JEE Main & Advanced Physics Communication System / संचार तंत्र Space Communication

The communication process utilising the physical space around the earth is termed as space communication.

Electromagnetic waves which are used in Radio, Television and other communication system are radio waves and microwaves.

The radio waves emitted from a transmitter anteena can reach the receiver antenna by the following mode of operation.

• Ground wave propagation
• Sky wave propagation.
• Space wave propagation.

(1) Ground wave propagation

(i) In ground wave propagation, radio waves travel along the surface of the earth (following the curvature of earth).

(ii) These waves induce currents in the ground as they propagate due to which some energy is lost.

(iii) The decrease in the value of energy (i.e. attenuation) increases with the increase in the frequency of radiowave.

(iv) As the ground wave propagates over the earth, it tilts over more and more due to diffraction. (This is another cause of attenuation of ground wave). After covering some distance, the wave just lie down which means it's death.

(v) Ground wave propagation can be sustained only at low frequencies \[\left( \tilde{\ }500\,kHz\,\,\text{to }1500kHz \right)\] or for radio broadcast at long wavelengths.

(2) Sky wave propagation

(i) These are the waves which are reflected back to the earth by ionosphere.

Ionosphere is a layer of atmosphere having charged particles, ions and electrons and extended above 80 km ? 300 km from the earth's surface.

(ii) These are the radio waves of frequency range 2 MHz to 30 MHz.

(iii) Sky waves are used for very long distance radio communication at medium and high frequencies (i.e. at medium waves and short waves).

(iv) The sky waves being electromagnetic in nature, changes the dielectric constant and refractive index of the ionosphere. The effective refractive index of the ionosphere is

\[{{n}_{eff}}={{n}_{0}}{{\left[ 1-\frac{N{{e}^{2}}}{{{\varepsilon }_{0}}m{{\omega }^{2}}} \right]}^{1/2}}={{n}_{0}}{{\left[ 1-\frac{80.5N}{{{f}^{2}}} \right]}^{1/2}}\]

where \[{{n}_{0}}=\] refractive index of free space, N = electron density of ionosphere, \[{{\varepsilon }_{0}}=\] dielectric constant of free space, \[e=\] charge on electron, m = mass of electron \[\omega =\] angular frequency of EM wave.

(v) As we go deep into the ionosphere, N increases so \[{{n}_{eff}}\] decreases. The refractions or bending of the beam will continue and finally it reflects back.

(vi) Critical frequency \[({{f}_{c}})\] : It is defined as the highest frequency of radio wave, which gets reflected to earth by the ionosphere after having been sent straight to it.

If maximum electron density of the ionosphere is \[{{N}_{\max }}\] per \[{{m}^{3}},\] then \[{{f}_{c}}\approx 9{{({{N}_{\max }})}^{1/2}}\]. Above \[{{f}_{c}},\] a wave will penetrate the ionosphere and is not reflected by it.

(vii) Maximum usable frequency (MUF) : It is the highest frequency of radio waves which when sent at some angle of incidence \[\theta \], towards the ionosphere, get reflected and return to the earth. \[MUF=\frac{{{f}_{c}}}{\cos \theta }\]

(viii) Skip distance : It is the smallest distance from a transmitter along the earth's surface at which a sky wave of a fixed frequency but more than \[{{f}_{c}}\] is sent back to the earth.

(ix) Fading : It is defined as the fluctuation in the strength of a signal at a receiver due to interference of two waves.

Fading is more at high frequencies. It results into errors in data transmission and retrieval.