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Radio wave propagation and the role of ionosphere
Sometimes relaying of a radio message becomes necessary. A station which is located closer to another station may not be able to contact it, yet it can contact a far away station! To answer these kind of situations, various theories had been put forward. Oliver Heaviside in England and A.E. Kenelly in America suggested that there must be some kind of reflecting medium in the upper atmosphere of the Earth that caused the radio waves to be returned to earth at considerable distances from the radio transmitter. This meant that except for very short distances, the waves did not follow the natural curvature of the earth as had been supposed.
Now it is a confirmed fact that under the action of solar radiation and the hail of meteorites, an ionized layer is formed in the upper part of the Earth's atmosphere. In this layer, the neutral air molecules are decomposed into ions and electrons and the whole layer presents a chaos of charged particles. Short wave radio signals are reflected from this layer just as light rays are reflected from the surface of a mirror, or sound wave from a barrier. Likewise this layer can be compared with the edge of a billiard table. And indeed communication specialists use this layer like the edge of a billiard table: if the ball does not go straight into the pocket, it can be directed on the rebound!
In the same way the short wave signals radiated by distant radio stations get to your receiver on the rebound. What is more they can continue travelling to several places round the world, for the Earth is also like the edge of a billiard-table.
Propagation by refraction, reflection and diffraction
Electromagnetic waves travel in straight lines until they are deflected by something. Radio waves are refracted, or bent, slightly when traveling from one medium to another. Refraction is caused by a change in the velocity of a wave when it crosses the boundary between one propagating medium and another. If this transition is made at an angle, one portion of the wave-front slows down (or speeds up) before the other, thus bending the wave slightly. Radio waves are commonly refracted when they travel through different layers of the atmosphere, whether the highly charged ionospheric layers 100 km and higher, or weather-sensitive area near the Earth's surface. When the ratio of the refractive indices of two media is great enough, radio waves can be reflected, just like light waves striking a mirror. The Earth is a rather lossy reflector, but a metal surface works well if it is several wavelengths in diameter.
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