Phase and amplitude fluctuations spectra of the HF point source field in the fluctuating reflection channel are investigated on the basis of Rytov's method generalized to the case of inhomogeneous ionosphere. Integral representations expressing the fluctuation spectra are transformed to a form that is more convenient for numerical estimations owing to the reduced number of integrals. The resulting expressions are evaluated numerically with controlled accuracy for the three-dimensional power type model of dielectric permittivity fluctuations. The results of calculations show the oscillatory behavior of frequency spectra apparently having Fresnel diffraction origin. The phenomenon is investigated in detail, and results are presented.