adio propagation via the ionosphere is a fascinating and important means of long-distance radio c... more adio propagation via the ionosphere is a fascinating and important means of long-distance radio communication. Thousands of hams and commercial operators use the ionosphere every day to make contacts over vast distances. To effectively use these propagation modes to their fullest, however, we must understand the physics behind the magic. Knowing when to listen, the best frequencies to use and where signals might come from enable experienced DXers to work stations when less-experienced operators come up short. In fact, knowledge of propagation and a " feel " for conditions and what each band might produce are valuable commodities for any radio operator. The Atmosphere Before we look at how signals are reflected by the ionosphere we must see where these reflections take place and how the reflecting areas are formed. The atmosphere can be split into a variety of different layers according to their properties. The most commonly used names are shown in Figure 1. Here we can see that the troposphere is the part of the atmosphere closest to the ground, extending to a height of about 10 km. At altitudes between 10 and 50 km we find the stratosphere, which contains the famous ozone layer at a height of about 20 km. For shortwave communication the ionosphere is the most im-Radio Waves and the Ionosphere Although hams are required to have a passing familiarity with the physics of the ionosphere, a more intimate understanding can make or break your enjoyment of the hobby. This primer will fill in some of the blanks and start you on a fascinating journey. portant, although the troposphere plays a key role in VHF and UHF communication. The ionosphere crosses several meteorological layers and extends from about 50 to 650 km. The Ionosphere The ionosphere is so named because it is a region in the atmosphere where ions exist. In most areas of the atmosphere molecules are in a combined state and remain electrically neutral. In the ionosphere, however, solar radiation (mainly ultraviolet light) is so intense that when it strikes gas molecules they split—ionize—and an electron is set free. What remains is a positive ion (a molecule that is " missing " an electron) and a free electron. Although ions give their name to the region, free electrons actually affect radio waves. The number of electrons starts to increase at an altitude of about 30 km, but the electron density isn't sufficient to affect radio waves until about 60 km. We often think of the ionosphere as having a number of distinct layers. This is convenient for many explanations, but it's not entirely accurate as the entire ionosphere contains ionized molecules (and free electrons). Instead, the layers are best thought of as peaks in ionization levels. To quickly identify the layers, peaks or regions, we refer to them by the letters D, E and F. (There is a C layer, but its level of ionization is so low that it has no effect on radio waves.) Layers The D layer is the lowest, at altitudes between 50 and 80 km. It is present during the day when radiation is beaming in from the sun. Because the density of the air is still high at this altitude, however, ions and electrons recombine relatively quickly. After R Figure 1—Areas of the atmosphere. Figure 2—Typical electron distribution (day and night).
adio propagation via the ionosphere is a fascinating and important means of long-distance radio c... more adio propagation via the ionosphere is a fascinating and important means of long-distance radio communication. Thousands of hams and commercial operators use the ionosphere every day to make contacts over vast distances. To effectively use these propagation modes to their fullest, however, we must understand the physics behind the magic. Knowing when to listen, the best frequencies to use and where signals might come from enable experienced DXers to work stations when less-experienced operators come up short. In fact, knowledge of propagation and a " feel " for conditions and what each band might produce are valuable commodities for any radio operator. The Atmosphere Before we look at how signals are reflected by the ionosphere we must see where these reflections take place and how the reflecting areas are formed. The atmosphere can be split into a variety of different layers according to their properties. The most commonly used names are shown in Figure 1. Here we can see that the troposphere is the part of the atmosphere closest to the ground, extending to a height of about 10 km. At altitudes between 10 and 50 km we find the stratosphere, which contains the famous ozone layer at a height of about 20 km. For shortwave communication the ionosphere is the most im-Radio Waves and the Ionosphere Although hams are required to have a passing familiarity with the physics of the ionosphere, a more intimate understanding can make or break your enjoyment of the hobby. This primer will fill in some of the blanks and start you on a fascinating journey. portant, although the troposphere plays a key role in VHF and UHF communication. The ionosphere crosses several meteorological layers and extends from about 50 to 650 km. The Ionosphere The ionosphere is so named because it is a region in the atmosphere where ions exist. In most areas of the atmosphere molecules are in a combined state and remain electrically neutral. In the ionosphere, however, solar radiation (mainly ultraviolet light) is so intense that when it strikes gas molecules they split—ionize—and an electron is set free. What remains is a positive ion (a molecule that is " missing " an electron) and a free electron. Although ions give their name to the region, free electrons actually affect radio waves. The number of electrons starts to increase at an altitude of about 30 km, but the electron density isn't sufficient to affect radio waves until about 60 km. We often think of the ionosphere as having a number of distinct layers. This is convenient for many explanations, but it's not entirely accurate as the entire ionosphere contains ionized molecules (and free electrons). Instead, the layers are best thought of as peaks in ionization levels. To quickly identify the layers, peaks or regions, we refer to them by the letters D, E and F. (There is a C layer, but its level of ionization is so low that it has no effect on radio waves.) Layers The D layer is the lowest, at altitudes between 50 and 80 km. It is present during the day when radiation is beaming in from the sun. Because the density of the air is still high at this altitude, however, ions and electrons recombine relatively quickly. After R Figure 1—Areas of the atmosphere. Figure 2—Typical electron distribution (day and night).
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