Q.What is the need for modulation?

Audio and other message signals are low-frequency signals that cannot be radiated efficiently or over long distances, and if sent directly all stations would occupy the same band and interfere.

[!ANSWER]

Modulation is needed to reduce the antenna height to a practical size, to increase the transmission range with less power, to allow many signals to share the channel without interference (multiplexing), and to reduce the effect of noise.

[!TLDR]

Modulation shifts a low-frequency message onto a high-frequency carrier so it can be radiated efficiently, over long range, without interference between stations.

A baseband message such as speech or music lies in the low-frequency range (about 20 Hz to 20 kHz). Such low frequencies cannot be transmitted directly as radio waves for several practical reasons, and modulation — impressing the message on a high-frequency carrier — overcomes them.

The main reasons modulation is essential are:

1. Practical antenna height: for efficient radiation the antenna length must be comparable to the signal wavelength (about $\lambda/4$). At low audio frequencies $\lambda$ is enormous (kilometres), needing an impossibly tall antenna; a high carrier frequency shortens $\lambda$ so a small antenna suffices.
2. Multiplexing / avoiding interference: by giving each station a different carrier frequency, many signals can be sent through the same medium at once without mixing.
3. Range and power: a high-frequency carrier travels farther for a given transmitted power, increasing the range.
4. Noise reduction: shifting the message to a suitable high frequency band reduces the effect of low-frequency noise and improves signal quality.

[!ANSWER]

Modulation is required to (i) make the antenna height practical, (ii) increase transmission range with reduced power, (iii) permit multiplexing so many stations share the channel without interference, and (iv) reduce noise.