Modes of Propagation of Electromagnetic Waves: Definitions, Types, Formulas

Modes of Propagation of Electromagnetic Waves: As you know, communication is very important in our daily lives because it is the transmission of information. For communication, we generally use electromagnetic waves to transfer information at different levels. Do you know how electromagnetic waves are produced? Neither static charges nor charges in uniform motion can be sources of electromagnetic waves.

The former produces only electrostatic fields, while the latter produces magnetic fields that do not vary with time. Maxwell’s gives an important result that accelerated charges radiate electromagnetic waves. In this article, we will introduce the concepts of communication, electromagnetic waves, and their properties. We will also discuss the different modes of propagation of electromagnetic waves.

Electromagnetic Waves

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

The direction of the propagation of such waves is perpendicular to the direction of the force of either of these fields, as seen in the below figure.

Properties of Electromagnetic Waves

Some important properties of the propagation of electromagnetic waves are given following:

1. Electromagnetic waves travel at the speed of light.
2. For propagation of these waves, a medium is not required.
3. These waves undergo interference and diffraction.
4. These waves do not get deflected by an electric or magnetic field.
5. These waves travel in transverse form.

Modes of Propagation of Electromagnetic Waves

As the wave propagates through the space. 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 electromagnetic waves travel away from the transmitter, the strength of the wave keeps on decreasing.

It is also important to understand the composition of the earth’s atmosphere as it plays a vital role in the propagation of electromagnetic waves. The mode of propagation of electromagnetic waves in the atmosphere and space may be divided into the following three categories that are given below:

1. Ground Waves
2. Sky Waves
3. Space Waves

1. Ground Waves

In the ground wave propagation, the radio waves from the transmitting antenna propagate along the earth’s surface to reach the receiving antenna. To radiate signals with high efficiency, the antennas should have a size comparable to the wavelength l of the signal (at least \(~1/4\)). At longer wavelengths (i.e., at lower frequencies), the antennas have a large physical size, located on or very near the ground. The ground has a strong influence on the propagation of the signal for these types of antennas.

As the wave glides over the surface of the earth, this mode of propagation is also called surface wave propagation, as shown in the figure below. A wave induces a current in the ground over which it passes, and it is attenuated due to the absorption of energy by the earth. When the frequency increases, then the attenuation of surface waves also increases very rapidly. Transmitted power and the frequency (less than a few \({\rm{MHz}}\)) determine the maximum range of coverage.

2. Sky Waves

The radio waves propagate from the transmitting antenna through the sky towards the receiver either directly or by reflection from the ground. Then, this kind of wave propagation is called skywave propagation. It is also called ionospheric wave propagation. It is used in short-wave broadcast services. It occurs in the frequency range from a few \({\rm{MHz}}\) up to \(30\) to \({\rm{40}}\,{\rm{MHz}}\).

By the ionospheric reflection of radio waves back towards the earth, we can also achieve long-distance communication. Due to the presence of many ions or charged particles in this atmosphere, it is so-called the ionosphere. It extends from a height of \(~65\,{\rm{Km}}\) to about \(400\;{\rm{km}}\) above the earth’s surface, as shown in the below figure.

When the ultraviolet and other high-energy radiation comes from the sun, it is absorbed by the air molecules, thus ionization occurs. The degree of ionization varies with height. The solar radiation is intense at great heights, but there are few molecules to be ionized.

Close to the earth, even though the molecular concentration is very high, the radiation intensity is low, so the ionization is again low. For a frequencies range of (\(3\) to \(30\,{\rm{MHz}}\)), the ionospheric layer acts as a reflector. When the electromagnetic waves of frequencies higher than \(30\,{\rm{MHz}}\) fall on this ionosphere, it penetrates and escapes.

3. Space Wave

In space wave propagation, the radio waves from the transmitting antenna propagate through space, around the ground, and reach the receiver either directly or by reflection from the ground. A space wave travels from the transmitting antenna to the receiving antenna in a straight line. For line-of-sight (LOS) communication and satellite communication, space waves are used. So it is also called the line of sight propagation.

Communication is essentially limited to line-of-sight paths at frequencies above \(40\,{\rm{MHz}}\). If the signal is to be received beyond the horizon, the receiving antenna must be high enough to intercept the line-of-sight waves. For calculating the maximum line-of-sight distance, let us consider the below figure:

If the transmitting antenna is at a height \(\left( {{h_{\rm{T}}}} \right)\), then you can show that the distance to the horizon \(\left( {{d_{\rm{T}}}} \right)\) is given as \({d_{\rm{T}}} = \sqrt {2R{h_{\rm{T}}}} \), where \(R\) is the radius of the earth (approximately \(6400\;{\rm{km}}\)). The term \(\left( {{d_{\rm{T}}}} \right)\) is also known as the radio horizon of the transmitting antenna. From the above figure, the maximum line-of-sight distance \(\left( {{d_{\rm{M}}}} \right)\) between the two antennas with heights as \({{h_{\rm{T}}}}\) and \({{h_{\rm{R}}}}\) respectively above the earth is given by

\({d_{\rm{M}}} = \sqrt {2R{h_{\rm{T}}}} + \sqrt {2R{h_{\rm{R}}}} \)

Where, \({{h_{\rm{T}}} = }\) height of the transmitting antenna, and

\({{h_{\rm{R}}} = }\) height of the receiving antenna.

Summary

An accelerating charge produces electromagnetic waves. Electric and Magnetic fields in an electromagnetic wave oscillate sinusoidally in space and time. The oscillating electric and magnetic fields are perpendicular to each other and also to the direction of propagation of the electromagnetic wave.

For transmission over long distances, a device called antennas radiates signals into the space, and these radiated signals propagate as electromagnetic waves. Electromagnetic waves propagate as surface waves near the surface of the earth. It is useful up to a few \({\rm{MHz}}\) frequencies. With the help of reflecting electromagnetic waves by the ionosphere, we can achieve long-distance communication between two points on the earth. Such waves are called sky waves. Up to a frequency of about \({\rm{30}}\,{\rm{MHz}}\), skywave propagation takes place, and above this frequency, electromagnetic waves essentially propagate as space waves. In line-of-sight communication and satellite communication, space waves are used.

If an antenna radiates electromagnetic waves from a height \(\left( {{h_{\rm{T}}}} \right)\), then the range \(\left( {{d_{\rm{T}}}} \right)\) is given by \({d_{\rm{T}}}\sqrt {2R{h_{\rm{T}}}} \), where \(R\) is the earth’s radius.

Modes of Propagation of Electromagnetic Wave – Sample Problems 

Q.1. What is the maximum distance for good communication between the transmitting antenna, whose height is given as \(32\,{\rm{m}}\), and the receiving antenna, whose height is given as \(50\,{\rm{m}}\) in LOS mode? Given the radius of the earth is \(6.4 \times {10^6}\;{\rm{m}}\).
Ans: The maximum line-of-sight distance \({d_{\rm{M}}}\) between the two antennas is given by:

\({d_{\rm{M}}} = \sqrt {2R{h_{\rm{T}}}} + \sqrt {2R{h_{\rm{R}}}} \)
\( \Rightarrow {d_{\rm{M}}} = \sqrt {2 \times 64 \times {{10}^5} \times 32} + \sqrt {2 \times 64 \times {{10}^5} \times 50} \,{\rm{m}}\)
\( \Rightarrow {d_{\rm{M}}} = 144 \times {10^2} \times \sqrt {10} \;{\rm{m}}\)
\( \Rightarrow {d_{\rm{M}}} = 45.5\,{\rm{Km}}\)

FAQs on Modes of Propagation of Electromagnetic Waves

Q.1. What is electromagnetic wave propagation?
Ans: The changing magnetic field, in turn, induces an electric field. When a series of electrical and magnetic oscillations combine, they produce a form that propagates as an electromagnetic wave.

Q.2. What are the different modes of propagation of electromagnetic waves?
Ans: There are three types of modes of propagation of electromagnetic waves: Ground wave propagation, Space wave propagation, and Skywave propagation.

Q.3. What are space waves?
Ans: Space waves are very high-frequency radio waves that lie between \(30\,{\rm{MHz}}\) to \(300\,{\rm{MHz}}\) or more. In space wave propagation, the radio waves from the transmitting antenna propagate through space, around the ground, and reach the receiver either directly or by reflection from the ground.

Q.4. Write all the types of waves in the electromagnetic spectrum PDF?
Ans: The EM spectrum is generally divided into seven regions, decreasing wavelength and increasing energy and frequency.
1. Radio Waves: Instant Communication.
2. Microwaves: Data and Heat.
3. Infrared Waves: Invisible Heat.
4. Visible Light Rays.
5. Ultraviolet Waves: Energetic Light.
6. X-rays: Penetrating Radiation.
7. Gamma Rays: Nuclear Energy.

Q.5. Give some examples of electromagnetic waves in our daily life.
Ans: Examples of Electromagnetic Waves in our daily life are given following:
1. Radio and Television
2. Microwave Ovens
3. Medical Examination
4. Sterilization
5. Communication Devices
6. RADAR
7. Military Equipment
8. Cancer Therapy

Now you are provided with all the necessary information on the modes of propagation of electromagnetic waves and we hope this detailed article is helpful to you. If you have any queries regarding this article, please ping us through the comment section below and we will get back to you as soon as possible.