From the beginning of 1976 the standard type CW Doppler sounder at Sugadaira has been used to study the spectra of frequency shifts of the Doppler echoes from the F-region ionosphere over central Japan. Some of the obtained data samples, with interpolated sections, are subjected to maximum entropy spectral analysis. The resultant power spectrum densities have revealed that the small-scale fluctuations of the Doppler signals contain several dominant periods in the range of 4 to 98min.