Studies of the Doppler frequency dispersion on a multiple high-frequency phase stable propagation link were made at a low-latitude site under a variety of geomagnetic disturbance conditions. Frequency stability of the system enabled us to measure ionospheric-induced Doppler frequency changes as low as 0.01 Hz. The average Doppler frequency changes during development of the reflecting region at dawn can be characterized consistently by an exponential decrease of positive frequency versus time as the critical frequency of the F-region increases. The power spectral density of the fluctuations in Doppler frequency was usually nearly white over the fluctuation period range 1 < τ < 20 minutes. The power spectral density magnitude correlates well with the planetary magnetic disturbance index Kp summed over the period of the observations. This result suggests that the observed effects are related to irregular movements of the F region. Occasional coherent-appearing spectral peaks were observed that may be a manifestation of wave energy in the acoustic window propagated to ionospheric altitudes. No obvious relationship between the appearance of these coherent disturbances and the tropospheric weather pattern could be found.