Abstract: Vacuum SHF apparatus comprising a means for forming a charged-particle beam, an accelerating tube for imparting relativistic energies to the charged-particle beam, and a deflection yoke for circular scanning of the beam at an input signal frequency, the accelerating tube and the deflection yoke being arranged coaxially with the beam forming means and successively along the path of the charged-particle beam. The apparatus further comprises an additional magnetic deflection yoke for deflecting the scanned beam outwardly from the longitudinal axis of the apparatus, and an annular travelling-wave resonator. The additional magnetic deflection yoke is preferably made as a cone whose vertex faces the charged-particle beam forming means.