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Communication System — Physics STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 SciencePhysicsCommunication System5 Marks
Question
Write the need of modulation in communication system.
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Answer

Need of Modulation : Modulation is necessary due to the following three reasons:
(i) Size of the antenna: For transmitting a signal, we need an antenna. This antenna should have a size comparable to the wavelength $\lambda$ of the signal (at least $\lambda/4$ in dimension) so that the antenna properly senses the time variation of the signal. For an electromagnetic wave of frequency 20 kHz the wavelength is 15 km. Obviously, such a long antenna is not possible to construct and operate. Hence direct transmission of such base band signals is not practical. We can obtain transmission with reasonable antenna lengths if transmission frequency is high (for example, if $v$ is 1 MHz, then $\lambda$ is 300 m). Therefore, there is a need of translating the information contained in our original low frequency base band signal into high or radio frequencies before transmission.
(ii) Effective power radiated by an antenna : A theoretical study of radiation from a linear antenna (length $l$) shows that the power radiated is proportional to $(l/\lambda)^{2}$ or directly proportional to the $v^{2}$. This implies that for the same antenna length, the power radiated increases with decreasing $\lambda$, i.e., increasing frequency. Hence, the effective power radiated by a long wavelength baseband signal would be small. For a good transmission, we need high powers and hence this also points out to the need of using high frequency transmission.
(iii) Mixing up of signal : If a number of transmitters are transmitting base band signals simultaneously, then all these signals cover the same frequency range (20 Hz to 20 kHz) and will get mixed up. If transmission is being done at a high frequency, different users may use adequate band width for that given high frequency.

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