Propagation

The last post on the 15 meter antenna brought up the radiation angle and the height of the ionosphere. I thought I would switch gears here and discuss some radio wave propagation. To make effective use of any antenna or to choose the right antenna it helps to know something about propagation or how a radio wave gets from here to there.
Most of our high frequency radio communication depends on the ionosphere bending the radio signal so it will return to earth. The ionosphere is an area of the upper atmosphere where the air is much less dense than here on the surface of the earth. It is also gets the full force of the radiation from the sun. It is the radiation from the sun that causes the air molecules to form ions. Positive and negative charges are created by the radiation from the sun. The strength of the radiation from the sun varies considerably. To put it simply, the radiation varies somewhat predictably with the number of sunspots following an eleven-year cycle. In other words there is usually a peak in the number of sunspots every eleven years. This peak in sun activity coincides with extremely good radio conditions on the higher frequency amateur bands such as 20, 15 and 10 meters.
In addition to the effect of the sun spot cycle, there is a seasonal and daily variation. The daily variation is mostly due to the fact that as the earth rotates, areas of the atmosphere or ionosphere are in view of the sun or being shielded by the sun. We find that during the day the ionosphere is “stronger” than at night. This is really a two edged sword. The higher daytime ionization helps return the higher frequencies such as 14, 21 and 28 MHz back to earth. It has a detrimental effect, however, on the lower frequencies (longer wavelengths). During the day a dense lower ionosphere layer forms. It is called the “D” layer. This strong daytime or “D” layer of the ionosphere absorbs the longer wavelengths. For this reason there is practically no sky wave propagation on 160 or 80 meters during the day. The 40 meter band is right in the middle. There is not much absorption on 40 meter signals so there is some sky wave propagation on 40 during the day. During the evening the “D” layer disappears and there is good sky wave propagation on the 160 and 80 meter bands and signals even get stronger on 40 meters.
Very high sun activity causes so much ionization that the 20, 15 and 10 meter bands can stay open for sky wave propagation 24 hours a day. Except for these periods of high sunspot activity, these bands are mostly considered “daytime” bands.
In general the higher the frequency, the more ionization necessary for sky wave propagation. This means that in the evening 10 meters will close first, then 15 meters will close and the last high band to close will be 20 meters. In the morning the reverse is true. 20 meters opens first, then 15 and then 10. The times all depend to how much radiation there is from the sun at any given time.