The Ionosphere.
In the upper air around 80km (50miles) and higher, radiation from the sun strips electrons from oxygen molecules causing the mole-cules to become ionized forming the ionosphere. The ionized oxygen molecules and its free electrons float in space forming radio-reflec-ting layers. Ionization of the ionosphere varies by the time of day, seasons of the year, and the sunspot cycle. The strength of ionization also varies from day to day and hour to hour. Since the height of the ionosphere varies, the higher the ionized layer becomes, the farther the skip will be. The part of the earths atmosphere called the iono-sphere is divided into three layers. T
he three layers are, from lowest to highest, the D-layer, the E-layer, and the F-layer. Each layer has a different effect on HF radio propagation.
Being at a lower altitude, the D-layer molecules are squeezed closer together by gravity than those in higher layers, and the free electrons reattach to the molecules easily. The D-layer requires constant radiation from the sun to maintain its ionization. Radio waves at lower frequencies such as the frequencies of the AM broadcast band cannot penetrate this layer and are absorbed. The higher frequency signals are able to pass through the D-layer. The D-layer disappears at night causing AM broadcast stations to reflect from the higher layers. This is
why AM broadcast signals only propagate by ground wave in the daytime and they can be received from great distances at night. Like the broadcast band, the D-layer absorbs signals on 160m and to a lesser extent 80m during the day making those bands go dead. During solar flares, the D-layer becomes ionized so strongly that all high frequency radio waves are absorbed, causing a radio blackout.
E-layer propagation is not well understood. Being at a lower altitude than F-layer, the E-layer is responsible for summertime short skip propagation on the higher HF bands. The skip zone is around 1500km (1000 miles), but at times when the E-cloud covers a wide area in the summer, double hops can be seen. A double hop occurs when the signal reflects from the ionosphere, then returns to the ground, reflects from the ground back to the ionosphere where it is reflected back to the ground. A double hop can propagate the signal 3000km (2000 miles) or more. The E-layer forms mostly during the day, and it has the highest degree of ionization at noon. The E-layer like the D-layer disappears at night. Even so, sporadic-E propagation
can and does form at night. There is a minor occurrence of sporadic-E propagation during the wintertime. On rare occasions, sporadic-E propagation can surprise you by occurring anytime regardless of the sunspot cycle or the season of the year.
The F-layer is the highest layer and it is divided into two levels: F1 and F2. At night the F1 and F2 merge into one layer. During the day, the F1-layer doesn't play a part in radio propagation, but F2 does. It is responsible for most high-frequency long distance propagation on 20m and above. However, the F-layer makes it possible for you to work DX on the lower bands at night. Sunspots are responsible for the ionization layers and in years with high sunspot numbers, worldwide contacts can be made easily on 10-20m by F2-layer propagation. In years of low sunspot numbers, working distant stations is difficult on those bands. Consequently, 10m and 15m will be completely dead most days and 20m will go dead at night. In years of low sunspot numbers DX contacts are easily made at night on 160m, 80m, and 40m. The sunspot numbers increase and decrease in 11-year average cycles.
Since the curvature of the earth averages about 5m (16ft) every 8km (5 miles), an object 8km (5 miles) from you on perfectly flat earth will be 5m (16ft) below the horizon. Because light travels in straight lines, you cannot see objects beyond the horizon. Radio waves travel in straight lines, but there are ways to get them beyond the horizon. This is referred to as propagation.
In the upper air around 80km (50miles) and higher, radiation from the sun strips electrons from oxygen molecules causing the mole-cules to become ionized forming the ionosphere. The ionized oxygen molecules and its free electrons float in space forming radio-reflec-ting layers. Ionization of the ionosphere varies by the time of day, seasons of the year, and the sunspot cycle. The strength of ionization also varies from day to day and hour to hour. Since the height of the ionosphere varies, the higher the ionized layer becomes, the farther the skip will be. The part of the earths atmosphere called the iono-sphere is divided into three layers. T
he three layers are, from lowest to highest, the D-layer, the E-layer, and the F-layer. Each layer has a different effect on HF radio propagation.Being at a lower altitude, the D-layer molecules are squeezed closer together by gravity than those in higher layers, and the free electrons reattach to the molecules easily. The D-layer requires constant radiation from the sun to maintain its ionization. Radio waves at lower frequencies such as the frequencies of the AM broadcast band cannot penetrate this layer and are absorbed. The higher frequency signals are able to pass through the D-layer. The D-layer disappears at night causing AM broadcast stations to reflect from the higher layers. This is
why AM broadcast signals only propagate by ground wave in the daytime and they can be received from great distances at night. Like the broadcast band, the D-layer absorbs signals on 160m and to a lesser extent 80m during the day making those bands go dead. During solar flares, the D-layer becomes ionized so strongly that all high frequency radio waves are absorbed, causing a radio blackout.E-layer propagation is not well understood. Being at a lower altitude than F-layer, the E-layer is responsible for summertime short skip propagation on the higher HF bands. The skip zone is around 1500km (1000 miles), but at times when the E-cloud covers a wide area in the summer, double hops can be seen. A double hop occurs when the signal reflects from the ionosphere, then returns to the ground, reflects from the ground back to the ionosphere where it is reflected back to the ground. A double hop can propagate the signal 3000km (2000 miles) or more. The E-layer forms mostly during the day, and it has the highest degree of ionization at noon. The E-layer like the D-layer disappears at night. Even so, sporadic-E propagation
can and does form at night. There is a minor occurrence of sporadic-E propagation during the wintertime. On rare occasions, sporadic-E propagation can surprise you by occurring anytime regardless of the sunspot cycle or the season of the year.The F-layer is the highest layer and it is divided into two levels: F1 and F2. At night the F1 and F2 merge into one layer. During the day, the F1-layer doesn't play a part in radio propagation, but F2 does. It is responsible for most high-frequency long distance propagation on 20m and above. However, the F-layer makes it possible for you to work DX on the lower bands at night. Sunspots are responsible for the ionization layers and in years with high sunspot numbers, worldwide contacts can be made easily on 10-20m by F2-layer propagation. In years of low sunspot numbers, working distant stations is difficult on those bands. Consequently, 10m and 15m will be completely dead most days and 20m will go dead at night. In years of low sunspot numbers DX contacts are easily made at night on 160m, 80m, and 40m. The sunspot numbers increase and decrease in 11-year average cycles.
Since the curvature of the earth averages about 5m (16ft) every 8km (5 miles), an object 8km (5 miles) from you on perfectly flat earth will be 5m (16ft) below the horizon. Because light travels in straight lines, you cannot see objects beyond the horizon. Radio waves travel in straight lines, but there are ways to get them beyond the horizon. This is referred to as propagation.
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