Propagation of electromagnetic waves

Propagation of electromagnetic waves

The propagation of electromagnetic waves depend on the properties of the waves and the environment. Radio waves ordinarily travel in straight lines except where the earth and its atmosphere alter their path. The useful ranges of the electromagnetic spectrum for communication are summarised in Table 10.1.

Radio wave is propagated from the transmitting to the receiving antenna mainly in three different ways depending on the frequency of the wave. They are :

1.     Ground (surface) wave propagation

2.     Space wave propagation

3.     Sky wave (or) ionospheric propagation

Ranges of electromagnetic  spectrum used for communication (NOT FOR EXAMINATION)

Name   Frequency        Wavelength

Extremely Low Frequencies (ELF)    30-300 Hz   10⁷    - 10⁶ m

Voice Frequencies (VF) 300-3000 Hz        10⁶    - 10⁵ m

Very Low Frequencies (VLF)  3-30 kHz     10⁵    - 10⁴ m

Low Frequencies (LF)    30-300 kHz 10⁴    - 10³ m

Medium Frequencies (MF)      300 kHz - 3 MHz 10³    - 10² m

High Frequencies (HF)   3 - 30 MHz 10²    - 10 m

Very High Frequencies (VHF) 30 - 300 MHz      10 - 1 m    

Ultra High Frequencies (UHF) 300 MHz - 3 GHz         1 - 10^-1 m

Super High Frequencies (SHF)         3 - 30 GHz 10^-1 - 10^-2 m

Extremely High Frequencies (EHF)30 - 300 GHz         10^-2 - 10^-3 m

1. Ground (surface) wave propagation

Ground or surface waves are the radio waves which travel along the surface of the earth as shown in Fig 10.1. Ground wave propagation takes place when the transmitting and receiving antennas are close to the ground. Ground wave propagation is of prime importance only for medium and long wave signals. All medium wave signals received during the daytime use surface wave propagation.

2 Space wave propagation

Radio waves propagated through the troposphere of the Earth are known as space waves. Troposphere is the portion of the Earth's atmosphere which extends upto 15 km from the surface of the Earth. Space wave usually consists of two components as shown in Fig 10.2.

1.     A component which travels straight from the transmitter to the receiver.

2.     A component which reaches the receiver after reflection from the surface of the Earth.

Space wave propagation is particularly suitable for the waves having frequency above 30 MHz.

3. Sky wave (or) ionospheric propagation

The ionosphere is the upper portion of the atmosphere, which absorbs large quantities of radiant energy like ultra violet rays, cosmic rays etc., from the sun, becoming heated and ionised. This ionised region contains free electrons, positive and negative ions.

Radio waves in the short wave band, radiated from an antenna at large angles with ground, travel through the atmosphere and encounters the ionised region in the upper atmosphere. Under favourable circumstances, the radiowaves get bent downwards due to refraction from the different parts of the ionised region and again reach the earth at a far distant point. Such a radio wave is called the sky wave and such a propagation of radio wave is known as sky wave propagation or ionospheric propagation. Long distance radio communication is thus possible through the sky wave propagation.

Reflection of electromagnetic waves by ionosphere

The electromagnetic waves entering into the ionosphere, are reflected by the ionosphere. In fact, the actual mechanism involved is refraction. The refractive indices of the various layers in the ionosphere do not remain constant and it varies with respect to electron density and the frequency of the incident wave. As the ionisation density increases for a wave approaching the given layer at an angle, the refractive index of the layer is reduced. Hence, the incident wave is gradually bent farther and farther away from the normal as shown in Fig 10.3 until some point. When the electron density is large, the angle of refraction becomes 90^o and the wave, then travel towards the Earth.

Skip distance and skip zone

In the skywave propagation, for a fixed frequency, the shortest distance between the point of transmission and the point of reception along the surface is known as the skip distance.

When the angle of incidence is large for the ray R₁ as shown in Fig. 10.4, the sky wave returns to the ground at a long distance from the transmitter. As this angle is slowly reduced, naturally the wave returns closer and closer to the transmitter as shown by the rays R₂ and R₃. If the angle of incidence is now made significantly less than that of ray R₃, the ray will be very close to the normal to be returned to the Earth. If the angle of incidence is reduced further, the radio waves penetrate through the layer as shown by the rays R₄ and R₅. For a particular angle of incidence, the distance between the point of transmission and the point of reception is minimum. The minimum distance between the transmitter and the ray like R₃ which strikes the Earth is called as the skip distance.

As we move away from the transmitter, the ground wave becomes lesser and lesser significant. A stage comes when there is no reception due to the ground waves. This point lies somewhere in the skip distance. The region between the point where there is no reception of ground waves and the point where the sky wave is received first is known as skip zone. In the skip zone, there is no reception at all.