PROPAGATION
The faithful reader of this column knows that the backbone of worldwide, regional, and area communications is the ionosphere. The magic of shortwave is that highfrequency radio waves may be refracted by the ionosphere, extending the reach of these signals. The magic — the science, of course — is the variables and the physics involved. Without the ionosphere, we would not have such a wealth of opportunity to have communications worldwide, with our stations armed only with an antenna and a transceiver.
The Ionosphere
Earth’s atmosphere is a mixture of gases held to the surface of the Earth by gravity. These gases vary in density and composition as the altitude above the surface increases. As the atmosphere extends outward from Earth, it becomes thinner and blends with the particles of interplanetary space.
The first 60 miles of Earth’s atmosphere consists of a homogeneous mixture of various gases. This region is called the homosphere. Above the homosphere, where gases are no longer uniformly mixed, lies the heterosphere. Relatively more of the heavy gas molecules, such as molecular nitrogen and molecular oxygen (N2 and O2), are found near
the bottom of the heterosphere, while relatively more of the lighter gases such as hydrogen and helium are found near the top.
The atmosphere is also divided into four regions according to temperature trends: The troposphere, stratosphere, mesosphere, and thermosphere. The lowest region is the troposphere, and it extends from the Earth’s surface up to about 6 miles. The gases in this region are heavier than those in higher altitudes and include O2 and N2. The highest mountains are within this region, as is the high-altitude jet stream. Terrestrial weather is confined to this lower region, and it contains 90% of the Earth’s atmosphere and 99% of the water vapor.
The atmosphere above the troposphere is called the stratosphere, starting at about 6 miles up. Gas composition changes slightly as the altitude increases, in this layer. This gas reaches a peak density of a few parts per million at an altitude of about 16 miles.
