Q.Why should broadcasting programs use different frequencies?

A carrier wave is a high-frequency electromagnetic wave used to carry a lower-frequency information signal (such as audio) over long distances, by having one of its own characteristics -- amplitude, frequency, or phase -- varied in accordance with the signal; this process of impressing the signal onto the carrier is called modulation, and it is performed at the transmitter.

Carrier waves and modulation are needed for several linked practical reasons. First, efficient transmitting and receiving antennas must be at least about a quarter-wavelength ($\lambda/4$) long, and a raw low-frequency audio signal (for example, at 15 kHz) would require an antenna roughly 5 km long -- clearly impractical -- whereas modulating that signal onto a much shorter-wavelength, high-frequency carrier brings the required antenna length down to a workable size. Second, the effective power an antenna can radiate depends strongly on frequency, generally increasing at higher frequency for a given antenna size, so a high-frequency carrier is radiated far more efficiently than a raw low-frequency signal could be. Third, if many transmitters tried to broadcast their raw low …