Please note this article first appeared in the August1995 edition of MARA NEWS

HOW TO UNDERSTAND PROPAGATION

We Are Entering The Sunspot Low--What Does It Mean For You? by Kurt Feldmesser*

Propagation of HF Radio Signals- High Frequency (or short wave) radio signals can travel much further than line-of-sight by reflecting (actually refracting) from layers of gas, hundreds of kilometers above the Earth's surface. The signals bounce off these layers, or regions, because the gases are ionized by the sun's rays. Because of this, their effectiveness depends on the time of day and the time of year.

The day-time and summer months are better for the higher frequencies (say, above 1OMHZ) and night-time and winter are better for the lower frequencies. Propagation of VHF Radio Signals- VHF signals (above 30 MHz) are less affected by solar activity although at 5O MHz very long distant propagation can occur at a sunspot maxima. Periodically, solar flares on the sun release streams of high energy particles. These particles create auroras (curtains of ionized gas) in polar regions of the earth. An aurora can propagate VHF signals over long distances for periods of a few hours at a time. What Are Sunspots? Sunspots are areas of the sun which are less hot (about 3000 degrees C) than the rest of the sun's surface (about 6000 degrees C) and therefore appear dark. They produce intense radiation which make the ionosphere reflect radio signals. The spots last from a few days to a month or two and the quantity of sunspots is directly related to the effectiveness of the ionosphere in reflecting short-wave signals.

How To View Sunspots Safely- IMPORTANT WARNING: Never look at the sun directly through a telescope or binoculars, even with a dark filter added. This can cause permanent severe damage to your eyes! The safe way to look at the sun is by projection. Put a pin hole in a piece of thick cardboard. The image of the sun is then projected onto a piece of white paper. Sunspots should be clearly visible using this method. You can make an "elegant version" of this scheme but putting a large cardboard box (with a pin hole on one side and a white sheet of paper pasted on the other inside surface. After the box is over your head and your eyes dilate (in about 3 minutes), you will see a fantastic, bright image of the sun.

Where Do They Come From? Sunspots have been observed and recorded for three centuries, ever since Galileo (1564 -1642) turned the newly invented telescope towards the sky. In the nineteenth century Dr Rudolf Wolf of the Zurich Observatory corresponded with a number of astronomers forming a network of solar observers. This helped to maintain their records even when the weather made observation impossible in some areas. The sunspot count tended to depend on the size of he astronomer's telescope (in other words, how big an instrument he could afford) and this resulted in some rivalry and disagreement as to the 'true' number. In order to get over this problem, Dr Wolf multiplied all counts by a factor which took telescope size into account and allowed a figure of 10 times for spot 'groups'. The system is still in use. Among this network of observers, a German apothecary, called Heinrich Samuel Schwabe, made meticulous observations of the sun for the best part of twenty years and at the end of that time, in 1843, announced that sunspot numbers peaked every ten years.

The effect of this announcement was electrifying. Hundreds of records of past observations were dug up and graphs plotted to see if Schwabe's findings held true. We now accept eleven years as the average time between peaks. You may ask how does radio fit into all that? Well, it's like this: When Marconi transmitted his famous three dots across the Atlantic from Cornwall, England, to Newfoundland, Canada, in 1901, he proved that radio waves could do something that light waves could not,go around the curvature of the Earth. Radio Amateurs, some twenty years later, proved that short waves were even better for that task than the long waves all professionals were using at that time. Because of this, Amateurs have frequency allocations throughout the short-wave spectrum. However, one snag with short-waves soon became apparent. Short radio waves rely upon reflection in the ionosphere where the air is extremely thin. Because of this thinness, the ionosphere is strongly influenced by the energy reaching it from the sun. Daylight and darkness have a great effect on this gas, and the time of year affects how much solar energy can reach the ionosphere.

Charged atomic particles, ejected by the sun in the "solar wind" can cause magnetic "storms" in the ionosphere. High intensity ultraviolet and X-ray radiation from solar flares can totally block all short-wave communication for perhaps half an hour on the sunlit side of the Earth. This is known as a Dellinger Fade-out. Luckily, flares can also cause parts of the ionosphere temporarily to reflect signals above 1OO MHz which would not normally be reflected. This is an effect known as aurora. It is not surprising that the effectiveness of the ionosphere as a "mirror in the sky" follows the sunspot number in its eleven year cycle. Since the early thirties of this century, the ionosphere observatory have measured the effectiveness of the ionosphere as a reflector of radio waves, by measuring its height and maximum usable frequency (MUF).

*The author is a member of the Radio Society of Great Britain Propagation Studies Committee