#### CHAPTER 15 COMMUNICATION SYSTEMS

Need for modulation

1. Size of the antenna or aerial

The antenna should have a size comparable to the wavelength of the signal (at least λ/4 in dimension) to sense the time variation of the signal. For an electromagnetic wave of frequency 20 kHz, the wavelength λ is 15 km. Hence direct transmission of such baseband signals is not practical due to large antenna requirement. We can transmit with reasonable antenna lengths if transmission frequency is high (for example, if ν is 1 MHz, then λ is 300 m). Therefore, there is a need of translating the information contained in the original low frequency baseband signal into high or radio frequencies before transmission.

2. Effective power radiated by an antenna

Power radiated by an antenna is related to length L of the antenna and wavelength λ as follows,

Therefore, for the same antenna length, the power radiated increases with decreasing λ, 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 the need of using high frequency transmission.

3. Mixing up of signals from different transmitters

If many people are talking at the same time or many transmitters are transmitting baseband information signals simultaneously, all these signals will get mixed up and there is no simple way to distinguish between them. Using communication at high frequencies and allotting a band of frequencies to each message signal for its transmission can solve the problem.