World Wide Communications and the Ionosphere

The High Frequency (HF) radio spectrum is defined to be from 2 to 30 MHz. It is this frequency range where radio waves reflect off the ionosphere. A station beaming a HF frequency radio wave at the horizon will have that wave reflected off the ionosphere and back down to earth about 1300 miles away. The earth also reflects HF waves. With 5 or 6 reflections a wave can reach the other side of the earth, thus allowing world wide communication.

The amount a wave is reflected depends on the intensity of the ions in the ionosphere, which in turn is dependent on the solar flux. That is, the amount of radiation being transmitted by the sun which is caused by sunspots. The more sunspots the more the sun radiation and the better HF waves travel around the earth.

There is a cycle of about 11 years in which the sunspots increase and decrease. We are now in the part of the cycle where the sunspots are once again high. Thus, it is relatively easy to communicate over distances which require wave reflections. The first graph below shows the solar flux from May 1990 to the July 2001. You will note that we have reached what would appear to be the peak of the solar flux. Thus the graph shows a complete solar sunspot cycle. The smooth line is a smoothed average (5th order regression).

Solar Flux -- May 1990 to July 1, 2001

The next graph shows data starting on January 1, 1995 which is near the minimum of solar flux for this above cycle. The smooth line is a smoothed average (5th order regression) as above.

Solar Flux -- January 1, 1995 to Present

Data as of August 18, 2002

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