Antenna is used at both transmitter and receiver end. Antenna height is an important parameter to be discussed. The height of the antenna must be a multiple of \(\frac{λ}{4}\),
h = \(\frac{λ}{4}\)...... (1)
where λ is wavelength (λ = \(\frac{c}{v}\)), c is the velocity of light and v is the frequency of the signal to be transmitted.
An example:
Let us consider two baseband signals. One signal is modulated and the other is not modulated. The frequency of the original baseband signal is taken as v = 10 kHz while the modulated signal is v = 1 MHz. The height of the antenna required to transmit the original baseband signal of frequency v = 10 kHz is
h1 = \(\frac{λ}{4}\) = \(\frac{C}{4v}\) = \(\frac{3 \times 10^8}{4 \times 10 \times 10^3}\) =7.5 km ......(2)
The height of the antenna required to transmit the modulated signal of frequency v = 10 kHz is
h1 = \(\frac{λ}{4}\) = \(\frac{C}{4v}\) = \(\frac{3 \times 10^8}{4 \times 10 \times 10^6}\) =75 km ......(3)
Comparing equations (2) and (3), we can infer that it is practically feasible to construct an antenna of height 75 m while the one with 7.5 km is not possible. It clearly manifests that modulated signals reduce the antenna height and are required for long distance transmission.
