Abstract
The variability of the critical frequency of the F2-layer (f0F2) has been investigated by finding the standard deviation (σ) of the hourly values of f0F2 from the monthly mean values (f). The ratio in low as well as in high sunspot years has been computed for three representative months and its daily variation studied at three stations, namely Kodaikanal, Ahmedabad and Tokyo. The proportionate variability in f0F2 is generally found to be more during night time than during day time. The deviations in f0F2 are found to be more in low sunspot years than those in high sunspot years for both day time and night time f0F2 at Ahmedabad, for night time f0F2 at Kodaikanal and for day time at Tokyo. It is indicated that the present practice of choosing optimum working frequency (OWF) as 85 per cent of the predicted maximum usable frequency (MUF) may not effectively take care of the day-to-day fluctuations in the MUFs. A higher margin of safety has to be considered for better reliability of transmissions via the F2-layer at mid latitudes.
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