In communication using radio waves, an antenna at the transmitter radiates of Electromagnetic waves (em waves), which travel through the space and reach the receiving antenna at the other end. As the em wave travels away from the transmitter, the other end. As the em wave travels away from the transmitter, the strength of the waves keeps on decreasing.
Several facy influence the propagation of em waves and the path they follow. At this point, it is also important to understand the composition of the earth's atmosphere as it plays a vital role in the propagation of em waves. A brief discussion on some useful layers of the atmosphere.
Ground wave
To radiate signal with high efficiency, the antennas should have a size comparable to the wavelength of the signal. At longer wavelength (i.e., at lower frequency), the antennas have large physical size and they are located on or very near ground. In standard AM broadcast, ground based vertical towers are generally used as transmitting antennas. For such antennas, ground has a strong influence on the propagation of the signal. The mode of propagation is called surface wave propagation and the wave glides over the surface of the earth. A wave induces current in the ground over which it passes and it is attenuated as a result y absorption of energy by the earth. The attenuated of surface waves increase very rapidly with increase in frequency.
The maximum range of coverage depends on the transmitted power and frequency (less then a few MHz).
Sky waves
In frequency range from a few MHz up to 30 to 40: long distance communication can be achieved by ionospheric reflection of radio waves back towards the earth.This mode of propagation is called sky waves propagation and is used by short waves broadcast services. The ionosphere particles.
It extends from a height of ~65 Km to about 40 Km above the and other high- energy radiation coming from the sun by air molecules.
The ionosphere is further subdivided into serveral layers. The degree of ionisation varies with the solar radiation is intense but there are few molecules to be ionised. Close to the earth 🌎, even through the molecular concentration is very high, at some intermediate height, there occurs a peak of ionisation density.
The ionospheric layers acts as a reflector for a certain range of frequencies (3 to 30 MHz). Electromagnetic waves of frequencies higher than 30MHz pentrate the ionosphere and escape. These phenomena are shown in the picture.The phenomenon of bending of em waves so that they are diverted towards the earth is similar to total internal reflection in optics.
