[en] The low frequency part of the spectrum of the electromagnetic radiation arising at a change of the velocity direction of an ultrarelativistic charged particle within a certain section of the force field of finite length l is investigated. It is shown that the correction to the spectrum-angular density of radiation corresponding to an instantaneous change of velocity has magnitude of an order of (l/Lsub(F)) where Lsub(F) = 2cy² /ω(1+y²theta=2) is the formation length, ω is the radiation frequency, y is the Lorentz-factor, theta is the angle between the longitudinal velocity of a particle and the direction of radiation. At the same time the correction to the spectrum density, after integrating over angles, turns out to be proportional to the length of an intermediate interval, but not to the square of this length. (orig.)