A review of present knowledge of the ionospheric propagation of very-low, low- and medium-frequency waves

The paper deals with the ionospheric propagation of waves of frequencies less than 3 Mc/s and is confined mainly to transmission over medium and long distances (greater than 500 km). The results of investigations on low and very-low frequencies (less than 300 kc/s) are summarized under the headings of field strength, phase and polarization measurement, the effects of ionospheric disturbances, the study of local ionospheric characteristics and theoretical studies. Since our knowledge of medium-frequency propagation was adequately summarized in a report64 in 1937, only work done since that date has been considered here. An important item of the recent work is the study of ionospheric cross-modulation. The paper concludes with a summary of our present knowledge of the subject and recommendations for further studies.