Very-long-wave phase differences between spaced aerial systems

The paper describes some investigations into the propagation of very-low-frequency radio waves, in which the variations of phase difference between signals received at two points about ten wavelengths apart were measured. Observations were made using two telegraph transmitters on frequencies of 17.2 kc/s and 18.4 kc/s at distances of 1030 km and 5650 km respectively; propagation with ground wave predominant, sky wave only, and both sky and ground wave, was observed. Two sets of measurements were made at each distance, to give an indication of seasonal variations. The results indicated that a navigational aid using these frequencies and a base line of about ten wavelengths would give a probable error of equivalent bearing of less than 0.2 deg near the median line of the system at these distances, and probably at most intermediate distances. Larger variations which were observed at certain times of the night appeared to be due to the sunrise or sunset line crossing the propagation path obliquely. An analysis of the periodicity of variations showed that observations would have to be averaged for a period of the order of half an hour to gain any appreciable increase in accuracy.