Physics PROPAGATION OF ELECTROMAGNETIC WAVE IN THE ATMOSPHERE

Electromagnetic Waves

Electromagnetic waves are those waves in which electric and magnetic field vectors changes sinusoidally and are perpendicular to each other as well as at right angles to the direction of propagation of wave.

Electromagnetic waves are produced by accelerated charge particles.

`text(Properties of EM Waves :)`

(i) These waves are transverse in nature.

(ii) These waves propagate through space with speed of light.

(iii) The energy in electromagnetic waves is divided equally between electric field and magnetic field vectors.

(iv) The electric vector is responsible for the optical effects of an electromagnetic wave.

(v) Intensity of electromagnetic wave is defined as energy crossing per unit area per unit time perpendicular to the directions of propagation of electromagnetic wave.

Propagation of Electromagnetic Waves

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.

The composition of the earth-s atmosphere as it plays a vital role in the propagation of em waves.

`text(Electromagnetic Spectrum :)`
The arranged array of electromagnetic radiations in the sequence of their wavelength or frequency is called electromagnetic spectrum.

Ground Wave

To radiate signals with high efficiency, the antennas should have a size comparable to the wavelength λ of the signal (at least ~ λ/4).

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).