. P ROPAGATION OF E LECTROMAGNETIC W AVES In communication using radio waves, an antenna at the transmitter radiates the 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 strength of the wave keeps on decreasing. Several factors 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 is given in Table . . .
. Ground wave To radiate signals with high efficiency, the antennas should have a size comparable to the wavelength of the signal (at least ~ # / ). At longer wavelengths (i.e., at lower frequencies), the antennas have large physical size and they are located on or very near to the 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 of absorption of energy by the earth. The attenuation of surface waves increases very rapidly with increase in frequency.
The maximum range of coverage depends on the transmitted power and frequency (less than a few MHz). Standard AM broadcast FM broadcast Television Cellular Mobile Radio Satellite Communication Service Frequency bands Comments - kHz - MHz - MHz - MHz - MHz - MHz - MHz - MHz . - . GHz .
- . GHz VHF (very high frequencies) TV UHF (ultra high frequencies) TV Mobile to base station Base station to mobile Uplink Downlink T ABLE . S OME IMPORTANT WIRELESS COMMUNICATION FREQUENCY BANDS . .
Sky waves In the frequency range from a few MHz up to to MHz, long distance communication can be