Explain the need for modulation related to the size of antenna (aerial).
1) For transmitting a signal, we need an antenna or an aerial. This antenna should have length comparable to the quarter wavelength of the signal (λ/4) so that the antenna properly senses the time variation of signal
2)For an electromagnetic wave of frequency 20 kHz, wavelength λ is 15 km. Thus, vertical antenna of this size required at audio frequencies (< 20 KHZ) is impracticable. Hence, direct transmission of such baseband signals is not practical.