Propagation of waves

Propagation of waves

Introduction : Different communication devices that we use in our daily life functions with the help of electromagnetic wave. This electromagnetic waves are transmitted to a large distance to communicate at larger distance. This is possible due to propagation of wave. We are going to see this topic in detail.

Before understanding propagation of waves let us study about Earth's atmosphere.

Propagation of wave : The process of communication involve the process of transmission of wave from one point to another. Transmission or propagation of wave is done mainly through three types.

1) Ground Wave propagation : The transmission of wave through the ground surface is called as ground wave propagation. In this method the wave can be transmitted through very short distance as the radio waves induces current in the ground and looses it's energy  and  thus it is not feasible to transmit it to long distance. This wave has a frequency of  2 MHz. This is used for TV signal and broadcasting.

2) Space Wave propagation : When the radio waves from the transmitting antenna reach the receiving antenna either directly along a straight line (line of sight) or after reflection from the ground or satellite or after reflection from troposphere, the wave propagation is called space wave propagation. TV signals which have high frequency, transmission over long distance is not possible by means of space wave propagation. The maximum distance over which a signal can reach is called its range. 

Mathematically,

Range of Transmission Antenna

 Let the height of the transmitting antenna (AA') situated at A be h. B represents the point on the surface of the Earth at which the space wave hits the Earth. The triangle OA'B is a right angled triangle. From ∆ OA' B we can write

 OA'² = AB'²  + OB2

 (R+h)²  = d²  + R²  or 

R² +  h²   + 2Rh = d²  + R²  

As h << R, we can ignore h²  and write d ≅ √2Rh 

3) Sky Wave propagation : In this type of propagation the wave is transmitted through till receiving antenna after reflection from ionosphere. This method is used to transmit signal to long distance. Mostly waves ranging from 3Mhz to 30MHz is transmitted through sky wave propagation.

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