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Ionosphere
The ionosphere forms the top layer of the atmosphere, extending from an altitude of about 50 miles to the point where the atmosphere thins out to the vacuum of space. In this region, atmospheric gases are at pressures too low to sustain weather phenomena or life, and energy from the Sun arrives nearly unimpeded. Ultraviolet light from the Sun strips nitrogen and oxygen atoms of some of their electrons, rendering them into electrically-active ions that alter the transmission paths of radio waves.
Radio Propagation
Radio waves pass through the lower atmosphere from the broadcast antenna to listener̵7;s receiving equipment. High frequencies, such as FM radio, television and radar, mostly pass through the ionosphere and into space. These transmissions are called ̶0;line of sight̶1; because they travel a straight line and do not follow the gradual curve of the Earth. If you happen to be farther than 60 miles from a transmission antenna, it is then beyond the horizon; the Earth itself blocks the signals. However, the ionosphere reflects lower-frequency waves such as AM and shortwave radio. These waves bounce back and forth between the Earth̵7;s surface and the ionosphere for long distances until the signal̵7;s power finally fades.
Multipath
The ionosphere is not a perfect, uniform layer; it has turbulence, and it warms and expands during the day and cools and contracts at night. The dynamic nature of the ionosphere means it ̶0;churns̶1; radio waves moving through it in complex ways. One ̶0;pocket̶1; in the ionosphere may allow radio waves to pass through it, and an adjacent zone may reflect the same frequencies back to Earth. At the receiving point, this creates a phenomenon called multipath, where waves from the same broadcast arrive at a receiver from many angles.
Fading Signals
Radio broadcasts coming directly from a transmission antenna move in even, coordinated waves like ripples in a pond. As radio signals move through different zones in the ionosphere, the waves in each zone shift. The waves arrive at the receiver from different directions, and when they do, sometimes they cancel each other out and you receive no signal. The ionosphere constantly changes the reflected angles of the waves, making distant radio programs fade in and out.