Q.Explain why is modulation needed.

[!TLDR] Modulation is needed because raw low-frequency signals cannot be transmitted efficiently over long distances (impractical antenna size, inefficient radiation, and unavoidable mixing between transmitters). [!ANSWER] To make practical-length antennas possible, radiate power efficientl …

As developed across sections 13.5.2 and 13.6, an audio (or similar low-frequency) signal below about 20 kHz cannot be transmitted directly over large distances for several compounding reasons: efficient antennas must be at least about $\lambda/4$ long, and a raw 15 kHz signal would need an impractical roughly-5-km antenna; low-frequency waves radiate power far less efficiently than high-frequency ones from any given antenna size; and if many transmitters broadcast their raw signals directly, those signals would inevitably overlap and mix together, making individual stations impossible to separate at a receiver. Modulation -- superimposing the low-frequency signal onto a high-frequency carrier wave by varying the carrier's amplitude, frequency, or phase -- solves all of these problems simultaneously: it allows a practically sized antenna, radiates the signal's power far more efficiently, and (by giving each transmitter a distinct carrier frequency) k …

Bring together the antenna-length, radiation-efficiency, and station-separation reasons from sections 13.5.2 and 13.6 into a sing …

Giving a single narrow reason (e.g. only 'to increase range') instead of the fuller set of linked reasons (antenna size, radiation efficiency, and sign …