays an important role in all options for increasing the oscillator efficiency. Numerical simulation using the macro-particle method and the experiments confirm the need to minimize the intense space charge fields. This key parameter decreases due not only to a decrease in the electron beam current, but also to an increase in the maximum transport current through the slow-wave structure of the backward-wave oscillator. This effect is achieved by bringing the electron beam closer to the structure walls in a nonuniform magnetic field. In this approach, partial current deposition on the slow-wave structure in the electron deceleration region has proven beneficial. The experiment employed the pulse-periodic regime of operation of the relativistic backward-wave oscillator with a clock frequency of up to 250 Hz at a carrier frequency of 3 GHz and a power of 1.0±0.1 GW in pulses with a duration of 23±1 ns. The corresponding degree of the beam–to–wave power conversion can exceed 50%.</p>