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Communication System — Physics STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 SciencePhysicsCommunication System5 Marks
Question
(i) Which mode of propagation is used by short wave broadcast services having frequency range from a few MHz to 30 MHz? Explain diagrammatically how long distance communication can be achieved by this mode.
(ii) Why is there an upper limit to frequency of waves used in this mode ?
✓

Answer

(i) Sky wave propagation is used by short wave broadcast services having frequency range from few MHz to 30 MHz.
Long Distance Communication by Sky Wave Propagation : The radio waves which travel from the transmitting to the receiving antenna after being reflected from the ionosphere are called 'sky waves' and the phenomenon is known as 'sky-wave propagation'. In this mode of transmission radio waves can travel through very large distances and can even travel around the earth.
Mechanism of reflection of Sky Waves from the ionosphere : The reflection of e.m. waves from the ionosphere is caused by the oscillating electric field of e.m. waves (say of frequency $\omega$) that changes the velocity of electrons in the ionosphere. It changes the effective dielectric constant $\in^{\prime}$ and also the refractive index $n^{\prime}$ as compared to free space with value of the above quantities as $\omega_0$ and $n_{0}$, respectively. These quantities are related as follows :
$n^{\prime}=\sqrt{\varepsilon^{\prime} \mu_0}$
$=\sqrt{\varepsilon_0 \mu_0\left[1-\left( N e^2 \varepsilon_0 m \omega^2\right)\right]}$
or $n^{\prime}=n_0\left[1-\left(N e^2 / \varepsilon_0 m \omega^2\right)\right]^{1 / 2}$ ...(1)
Image
where $e$ is the electronic charge, $m$ is the mass of electron and $n$ is the density of electrons in ionosphere. The ionosphere behaves as a rarer medium because its refractive index is less than its free space value of no. Therefore, the wave turns away from the normal when it enters the ionosphere. As we go deep into the ionosphere (the density of electrons, N is large), the refractive index keeps on decreasing. The bending of the beam will continue till it reaches the critical angle after which it is reflected back. It is seen from equation (1) that different frequencies (ω) will be reflected from different regions of the ionosphere having different values of N. Therefore, different points on earth receive signals reflected from different depths of the ionosphere. In case, the frequency is too high, then after a certain value the electron density N may never be so high as to produce enough bending for attaining critical angle. This is called critical frequency. If the maximum electron density of the ionosphere is Nmax per m³, then the critical frequency $f_c$ is approximately calculated by :
$f_c=9\left(N_{\max }\right)^{1 / 2}$
The $f_c$ ranges approximately from 5 to 10 MHz . If the frequencies are higher than this range, they cross the ionosphere and do not come back to the earth.
(ii) There is an upper limit because for frequencies above 30 MHz radiowaves penetrate through ionosphere and escape i.e. never come back to earth.

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