For a long distance transmission of the message signal, we need modulation because of the following reasons.
(i) Low energy: The message signals when converted to e.m. waves, do not have the sufficient energy to travel up to the long distance, because of their low frequency. Hence these message signals are modulated with high-frequency carrier signals before sending because carrier signals have high energy for long distance transmission.
(ii) Size of Antenna: For the effective Transmission by an antenna, the size of the antenna should be at least of the size λ/4 where λ is the wavelength of the signal to be sent. Thus for an em wave of the audio signal of frequency is 20 kHz, we need an antenna of size λ/4 i.e. nearly 3.75 km high, which is practically impossible. Hence, these low-frequency signals first modulated to high-frequency signals before transmission to get the proper size of the antenna.
(iii) Mixing of signals:**-** When a number of signals are transmitted simultaneously, all these signals will get mixed up and at the end, we get a mixed signal, which is very difficult to separate. Therefore to remove this limitation, transmission is done at high frequency and a band of frequency is allotted to each user at the end, as done for radio and TV channels.
