Answer Posted / reza r

Modulation is used in order to achieve some of the followings:

1- To put the signal on a carrier frequency we have, for
example we have a channel that can only carry frequencies
between 30~31MHz, an we have to transfer our speech over
this channel (Bandwidth of speech is 0~4KHz), so we use
modulation to mount the signal over the available channel.

2- To minimize the destructive channel effects such as
noise, interference and so on. destructive effects will vary
on the different frequencies.

3- in order to decrease the size of the antenna! (antenna
size depends on the Landa (wavelength of the signal). The
higher the frequency, the lower the wavelength. The lower
the wavelength, the smaller the antenna size!

also there is some other benefits in modulation...

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