This paper outlines a method for obtaining the relation between the singly scattered signal and the Fourier spectrum of medium dielectric permittivity fluctuations, with regard for the fact that the scattering volume is determined by antenna patterns and is not small. On the basis of this equation we obtained the radar equation relating the scattered signal spectrum to the spatial spectrum of fluctuations. Also, a statistical radar equation is obtained that relates the mean statistical power of the scattered signal to the spectral density of the dielectric permittivity fluctuations without a classical approximation of the smallness of the irregularities' spatial correlation radius. The work deals with the bistatic sounding case, when the exact forward scattering and exact backward scattering are absent and sounding signal has sufficiently narrow spectral band for scattered volume to change slowly on ranges of Fresnel radius order. The statistical radar equations obtained differ from the classical ones in the presence of coherent structures with big correlation radii, and so the received signal spectrum can differ from the intrinsic spectrum of irregularities.