Temporal-frequency spectra for plane and spherical waves in a millimetric wave absorption band

Complete analytical expressions for the temporal power spectral density functions in a millimetric wave absorption region for plane and spherical waves have been developed for both amplitude and phase fluctuations due to atmospheric turbulence. Asymptotic expressions for both high and low scintillation frequencies are derived. Theoretical expressions for the differential phase power spectrum (i.e. the phase difference between two frequencies) are also presented. Experimental results of amplitude and differential phase scintillations measured on a 4.1-km link across central London, are presented. Results show that the plane wave case gives the best agreement with theory for this particular link. It is also shown that neglecting the cross-spectral density term at the higher scintillation frequencies for the differential phase spectrum can lead to a large difference between the theoretical and experimental power spectra. In particular, for a small frequency separation and a large value of the outer scale of turbulence, the highest scintillation frequencies are too low to neglect the cross term