The questions in this section were unchanged, but I rewrote this section a little in order to make it clearer.
The ionosphere is what makes long-distance radio communications possible on the shortwave bands. The ionosphere is made up of three layers of charged particles, labelled D, E, and F. Where the Sun is overhead, ionospheric layers reach their maximum height. (G3C02)
During the day, there are two F layers, F1 and F2. The F2 region is mainly responsible for the longest distance radio wave propagation because it is the highest ionospheric region. (G3C03) 2,500 miles is the approximate maximum distance along the Earth’s surface that is normally covered in one hop using the F2 region. (G3B09) At night, F1 and F2 combine into a single F layer.
While the F2 layer is the layer that normally reflects HF radio signals, the E layer can also reflect signals under some conditions. 1,200 miles is the approximate maximum distance along the Earth’s surface that is normally covered in one hop using the E region. (G3B10)
The ionospheric layer closest to the surface of the Earth is the D layer. (G3C01) The D layer is the ionospheric layer that is the most absorbent of long skip signals during daylight hours on frequencies below 10 MHz. (G3C12) Long distance communication on the 40, 60, 80 and 160 meter bands is more difficult during the day because the D layer absorbs signals at these frequencies during daylight hours. (G3C05)
One factor that affects how well the ionosphere will reflect a signal is the angle at which the signal impinges upon it. If the angle is too high, it will pass right through the ionosphere and not be reflected back to earth. The highest takeoff angle that will return a radio wave to the Earth under specific ionospheric conditions is called the critical angle. (G3C04)
Antennas used for DXing should have low takeoff angles. One thing that affects the takeoff angle of an antenna is its height above ground. A horizontal dipole placed between 1/8 and 1/4 wavelength above the ground will be most effective for skip communications on 40 meters during the day. (G3C11)
One interesting propagation phenomenon is scatter propagation. Scatter propagation allows a signal to be detected at a distance too far for ground wave propagation but too near for normal sky-wave propagation. (G3C09) An indication that signals heard on the HF bands are being received via scatter propagation is that the signal is heard on a frequency above the Maximum Usable Frequency. (G3C10) HF scatter signals in the skip zone are usually weak because only a small part of the signal energy is scattered into the skip zone. (G3C08)
A characteristic of HF scatter signals is that they have a wavering sound. (G3C06) HF scatter signals often sound distorted because energy is scattered into the skip zone through several different radio wave paths. (G3C07)
Another interesting phenomenon is Near Vertical Incidence Skywave propagation. Near Vertical Incidence Sky-wave (NVIS) propagation is short distance HF propagation using high elevation angles. (G3C13) Basically what happens is that the antenna sends the signal at an angle of close to 90 degrees, and if conditions are right, the ionosphere reflects that signal back to the earth at a very short distance from the transmitting station.
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