Given three reasons why modulation of a message signal is necessary for long distance transmission.

Explanation

Three reason behind modulating message signal for long distance transmission are:
The audio/audiovisual signals when converted into electromagnetic waves do not have sufficiently high energy to travel up to long distances, because of their lower frequency. Hency these signals must be modulated with high frequency carrier waves, before being sent.
For effective transmission by an antenna, its size should at least be of the order `(lambda)/(4)`where `lambda` is the wavelength of the signal to be sent. For an electromagnetic wave of frequency 20 kHz, we need an antenna of size `(lambda)/(4)` i.e., 3.75 km high, which is practically impossible. Hence these low frequency signals are first converted into high frequencies so that transmission can be obtained with reasonable antenna lengths.
(iii) The power radiated by an antenna is related to its length l and wavelength `lambda` of the signal as `P prop (l^(2))/(lambda^(2))`.
The relation shows that for the same antenna length, power (P) radiated increases with decreasing wavelength or increasing frequency. Hency there is a need of higher frequency conversion for effective power transmission by the antenna.