Q.Define modulation. Why is it necessary?

Modulation is the process of superimposing a low-frequency information (baseband) signal onto a high-frequency carrier wave before transmission, and three separate arguments show why it is essential rather than optional. First, antenna size: efficient radiation needs an antenna comparable to the signal's wavelength (at least $\lambda/4$), and a 20 kHz baseband signal has a 15 km wavelength - physically impossible to build an antenna for - whereas raising the frequency to, say, 1 MHz shrinks the required antenna to a practical 300 m. Second, radiated power: for a fixed antenna length, radiated power is proportional to $(l/\lambda)^2$, so a low-frequency (long-wavelength) signal would radiate only very little effective power. Third, signal mixing: if many transmitters sent thei …