This paper is devoted to theoretical and experimental investigations of high power microwave Cherenkov oscillators using of high current nanosecond electron beams. The influence of non synchronous waves and additional (non Cherenkov) resonance on the process of microwave generation in 3 cm relativistic BWO are discussed. Experimental data presented in this paper show that the efficiency of 3 cm relativistic BWO can achieve 40% and pulsed microwave power - 500 MW. The enhancement of microwave power and efficiency of microwave generation was reached by creating of the non uniformity of the synchronous wave phase velocity and that of the coupling coefficient between the electrons and the electromagnetic wave. We experimentally studied the oversize Cherenkov oscillators in mm wavelength. One of them was 8 mm 120 MW orotron with TM$-13$/-mode of the circular waveguide. Another one was two stage two electron beams 8 mm Cherenkov oscillator. The maximum of microwave power in this experiment was 600 MW, pulse duration was 10 - 15 ns. All experiments were carried out by high current electron accelerators of type SINUS. The performance parameters of our accelerators are the following: electron energy from 100 to 2000 keV, beam current from 1 to 20 kA, pulse duration from 5 to 50 ns, repetition rate up to 200 - 300 pps.
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