Propagation Of Electromagnetic Waves - Practice Questions & MCQ

Propagation of Electromagnetic Waves

Electromagnetic Waves are basically defined as superimposed oscillations of an Electric and Magnetic Field in space with their direction of propagation perpendicular to both of them. Electromagnetic waves are oscillations produced due to the crossing over of an electric and a magnetic field.

The direction of the propagation of such waves is perpendicular to the direction of the force of either of these fields as shown in the diagram below.

In communication using radio waves, an antenna at the transmitter radiates the Electromagnetic waves (em waves), which travel through 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. There are several factors which can influence the propagation of em waves and the path they follow.

Ground wave: These waves are used for a low-frequency range transmission, mostly less than 1 MHz. This type of propagation employs the use of large antenna order which is equivalent to the wavelength of the waves and uses the ground or Troposphere for its propagation. Signals over large distances are not sent using this method.  It causes severe attenuation which increases with the increased frequency of the waves.

SkyWave: Used for the propagation of EM waves with a frequency range of 3 – 30 MHz.  they are present in the ionosphere region of charged ions about 60 to 300 km from the earth's surface. These ions provide a reflecting medium to the radio or communication waves within a particular frequency range. We use this property of the ionosphere for long-distance transmission of the waves without much attenuation and loss of signal strength.

Another thing to consider is the angle of the emission of these waves from the ground. The transmitter emits the EM Waves at a critical angle to ensure total reflection to the ground just like the total internal reflection of optic waves otherwise the waves may escape into space. Skip Distance is the distance between the 2 points between which the wave transmission happens.

Space Wave: It is used for a line of Sight communication also known as LoS. Space satellite communication and very high-frequency waves use this method of propagation. It involves sending a signal in a straight line from the transmitter to the receiver. One must ensure that for very large distances, the height of the tower used for transmission is high enough to prevent waves from touching the earth's curvature thus preventing attenuation and loss of signal strength.

The  important relationship for determining the height of the antennas and their corresponding distance of transmission is given by:

$$
d_M=\sqrt{2 R h_T}+\sqrt{2 R h_R}
$$

where,
$\mathrm{d}_{\mathrm{m}}=$ distance between 2 antennas;
$R=$ Radius of earth $=6400 \mathrm{~km}$
$\mathrm{h}_{\mathrm{T}}=$ Height of transmission antenna;
$h_R=$ Height of receiver antenna
Another important relation for determining the range of transmission(Dt) for a given antenna of height $H_t$ is:

$$
d_T=\sqrt{2 R h_T}
$$