Interference and Sharing at Medium Frequency: Skywave Propagation Considerations

A considerable amount of work dealing with nighttime skywave propagation has been done; however, some areas remain to be explored. For example, short-term variation of field strengths has been overlooked. Daytime propagation is far from being quantitatively understood. Also, it is not quite possible to determine interfering signal levels which may be present for a small percentage of time with high accuracy. During sunspot cycles 18 and 19, the Federal Communications Commission conducted an extensive field-strength measurement program in the midlatitude areas of North America. These and other data are revisited with a new emphasis, namely interference and sharing. This paper discusses, among other things, determination of field strengths exceeded for different percentages of time (1 to 99%), diurnal variation of field strengths, favorable conditions for daytime skywave propagation, effect of magnetic storms, frequency dependence as observed at daytime and nighttime. *Views and opinions expressed in this paper are those of the author and should not be interpreted as official FCC policy.