The Dwindling High-Frequency Spectrum

Abstract

The bulk of the world's long-distance communications presently takes place in the high-frequency radio spectrum (between approximately 3 and 30 Mc), by means of ionospheric reflection. The ionosphere is formed mainly by ultraviolet radiation emitted from the sun, and its capability to reflect HF radio waves varies diurnally, seasonally, geographically and throughout the sunspot cycle. While the physical nature of sunspots and the cause of the 11-year sunspot cycle are not yet clearly understood, it is known that solar activity is a reliable index of the total available bandwidth, or propagationally useful capacity of the HF radio spectrum. During periods of high solar activity, as occurred between 1957 and 1959, the ionosphere is capable of reflecting radio waves over a very wide range of frequencies between approximately 3 and 50 Mc; when the solar cycle declines to low levels, frequencies above 20 Mc may seldom be reflected. This paper points out that the present sunspot cycle, which climbed to an unprecedented peak during early 1958, is now declining. This decline is expected to continue until the cycle reaches a minimum sometime during 1965. There are also indications that the present cycle may be followed by three cycles of relatively low maxima, which could result in unusually low solar activity for the remainder of the century. Such a drastic reduction in solar activity would be accompanied by a similar reduction in the amount of propagationally useful HF spectrum. The spectrum will be compressed more or less linearly towards the lower frequency end as the solar cycle declines. During the forthcoming years of low solar activity the HF spectrum may be reduced by more than one-half that available during the recent years of high sunspot count (1957-1959). Such a reduction in the traffic handling capability of the HF spectrum, coupled with the ever increasing world-wide demands for additional HF circuits, leads to the conclusion that the HF radio spectrum will become progressively less useful for communications during the years ahead, and that the amount of available useful spectrum will fall far short of meeting the anticipated demands. To avoid communications chaos which could easily result from such a condition, this paper concludes by urging that space satellite communication systems, intercontinental microwave links and expanded underwater multichannel cables as well as other communication systems not dependent upon the vagaries of the ionosphere, be developed at an accelerated pace.