Q.In the broadcast of communication modulation is necessary.

Modulation superimposes a low-frequency message onto a high-frequency carrier; it is essential mainly because low-frequency signals need impractically large antennas and radiate very little power, and because it lets many signals be sent without interfering with each other.

1. What is modulation: Modulation is the process in which some characteristic (amplitude, frequency, or phase) of a high-frequency carrier wave is varied in accordance with the instantaneous value of a low-frequency message (baseband/audio) signal. This produces a modulated wave that carries the message signal's information but at a much higher frequency, suitable for transmission.
2. Why modulation is necessary (any two reasons):

1. Practical size of antenna: Efficient radiation of a signal requires an antenna length comparable to (at least a quarter of) its wavelength, $l \sim \lambda/4$. Audio signals have frequencies of only 20 Hz–20 kHz, corresponding to wavelengths of many kilometres — an antenna of that size is physically impossible to build. Modulating the audio signal onto a high-frequency carrier (MHz range) reduces the required wavelength, and hence the required antenna length, to a few metres — a practical size. …