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Radio Wave Propagation - Electronics .. Physics

Radio Wave Propagation

Radio waves propagate in the atmosphere in two different ways: The first way, in which the waves are called ground waves, and they travel directly above the Earth's surface. However, because they behave in straight lines, they are unable to provide communications except over relatively short distances.

The second way is done by indirect waves called sky waves, and they can provide communications between two distant places that are not located on a single direct line. This type of communication depends primarily on the presence of a layer of stray gas in the Earth's atmosphere called the stray layer, which reflects indirect waves to the Earth.

The reason for the scattering of the atoms of this atmosphere is the presence of ultraviolet rays and X-rays that come from the sun, and their effect therefore differs between night and day. During the night, and due to the absence of sunlight, the scattering rate decreases in the lower scattering section called the Havisside atmosphere (or layer H, which ranges in height from the Earth's surface between 90 and 150 km), while the ions and electrons tend to combine with each other. As for the upper layer, called the Appleton atmosphere (or layer H, which extends at a height ranging between 150 and 440 km), it remains in a state of scattering for a longer period, due to its low density and therefore - the few collisions that occur between the ions and electrons inside it.

It is known that both the very high frequencies and the ultra-high frequencies used in television transmission operations are able to penetrate these two atmospheres to a large extent, and only a small amount is reflected from them. This is what makes television broadcasting limited to short distances only.

This problem has been overcome by satellites placed in fixed orbits around the Earth. A fixed orbit is any orbit in which the satellite rotates at the same speed as the Earth, and belongs to a plane containing the equator, which allows it to remain in place relative to the Earth. Any satellite in a fixed orbit over the middle of the Atlantic Ocean can receive any signal coming from the American continent and rebroadcast it to the European continent.